Biology Mock Test - 2

The correct answer is Gametogenesis.

Explanation:

• The formation of the gamete is the initial step of sexual reproduction.
• Gametes are the haploid egg cells in females and sperm in males.
• Gametogenesis is the process of gamete formation in the gonads (testis and ovaries) in sexually reproducing animals.
• Gonads refer to the gamete-forming organs in the body, i.e., testis in males and ovaries in females.
• Gametes are haploid in nature as they are formed from diploid cells by meiosis.
• The two processes of gametogenesis are:

1. Spermatogenesis:​

• ​​The process of formation of haploid spermatozoa (sperms) from diploid spermatogonia is known as Spermatogenesis.

      2. Oogenesis:

• The process of formation of a mature female gamete (haploid ovum) from the diploid oogonia in the ovary is known as Oogenesis.

Mistake Points

•  Please note that spermatogenesis and oogenesis cannot be chosen as correct answer because they are specific processes.
• The question asks about 'gamete' in general and so 'gametogenesis' is the correct answer.

Thus, Formation of gametes in the gonads is known as Gametogenesis.

The correct answer is Oviparous.

• The animals which lay eggs are called oviparous animals.
• Development of the embryo is through external means(Internal fertilization).
• Egg-laying animals are referred to as oviparity.
• Examples of oviparous animals are:
  • Hen.
  • Frog.
  • Lizard.
  • Butterfly.
  • Fishes.
  • Reptiles.
  • Crow or any other bird.
• The animals which give birth to young ones are called viviparous animals.
• Development of the embryo and fertilization both occurs inside the female body.
  • Humans.
  • Elephant.
  • Dog.
  • Cow.
  • Cat.

Concept-

• The flower is the reproductive unit of a plant.  It is meant for sexual reproduction.
• The flower may contain essential and non-essential whorls.
• The vegetative part of a flower consists of Petals and Sepals.
• The reproductive parts of a flower contain:
• Stamen, which is the male reproductive organ and is also known as Androecium. It consists of two parts: anther and filaments.
• Pistil, which is the female reproductive organ and is also known as Gynoecium.

Explanation-

• A typical flower has four main parts - or whorls - known as the calyx, corolla, androecium, and gynoecium.
• The innermost group of structures in the flower is the gynoecium, or the female reproductive component(s).
• The innermost whorl is the whorl of the carpel and is called gynoecium.
• The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary.
• A flower may have one or multiple carpels.

Thereby gynoecium represents the whorl of the carpel.

Additional Information

• The whorl of stamens is called the androecium.

The correct answer is Anther.

• Pollination: To prevent pollen grains from drying up they generally have a tough protective coat.
  • They can be carried by wind or water as pollen grains are light in weight.
  • Insects carry away pollen on their bodies when they visit flowers to suck nectar.
  • Thus pollination is a process of transfer of pollen from the anther to the stigma of a flower.
  • It is called self-pollination if the pollen lands on the stigma of the same flower.
  • it is called cross-pollination if the pollen of a flower lands on the stigma of another flower of the same plant, or that of a different plant of the same kind.

Additional Information

Table: Description of parts of a flower and their functions:

The correct answer is 1 - B, 2- A, 3 - C.

Key Points

Autogamy

• In this type, pollination is achieved within the same flower.
• Transfer of pollen grains from the anther to the stigma of the same flower.
• In a normal flower that opens and exposes the anthers and the stigma, complete autogamy is rather rare.
• Autogamy in such flowers requires synchrony in pollen release and stigma receptivity and also, the anthers and the stigma should lie close to each other so that self-pollination can occur.
• Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers: chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all.
• In such flowers, the anthers and stigma lie close to each other.
• When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination.
• Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma.
• Cleistogamous flowers produce assured seed-set even in the absence of pollinators. 

Geitonogamy

• Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
• Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

Xenogamy

• Transfer of pollen grains from the anther to the stigma of a different plant.
• This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Concept:

• The endosperm is the food-laden tissue formed during the development of angiospermous seed which provides essential nutrients to the growing embryo and also to the young seedling at the time of seed germination. 
• In angiosperms, the endosperm develops from a triploid (3n) primary endosperm nucleus which is formed as a result of the fusion of a male gamete with the secondary nucleus of the central cell.
• Based on the first and subsequent divisions of the primary endosperm nucleus, the endosperm is of three types free nuclear, cellular, and helobial.

Explanation:

• Albuminous seeds: The seeds in which the endosperm is not completely utilized during the development of the embryo and they retain some portion of it are called albuminous seeds or endospermic seeds.
• Examples: Wheat, Maize, Barley, Castor & Sunflower
• Non-albuminous seeds: These seeds do not have any residual endosperm, so they are called non-albuminous or non-endospermic seeds.
• Examples: Pea, Groundnut

​Important Points

• Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.

The correct answer is Umbilical cord.

Key Points

• Umbilical cord
  • It is also known as a birth cord or funiculus umbilicalis and is found in placental mammals.
  • It is connecting structure between the placenta and the foetus.
  • During fetal development, the umbilical cord is physiologically and genetically part of the foetus.
  • In humans, it contains two umbilical arteries and one umbilical vein.
  • Function - The umbilical vein is associated with the function of supplying oxygenated.
    • Nutrient-rich blood from the placenta to the foetus.
    • The umbilical arteries carry the deoxygenated, nutrient-depleted blood from the foetus to the placenta.

Additional Information

• Amnion​
  • It is one of the four protective embryonic membranes found in amniotes (reptiles, birds, and mammals).
  • The other three embryonic membranes are - chorion, allantois, and yolk sac.
  • An amniotic cavity is found between the embryo and the amnion and is filled with fluid.
  • The amniotic fluid protects the embryo from external shock and temperature fluctuations.
• Bulbourethral gland
  • They are also known as Cowper's glands.
  •  It is an exocrine gland and is part of the male reproductive system.
  • The secretion (Cowper's fluid) of the gland is alkaline in nature.
  • Function - The fluid serves to lubricate the passage of semen through the urethra.
    • The fluid neutralizes the acid that may be present in the urethra
    • The fluid also lubricates the female's vagina and protects sperms from abrasion against the vaginal wall.
• Chorion
  • It is the outermost embryonic membrane found in amniotes.
  • It together with the amnion forms the amniotic sac.
  • The space between the amnion and the chorion is known as an extraembryonic coelom.

Concept-

 There are several accessory glands that play a major role in the male reproductive system.

Explanation:

      1.Seminal Vesicles-

• seminal vesicles are sac-like pouches that attach to the vas deferens near the base of the bladder.                       
• The seminal vesicles make a sugar-rich fluid (fructose) that provides sperm with a source of energy and helps with the sperm's ability to move (motility).

      2.Prostate gland-

• The prostate gland is a walnut-sized structure that is located below the urinary bladder in front of the rectum.
• The prostate gland contributes additional fluid to the ejaculate. Prostate fluids also help to nourish the sperm.

      3.Bulbourethral gland-

• The bulbourethral glands, or Cowper’s glands, are pea-sized structures located on the sides of the urethra, just below the prostate gland.
• These glands produce a clear, slippery fluid that goes directly into the urethra. This fluid serves to lubricate the urethra and to neutralize any acidity that may be present due to remaining drops of urine in the urethra.

Additional Information

• Male reproductive system also consists of the penis, scrotum, testes, epididymis, vas deferens.

The correct answer is the Formation of sperms.

Key Points 

• Estrogen is the female sex hormone produced in the adrenal glands, ovaries, and fatty tissue, which is mainly responsible for the development of breasts in puberty and the growth of the uterus lining during the menstrual cycle.
• In females, the estrogen hormone prepares the uterus and other reproductive organs for pregnancy.
• It also regulates the menstrual cycle, growth of pubic and underarm hair and also maintains bone strength by collaborating with calcium, vitamin D, and other essential vitamin and minerals.
• It is not responsible for the formation of sperms.

Important Points

• Functions of Estrogen Hormone​: 
  1. It keeps cholesterol in control.
  2. Regulation of the menstrual cycle.
  3. Regulates the development and functioning of the uterus.
  4. It is mainly responsible for the changes that take place during pregnancy.
  5. It is also responsible for secondary sexual characteristics and reproductive development during puberty.
  6. It plays a vital role in the growth and development of the uterus, changes in the uterus during the menstrual cycle, widening of the pelvis, breast, increasing growth of the vagina and hair on the body.​

Important Points

• Penis carries the sperms up to the site of fertilization
• The sperm cell is made up of 3 parts
  • Head- Stores genetic information.
  • Middle Part- Has mitochondria for the generation of energy.
  • Tail- This is a hair, like a flagellum which provides motility to sperm.

Key Points

• The sequence of Spermatogenesis:
  • Spermatogonia- Primary Spermatocytes- secondary spermatocytes- Spermatid- Spermatozoa (Sperm).
• The temporary storage of immature sperm occurs in epididymis with 1 to 2 months these become mature and able for fertilization.
  • This process is known as 'capacitation'.
• The prostate gland is found only in males, there is no gland in females homologous to the male Prostate gland.
• Sertoli cells in testes secrete ABH (Androgen Binding Hormone).
• This hormone is useful for the storage of a large amount of 'Testosterone' in testes.

Concept:

• The devices that are used to prevent unwanted pregnancies are called Birth control devices or Contraceptives.
• Various Contraceptive methods are:

1. Natural/traditional methods
2. Barrier methods - Condoms, Diaphragms, Cervical caps
3. Intra uterine devices (IUDs) - Lippes loop, Copper T, Copper 7
4. Oral contraceptives - Saheli
5. Injectables and Implants 
6. Surgical Methods - Vasectomy & Tubectomy

Explanation:

Natural/traditional methods: These are natural methods that work on the principle of avoiding the meeting of ovum and sperm.

a.) Periodic abstinence is a method in which a couple avoids or abstains from coitus from day 10-17 of the menstrual cycle when ovulation is expected to occur

b.) Coitus interruptus or withdrawal is a method in which a male partner withdraws his penis from the vagina just before ejaculating so as to avoid insemination

Additional Information 

• Condoms are barriers made of thin rubber/latex sheath used to cover the penis in the male or vagina and cervix in females. It prevents the deposition of ejaculated semen into the vagina of the female.
• Diaphragms are the barrier made of rubber that is inserted into the female reproductive tract to cover the cervix during coitus.
• Tubectomy: It is a surgical method, a small part of the fallopian tube is removed or tied up through a small incision in the abdomen through the vagina.

Concept:

The methods that are used to prevent unwanted pregnancies are called birth control methods.

• Various Contraceptive methods are:
• Natural/traditional methods: These are natural methods that work on the principle of avoiding the meeting of ovum and sperm.
• a.) Periodic abstinence is a method in which a couple avoids or abstains from coitus from day 10-17 of the menstrual cycle when ovulation is expected to occur
• b.) Coitus interruptus or withdrawal is a method in which a male partner withdraws his penis from the vagina just before ejaculating so as to avoid insemination
• Barrier methods - Condoms, Diaphragms, Cervical caps
• Intra-uterine devices (IUDs) - Lippes loop, Copper T, Copper 7
• Oral contraceptives - Saheli
• Hormonal methods - Injections and Implants
• Surgical Methods - Vasectomy & Tubectomy

Explanation:

• Medical termination of Pregnancy (MTP):
• It is defined as the intentional or voluntary termination of pregnancy before full term.
• MTP was legalized by the Government of India in 1971 under strict conditions to avoid misuse.
• MTP is done in the following situations:

1. Failure of Contraceptive used during coitus or rapes.
2. When continued pregnancy is harmful or fatal to either mother or fetus or both.

• It is performed during the first trimester, i.e., up to 12 weeks of pregnancy

Concept-

• Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses.
• Today these rules are called the Principles or Laws of Inheritance the First Law or Law of Dominance and the Second Law or Law of Segregation.

• According to the law of dominance.
  • Characters are controlled by discrete units called factors.
  • Factors occur in pairs.
  • In a dissimilar pair of factors, one member of the pair dominates the other.

Explanation-

• The dihybrid cross involves two characters in a single plant.
• The parent pea plants are of two types.
  • Tall plants with violet flowers.
  • Short plants with white flowers.
• So, the genotype of the parents could be-
  • Tall plants with violet flowers: TTWW or TtWw
  • Short plants with white flowers: ttww
• Since all the progeny bore violet flowers, it implies that the tall plant with violet flowers has genotype 'WW' for violet flower color. 
• Since the progeny acquired is both tall and short, the parent plant was not a pure tall plant and carry genes that determine the short height of the plant.
• Therefore, the genotype of the plant with respect to height would be 'Tt'.

So, if a cross is carried out between a tall parent with violet flowers (TtWW) and a short parent with white flowers (ttww), the progeny obtained is TtWw (8) : ttWw (8).

Additional Information

• Law of Segregation
  • This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage. 
  • Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.
  • Of course, a homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion.

Key Points 

• The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example of incomplete dominance.
• In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F1 (Rr) was pink.
• When the F1 was self-pollinated the F2 resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White.
• Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant: recessive ratio.
• This happened because the R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).

Additional Information 

Key Points

• Sickle-cell anaemia is caused by a point mutation that causes a substitution of amino acid.
• The β-globin chain of haemoglobin (Hb) has Glutamic acid at the 6th position of its peptide chain.
• This Glutamic acid (Glu) is substituted by Valine (Val) due to the point mutation.
• A single base pair of A-T in the normal Hb gene (Hb^A) is replaced by T-A in the mutated gene Hb^S.
• Thus, the DNA sequence that is transcribed changes from CTC to CAC.
• The CAC codon is transcribed to GUG in the mRNA.
• The GUG codon of mRNA is then translated into Valine (Val).
• Therefore, the Glu is changed to Val in haemoglobin of sickle-cell anaemic patients.

Important Points

• Sickle cell anaemia is a genetic disorder that affects the red blood cells (RBCs) of the body.
• The shape of the RBCs changes from biconcave disc to an elongated sickle-like shape.
• This effect does not allow enough oxygen to be carried to different organs for normal functioning.
• It is common in the African population.
• The malarial pathogen Plasmodium uses the RBCs to reproduce asexually and rupture them, causing malarial fevers.
• The change in shape of RBCs restricts the infection of Plasmodium.
• Thus, it is believed that people with sickle-cell anaemia is less prone to malaria.

Key Points

• Sex determination is the mechanism which determines the biological sex of an individual.
• Sex determination in most of the organisms is determined by the sex chromosomes.
• However, in some organisms sex determination is based on the number of sets of chromosomes in an individual.
• One such example is honeybee, where males are haploid (n) and females are diploid (2n).
• This is known as the haplodiploidy or haplodiploid system of sex determination.
• The female honeybees have 32 chromosomes, while males have 16 chromosomes.

Explanation:

• As females are diploid, they produce haploid gametes (eggs) by meiosis.
• Males are haploid and so they produce haploid gametes (sperms) by mitosis.
• Fusion of a sperm and egg forms a diploid zygote.
• This zygote develops into a female honeybee.
• Male honeybee develops from a haploid egg by parthenogenesis, without any fertilization.
• Therefore, males have 16 chromosomes and females have 32 chromosomes in honeybees.
• This haplodiploid sex determination system also indicates some peculiar features:
  • Male gametes are produced by mitosis.
  • Males do not have a father and thus cannot have sons.
  • But males do have a grandfather and can have grandsons.

Additional Information

Sex determination methods differ from organism to organism:

• X : A ratio - It is the ratio of X-chromosome and the autosomes that determines the sex of the individuals in Drosophila.
• ​The presence of Y chromosome​ - This is the system found in humans, where presence of Y-chromosome makes an individual male and its absence confers female sex.
• Environmental temperature - It is observed in reptiles where environmental temperature during developmental phase determines sex of the individual.

Concept:

• A normal human cell consists of 46 chromosomes (23 pairs), which is represented as 2n = 46
• So, a normal human cell is diploid.
• If by chance due to chromosomal mutation there is an addition or loss of one or a few chromosomes, this condition is called Aneuploidy.

Explanation:

The Correct option is 2 n - 1 = Monosomy, 2 n + 1 = Trisomy. This can be explained as

• Aneuploidy is of two types:

1. Hypoaneuploidy (loss)

• 2n - 1 = monosomy (loss of one chromosome only) i.e. the presence of a single copy of a particular chromosome. Eg - Turner's syndrome.
• 2n - 2 = Nullisomy (loss of a pair of the homologous chromosome)

      2. Hyperaneuploidy (addition)

• 2n+1 = Trisomy: addition of one chromosome in one set. Eg - Down's syndrome
• 2n + 2 = Tetrasomy: addition of two chromosomes in one set.

Key Points

• The nucleosome model was proposed by Roger Kornberg for the DNA packaging in eukaryotic chromosome.
• Under electron microscopes, chromosomes were found to look like 'beads on a string' because of the series of spherical particles connected by a fine filaments.
• These 'beads' were termed as nucleosomes.
• Nucleosome is a complex of DNA and Histone proteins to condense the DNA into smaller volume inside the nucleus.

Important Points

• Histones are positively charged, basic proteins which are rich in Lysine and Arginine residues.
• The histones H2A, H2B, H3 and H4 are involved in the nucleosome structure, while the H1 protein acts like a linker molecule between two nucleosomes.
• 2 molecules of H3 and H4 form a core of tetramer (as a pair of dimers).
• This (H3)₂.(H4)₂ tetramer is associated with 2 independent dimers of H2A-H2B.
• DNA being negatively charged, wraps around the histone octamer and forms the nucleosome.
• DNA enters and leaves the same side and the two turns are sealed by the H1 protein.
• A nucleosome contains about 200bp of DNA, out of which 146bp is involved in the coiling of octamers and the rest is associated with the linker molecule.
• It forms the 11nm wide interphase fibre.

Additional Information

• Nucleosomes are the first level of DNA condensation involved in DNA packaging.
• Nucleosomes are further condensed into the 30nm solenoid which is again organized into the tightly packed metaphase chromosomes.
• Non-histone proteins (NHP) can also bind to DNA and are present in the nucleus, but they do not play any role in nucleosome formation.
• NHPs may have various functions related to higher level of DNA packaging.

Concept -

Bacterial Transcription: 

• Transcription is the process of copying (transcription) a gene's DNA sequence to create an RNA molecule.
• The main transcription enzyme is RNA polymerase.
• Transcription in bacteria follows three steps - 

1. Initiation: 
   • When RNA polymerase connects to a promoter sequence near the start of a gene, transcription begins (directly or through helper proteins).
   • To build a new, complementary RNA molecule, RNA polymerase employs one of the DNA strands (the template strand) as a template.
   • To start transcribing a gene, RNA polymerase binds to the promoter region of the gene's DNA.
   • The promoter basically instructs the polymerase "where" to "sit down" on the DNA and start transcribing.
   • Each gene (or group of genes transcribed together in bacteria) has its own promoter.
   • DNA sequences in a promoter allow RNA polymerase or its helper proteins to connect to the DNA.
   • The polymerase can begin transcription after the transcription bubble has formed.
2. Elongation: 
   • The following step of transcription, elongation, can begin once RNA polymerase is in place at the promoter.
   • Elongation is the process through which the RNA strand lengthens due to the insertion of additional nucleotides.
   • RNA polymerase "walks" along one strand of DNA, known as the template strand, in the 3' to 5' direction during elongation.
   • RNA polymerase adds a complementary (matching) RNA nucleotide to the 3' end of the RNA strand for each nucleotide in the template.
3. Termination: 
   • In bacteria, there are two types of termination strategies: Rho-dependent and Rho-independent.
   • Termination is the last step in the transcription process.
   • Termination is determined by RNA sequences that signal the completion of the transcript.

​

Explanation -

Bacterial transcription is a three-step process.

• It starts with initiation where the RNA polymerase finds a promoter sequence that initiates the process of transcription.
• Then comes elongation where the RNA strand lengthens due to the insertion of additional nucleotides. 
• The third and final step is termination which takes place when the signal for the completion of transcription of the gene is sent and the process is stopped. 

As the initiation starts with RNA polymerase which finds a promotor sequence, to begin with, transcription in bacterial transcription,  hence, it is the correct answer to this question. 

Key Points

• Central dogma of molecular biology states that flow of genetic information takes place from DNA to RNA to proteins.
• Transcription of DNA to RNA is based on the complementarity of their strands.
• However, no such complementarity exists between the RNA and the proteins for translation to take place.
• Evidences show that changes in nucleic acids also caused changes in the amino acids of the proteins.
• This suggested the existence of a Genetic Code that directs the synthesis of amino acids that form the proteins.
• It was George Gamow who suggested that if the 4 nitrogen bases of nucleic acids have to code for the 20 non-essential amino acids produced in the body, the code should constitute a combination of bases.
• The nitrogen bases include Adenine (A), Guanine (G), Cytosine (C) and Uracil (U) for an RNA.
• A permutation combination of 2 bases would give: 4² = 16 amino acids only, which is not sufficient.
• So, it has to be a combination of 3 bases giving: 4³ = 64 codons.
• Thus it was concluded that 64 codons would code for the 20 amino acids with some codons being signal codons too.

Important Points

• We can find the correct amino acids for the given genetic codes from the above table.
  • GCA - Alanine (Ala)
  • CGA - Arginine (Arg)
  • ACG - Threonine (Thr)
  • GAC - Aspartic acid (Asp)
• In the given question, CGA has been wrongly matched with Proline (Pro).
• Thus the correct answer is option (2).

The correct answer is Both A and R are true and R explains A.

• DNA Finger Printing has become an important test to establish the paternity and identity of criminals of rape cases.
• Small samples such as hair, dried blood, and semen are adequate for DNA analysis.

​Key Points

• DNA Finger Printing:
  • DNA fingerprinting is a technique that explains the genetic makeup of living things.
  • It is a method of finding the difference between the satellite DNA regions in the genome.
  • Any piece of DNA sample found at a crime scene can be analyzed. Hence the statement A is true.
  • After the DNA profile is traced, it becomes easier to find the criminal by performing the DNA fingerprinting for the suspects.
  • Apart from crime scenes, Fingerprinting applications also prove useful in finding the parents of an unclaimed baby.
  • It can be done by conducting a paternity test on a DNA sample from the baby. Hence the statement R is true.

​Additional Information

• DNA Fingerprinting Steps:
  • Alec Jeffreys developed this technique. 
  • He used satellite DNAs.
• Following are the steps involved in DNA fingerprinting:
  • ​Isolating the DNA
  • Digesting the DNA with the help of restriction endonuclease enzymes.
  • Separating the digested fragments as per the fragment size by the process of electrophoresis.
  • Blotting the separated fragments onto synthetic membranes like nylon.
  • Hybridizing the fragments using labeled VNTR probes.
  • Analyzing the hybrid fragments using autoradiography.

The correct answer is 40%.

Key Points

• In DNA, adenine (A) always bonds with thymine (T), while guanine (G) always bonds with cytosine (C). 
• So, the percentages of A and T will be equal, while the percentages of G and C would also be equal.
• In this sample, there exist 10% of guanine, therefore there's also 10% of cytosine.
• Therefore, thymine and adenine would make up a total of 100% −10% −10% = 80% of the DNA.

• As they must exist in equal amounts, the percentage of thymine in this DNA sample will be 80% ÷ 2 = 40%.

Additional Information

• Adenine belongs to the purine family, occurring free in tea or combined in many substances of biological importance, including the nucleic acids, which govern hereditary characteristics of all cells. 
• Thymine belongs to the pyrimidine family that is a constituent of deoxyribonucleic acid (DNA). DNA, along with RNA (ribonucleic acid), regulates hereditary characteristics in all living cells.
• Guanine belongs to the purine group, a class of compounds with a characteristic two-ringed structure, composed of carbon and nitrogen atoms, and occurring free or combined in such diverse natural sources as guano (the accumulated excrement and dead bodies of birds, bats, and seals), sugar beets, yeast, and fish scales.
  • It is a component of nucleic acids, the cell constituents that store and transmit hereditary traits.
• Cytosine belongs to pyrimidine that occurs in nucleic acids, the heredity-controlling components of all living cells, and in some coenzymes, substances that act in conjunction with enzymes in chemical reactions in the body.

The correct answer is Survival of the fittest.

Concept-

• Charles Darwin made a sea voyage around the world in a ship named H.M.S. Beagle.
• When he arrived at an island called the Galapagos island, he saw some finches and observed differences in the shapes of their beaks.
• He concluded that this change is due to the different eating habits of the finches thus, he gave the concept of Adaptive radiation.

Explanation-

• The original idea of survival of the fittest was proposed by Herbert Spencer.
• Acco​rding to Darwin, the most suitable and fit individuals are successful in the struggle for existence.
• The individuals with the most favorable adaptations are able to lead the most successful life and are able to win over their mating partners.
• Darwin called it Sexual Selection.
• In the struggle for existence, only those members survive which possess useful variations means nature selects fit individuals.
• This was called Natural Selection.
• Fitness is the end result of the ability to adapt and get selected by nature.

Thereby the most convincing in Darwin's theory is survival of the fittest.

Additional Information

Speciation

• The formation of one or more new species from an existing species is called speciation. 
• Speciation is of 2 types.
  • Divergent speciation and transformation speciation.

Concept:

• Life on our Earth originated millions of years ago. The origin of life is considered a unique phenomenon that has ever occurred in the history of the universe.
• Non-living molecules like amino acids and water originated on Earth prior to the onset of life on Earth.  Some non-living molecules containing precursors for life were formed known as Probionts.
• Life on Earth originated in water in the form of a unicellular organism. Gradually it evolved to form such a diverse variety of life forms on Earth.
• Evolution is the gradual and continuous change that occurs to form complex life forms on present-day Earth from the pre-existing lifeforms. These evolutionary changes are irreversible changes.
• Various evidence supports the theory of the evolution of life forms on our planet Earth. Various branches of biology like paleontology, molecular biology, embryology, comparative anatomy provide evidence for the process of evolution.

Explanation:

• Comparative anatomy and morphology similarities and differences among organisms of today and those who existed years ago. By interpretation of these similarities and differences, biologists can trace if such organisms have common ancestors or not.
• One of the very well-studied examples is the pattern of bones of forelimbs of animals like whales, tigers, bat, and man. Through these forelimbs, these animals perform different functions but the anatomical structure of these organs is very similar. All of them have humerus, radius, ulna, carpels, metacarpals, & phalanges.
• Hence these animals have the same structure developed in different ways to adapt to different needs. These structures are known as homologous organs. These homologous organs prove to be important evidence in support of the theory of evolution. Homology indicates common ancestry.

so, Forelimbs of the whale, bat, tiger, and man are examples of Homologous organs

Additional Information

Some other evidences of evolution are:

1. Molecular Evidences:  Similarities in proteins and genetic material performing a similar function among diverse organisms gives evidence of common ancestry. ATP is the energy source in all living organisms.
2.  Paleontological evidence:  The study of fossils provides the most convincing and direct evidence of evolution. Fossils are the dead remains of plants and animals that lived in past in various geological layers.

Explanation:

• Man has always been curious about how this world has been formed and also about the existence of life on our Earth. As far as the origin of life is considered, it has remained the curiosity for intellectuals at all times. Despite advancements in various fields like biochemistry, astrobiology, geography, molecular biology, etc. scientists are unable to ascertain the truth. Various theories and hypotheses have been proposed to find the probable answer to this question.
• Stanley L.Miller and his teacher Harold C.Urey provided the first experimental evidence in support of the chemical evolution theory of Oparin. They designed a glass-apparatus called spark-discharge apparatus.
• The apparatus was first sterilized and evacuated. Methane, ammonia, and hydrogen gases were pumped in the proportion of 1:2:2 into the glass chamber.
• A tube carrying water vapour was also connected to the chamber. The lightning effect was mimicked by the electric discharge carbon arc spark in the chamber.
• Process of evaporation and precipitation was also activated by the use of heating mantle and condenser respectively. The mixture of CH4, NH3, H2 was exposed
  continuously to electric discharge for several days causing the gases to interact, after which these were condensed.
• This experiment strongly supports the presence of organic substances present in the earth's pre-historic environment. It acts as a building block for the life on the Earth.

Experiment to prove that synthesis of the organic compound formed the basis of life was performed by Miller.

The diagram showing the set up for miller's experiment.

The correct answer is Both.

Concept:

• G.H. Hardy and Wilhelm Weinberg independently described the basic principle of population genetics that is known as the Hardy-Weinberg principle.
• Hardy-Weinberg principle states that gene frequency in a population remains stable and constant from generation to generation.
• The gene pool is constant and this is called Hardy-Weinberg equilibrium or genetic equilibrium.
• Gene pool - total number of genes and their alleles in a population.
• It can be expressed by a mathematical equation:

​p² + 2pq + q² = 1

where p and q are frequencies of the dominant and recessive allele respectively.

• This equation can also be used to calculate the genotypic frequencies in a population at equilibrium.
  • ​p² - homozygous dominant frequency
  • q² - homozygous recessive frequency
  • 2pq - heterozygous frequency

Explanation:

• Hardy-Weinberg equilibrium is affected by Mutation, Natural selection, Genetic drift, Genetic recombination and Gene migration or gene flow.

1. Mutation-
   • Advantageous mutations lead to new phenotypes and over few generations, result in speciation.
2. Natural selection-
   • It is a process in which heritable variations enhance resource utilization for few individuals, enabling them to survive and reproduce better.
   • This leads to a change in the characteristics of the population, giving rise to new forms.
   • Natural selection can have the following three effects:
   1. Stabilizing: Large number of individuals acquire mean character value so peak gets higher and narrower.
   2. Directional: Large number of individuals acquire value other than mean character value so peak shifts in one direction.
   3. Disruptive: Large number of individuals acquire peripheral character values at both ends of the distribution curve and hence two peaks are formed.

Concept:

• Plasmodium is a protozoan that causes the disease malaria. There are different plasmodium varieties like P. vivax, P. malariae, and P. falciparum.
• P. falciparum is the most fatal one.
• Plasmodium requires two hosts to complete its Life cycle:

1. Human
2. Female Anopheles Mosquito

Explanation:

The life cycle of Plasmodium: 

Ist phase in the Human:

• The infected female Anopheles mosquito transfers the sporozoites (an infectious form of plasmodium) to the human body by biting.
• The sporozoites reach the liver cells and multiply themselves.
• This is now followed by their attack on the red blood cells.
• They reproduce asexually in the RBCs producing male & female gametes.
• After this, they rupture the RBCs and release a toxin called hemozoin, which is responsible for high recurring fever, chills, and shivering.

IInd phase in Mosquito:

• The plasmodium again enters the Female Anopheles mosquito when it bites another infected person.
• Now the male and female gametes get fertilized in the mosquito's intestine and migrate to the salivary glands.
• This is how plasmodium completes its life cycle, so the production of gametes takes place in Man​

Additional Information

• Male Anopheles mosquito do not cause Malaria 

Concept:

• A disease is an infection or an abnormal condition that negatively affects the structure and capacity of all parts or in particular specific parts of organisms.
• It is not due to the immediate physical injury. 
• Infections are frequently known to be ailments that are related to explicit signs and side effects.
• There are different types of diseases such as: 
  • Infectious diseases
  • Deficiency diseases
  • Hereditary diseases 
  • Physiological diseases     
• ​Diseases can also be classified into Communicable and Non-communicable diseases. 
• It can spread through the direct transfer of bacteria, viruses, or other germs from one person to another or through mosquito bites, etc.

Explanation:

Dengue:

• Dengue is carried by the female mosquito Aedes.
• Its symptoms are nausea, vomiting, aches and pains, rash.
• Whereas, vomiting blood, bleeding from the nose or gums are symptoms of severe dengue.
• It can be prevented by the cleanliness and preventing oneself from mosquito bites.
• It can be cured by taking paracetamol, drinking fluids.

Thus, the carrier of dengue disease is the Aedes mosquito.

Concept:

• There are various organs in the human body that produce specific substances like hormones.
• These are known as glands, which can be endocrine or exocrine.
• Endocrine glands are ductless glands that release their hormones directly into blood, to be carried to the target organ.
• Exocrine glands have ducts via which their secretions are released.
• The pineal, pituitary, thymus and thyroid glands are all endocrine glands.

Key Points

• Thymus is a lobed organ located between the lungs behind the sternum on the ventral side of aorta.
• It is quite large in size at birth but reduces with ageing.
• By puberty, it attains a very small size.
• It is one of the primary lymphoid organs along with bone marrow.
• It produces the peptide hormones called thymosins, which play important role in developing the immune system.
• Functions of thymosins:
  • They help in the differentiation of T-lymphocytes, which are vital for cell-mediated immunity.
  • They promote the production of antibodies that provide humoral immunity.
• The thymus gland is degenerated in older persons, which results in their weakened immune system.

Additional Information

• Pineal - It secretes the hormone melatonin which maintains the diurnal rhythm of the body.
• Pituitary - It is one of the major glands as it secretes several hormones like FSH, LH, TSH, prolactin, etc.
• Thyroid - It produces thyroxine (T4) and triiodothyronine (T3), which play a vital role in controlling the basal metabolic rate.

Concept-

• Plant breeding as technology has helped increase yields to a very large extent.
• The development of several high-yielding varieties of wheat and rice in the mid-1960s, as a result of various plant breeding techniques, led to a dramatic increase in food production in our country. This phase is often referred to as the Green Revolution.
• Green Revolution was responsible for our country to not merely meet the national requirements in food production but also helped us even to export it.
• The green revolution was dependent to a large extent on plant breeding techniques for the development of high-yielding and disease-resistant varieties in wheat, rice, maize, etc.

Explanation-

• During the period 1960 to 2000, rice production went up from 35 million tonnes to 89.5 million tonnes.
• This was due to the development of semi-dwarf varieties of wheat and rice. 
• Semi-dwarf rice varieties were derived from IR-8, (developed at International Rice Research Institute (IRRI), Philippines) and Taichung Native-1 (from Taiwan).
• The derivatives were introduced in 1966.
• Later better-yielding semi-dwarf varieties Jaya and Ratna were developed in India.

Therefore the semi-dwarf rice varieties introduced in India during the green revolution were derived from IR-8 and Taichung Native-1.

Additional Information

• During the period 1960 to 2000, wheat production increased from 11 million tonnes to 75 million tonnes.
• This was due to the development of semi-dwarf varieties of wheat and rice.
• Nobel laureate Norman E. Borlaug, at International Centre for Wheat and Maize Improvement in Mexico, developed semi-dwarf wheat.
• In 1963, several varieties such as Sonalika and Kalyan Sona, which were high yielding and disease resistant, were introduced all over the wheat-growing belt of India.

Explanation:

Artificial Selection or Selective breeding -

• It is a process in which humans identifies animals or plants having desirable phenotypic traits.
• These are then mated to obtain the progeny having the desired character.
• In other words, it is the process of selecting parents for obtaining special characters in their offspring.
• In this animals or plants with commercially useful traits are selected, the others are rejected.
• It is the first step in any genetic hybridization technique.
• Animal husbandry and plant breeding programmes are examples of Artificial selection.

Artificial selection in animals -

• A mule can be obtained by crossing a male donkey and a female horse, it has the characters of both Donkey and Horse. A mule can carry heavyweights like a donkey and can walk long distances like a horse.

Artificial Selection in plants -

• Resistance to the yellow mosaic virus in bhindi (Abelmoschus esculentus) was transferred from a wild species and resulted in a new variety of A. esculentus called Parbhani Kranti.

Additional Information

Mutation -

• Mutations are permanent irreversible inheritable changes i.e. they are carried forward in the next generation & cannot be reversed thus, mutations are referred to as the ultimate source of organic variation 

Natural selection -

• Natural selection was given by Charles Darwin, according to it those populations which are a better fit (reproductively fit) in an environment will be selected by nature and will survive more.

Concept-

• Addiction is the state of being given up to some habit, drug, alcohol, tobacco, etc.
• Addiction is of three types- Drug abuse, Tobacco addiction, and Alcohol addiction.
• Drug abuse is taking drugs for a purpose except for clinical use.
  • Drug abusers are likely to require drugs again and again.
  • Drug alter perception, behaviour and consciousness.

Explanation-

• Cannabinoids are a group of chemicals, which interact with cannabinoid receptors present principally in the brain.
• Four drugs namely, bhang, ganja, charas, and marijuana are obtained from the inflorescences of the plant Cannabis sativa.
• Generally taken by inhalation and oral ingestion, these are known for their effects on the cardiovascular system in the body.

So, The drugs that are known for their effect on the cardiovascular system in the body are cannabinoids.

Additional Information

• Opioids are drugs, which bind to specific opioid receptors present in our central nervous system and gastrointestinal tract.
• Amphetamines are strong CNS stimulants and produce increased self-confidence and flow of ideas.
• Barbiturates are strong CNS depressant.

The Correct Answer is Option 1 i.e C.

• Lactobacillus:
  • It is a genus of Gram-positive bacteria.
  • These are rod-shaped and can be found in fermented foods and supplements.
  • It is the bacteria that is used to convert milk into curd. 
  • Conversion of milk into curd is done by the process called fermentation.
• Streptomyces:
  • It is a genus of Gram-positive bacteria.
  • It grows in various environments like soil etc.
  • Its shape resembles the filamentous fungi.
• Vibrio:
  • Vibrio is a genus of Gram-negative bacteria.
  • They are in the shape of curved-rod or comma.
  • Cholera is a disease that is caused by the bacteria Vibrio cholerae.
• Spirilla:
  • It is a genus of G  ram-negative bacteria.
  • It has a rigid spiral structure.  

Concept:

• A huge amount of waste water is produced everyday.
• It contains organic matter as well as microbes, which may even be pathogenic.
• This waste water needs to be treated in sewage treatment plants (STPs) before disposing it into water bodies.
• This treatment is carried out with the help of the heterotrophic microbes present in the sewage.
• The treatment takes place in 2 stages - primary and secondary treatments.

Important PointsSewage Treatment Process:

1. Primary Treatment -
   • ​Floating debris is removed by sequential filtration.
   • The grit (soil and small pebbles) are removed by sedimentation.
   • The solids settle as primary sludge and the supernatant as effluent.
   • The effluent moves from primary settling tank to large aeration tanks for secondary treatment.​
2. ​Secondary/Biological Treatment -
   • ​​The effluent is continuously agitated mechanically and air is pumped into it.
   • This allows vigorous growth of aerobic microbes into flocs.
   • These microbes use up most of the organic matter for their growth.
   • This reduces the BOD (biological oxygen demand) of the effluent.
   • Effluent is then passed into a settling tank, where the bacterial flocs sediment to form the activated sludge.
   • Most of the activated sludge is moved into large tanks called anaerobic sludge digesters, and the rest is used as inoculum in the aeration tanks.
   • Next, the anaerobic bacteria digest the other bacteria and fungi in the sludge.
   • The effluent is then discharged into water bodies.

Therefore, in the secondary treatment, the activated sludge is pumped into anaerobic sludge digesters for further treatment.

• Restriction enzymes or restriction endonucleases are endonucleases that have the ability to cut at a specific site within DNA.
• Examples - EcoRI, Hind III.
• The sites at which they cut are called restriction sites.
• These enzymes also have a specific recognition sequence which is a palindromic nucleotide sequence.
• These sequences are base pair sequences that read the same on both the strands of DNA when orientation of reading is same, i.e., either 5' to 3' or 3' to 5'.
• The action of restriction endonucleases on DNA is known as restriction digestion as it cleaves the DNA.
• Restriction endonucleases cut at the same points on both the strands.
• They may produce 2 types of fragment ends:
  • Sticky/Cohesive End - It is produced by offset cuts of DNA i.e., not in the middle of the recognition sequence.
  • Blunt End - It is produced when the cut is exactly at the middle of the palindrome sequence.

Important Points

• Cohesive ends have an overhanging single-stranded end of DNA.
• This overhanging end can form H-bonds with its complementary sequence, if available.
• This property is utilized for joining 2 different DNA fragments with same overhanging ends.
• Hence, they are termed as sticky or cohesive ends.
• Restriction enzymes create same cohesive ends on different DNA fragments.
• These 2 cohesive ends can then be joined together by ligase enzyme.
• This is known as end to end cloning.
• It produces a recombinant DNA molecule, which can then be inserted into a host cell for cloning.

The correct answer is Option 4, i.e. 1, 2 and 3.

Key Points

• Vectors used at present, are engineered in such a way that they help easy linking of foreign DNA and selection of recombinants from non-recombinants.
• The following are the features that are required to facilitate cloning into a vector.

1. Origin of replication (ori):
   • This is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells.
   • This sequence is also responsible for controlling the copy number of the linked DNA.
   • So, if one wants to recover many copies of the target DNA it should be cloned in a vector whose origin support high copy number.
2. Selectable marker:
   • In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.
3. Cloning sites:
   • In order to link the alien DNA, the vector needs to have very few, preferably single, recognition sites for the commonly used restriction enzymes.
   • The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes.

Explanation:

• For example, the above figure is of plasmid pBR322, which is a cloning vector.
• It contains an origin of replication (ori).
• It has several cloning sites like PstI, PvuI, EcoRI, BamHI and SalI.
• The selectable markers here are ampicillin resistance (amp^R) and tetracycline resistance (tet^R).

The correct answer is A → B → F → G → E → H → C → D.

Key Points

• The complete process of recombinant DNA technology includes multiple steps, maintained in a specific sequence to generate the desired product
  • Identification and isolation of the genetic material: The first and the initial step in Recombinant DNA technology is to isolate the desired DNA in its pure form i.e. free from other macromolecules.
  • Fragmentation of DNA: The restriction enzymes play a major role in determining the location at which the desired gene is inserted into the vector genome. These reactions are called ‘restriction enzyme digestions’.
  • Isolation of desired DNA fragments.
  • Amplification of gene of interest: It is a process to amplify a single copy of DNA into thousands to millions of copies once the proper gene of interest has been cut using the restriction enzymes.
  • Ligation of DNA fragmentation into the vector: In this step of Ligation, joining of the two pieces – a cut fragment of DNA and the vector together with the help of the enzyme DNA ligase.
  • Transfer of Recombinant DNA into the host cell/organism: The recombinant DNA is introduced into a recipient host cell. This process is termed Transformation. Once after the insertion of the recombinant DNA into the host cell, it gets multiplied and is expressed in the form of the manufactured protein under optimal conditions.
  • Obtaining the foreign gene product: Multiplication/expression of the introduced gene in the host.
  • Downstream processing: Selection of the transformed host cells and identification of the clone containing the desired gene/DNA fragment.

Concept:

• Emphysema is a lung disease due to which one may face shortness of breath.
• The alveolar sacs are damaged in emphysema patients.
• It is usually caused by smoking, but can also be caused by other air pollutants or respiratory infections.
• It may also be caused by the deficiency of the protein alpha-1 antitrypsin.
• Thus, this protein is used to treat the patients with emphysema.

Important Points

• Medicines for certain human diseases contain biological compounds that are produced by genetic engineering.
• This has a lot of advantages:
  • It can be produced in large amount.
  • It would not have any unwanted immune-response in the body.
• Transgenic animals are usually produced by genetic engineering processes for such purposes.
• Alpha-1 antitrypsin (AAT) is one such biological product that is produced by transgenic animals.
• Attempts are also being made for the treatment of phenylketonuria and cystic fibrosis.
• The protein α-1 antitrypsin is an example of application of biotechnology in the field of medicine.

Additional Information

• Cancer, rheumatoid arthritis and Alzheimer's disease are some diseases that are being studied extensively for understanding the development of such diseases.
• Transgenic models exist for such diseases, which helps in the investigation of new treatment methods for them.

The correct answer is 1, 2, 3, and 4.

Key PointsGenetically Modified plants 

• Plants, bacteria, fungi, and animals whose genes have been altered by manipulation are called genetically modified organisms (GMO). GM plants have been useful in many ways.
• Genetic modification has:
  1. Made crops more tolerant to abiotic stresses (cold, drought, salt, heat). Hence, statement 1 is correct.
  2. Reduced reliance on chemical pesticides (pest-resistant crops). Hence, statement 2 is correct.
  3. Helped to reduce post-harvest losses.
  4. Increased efficiency of mineral usage by plants (this prevents early exhaustion of fertility of soil). Hence, statement 3 is correct.
  5. Enhanced nutritional value of food, e.g., Vitamin 'A' enriched rice. Hence, statement 4 is correct.​
• ​In addition to these uses, GM has been used to create tailor-made plants to supply alternative resources to industries, in the form of starches, fuels and pharmaceuticals.
• Some of the applications of biotechnology in agriculture are the production of pest-resistant plants, which could decrease the amount of pesticide used.
• The recombinant DNA technological processes have made immense impact in the area of healthcare by enabling mass production of safe and more induced unwanted immunological responses as is common in the case of similar products isolated from non-human sources.

Concept-

• A transgenic crop is a crop whose genes have been altered by manipulation. This crop is known as genetically modified crops (GM crops).
• GM plants can reduce reliance on chemical pesticides (pest-resistance crops).
• Transgenic means that one or more DNA sequences from another r species have been introduced by artificial means.
• Transgenic plants can be made by introducing foreign DNA into a variety of different tissues.
• It is a plant that has a novel combination of genetic material obtained through biotechnology that adds a new quality to the parent crop like herbicide-tolerance. 

Key Points

The first genetically modified plant was produced in 1982, an antibiotic-resistant tobacco plant.

• A nematode Meloidogyne incognita infects the roots of tobacco plants and causes a great reduction in yield.
• A novel strategy was adopted to prevent this infestation which was based on the process of RNA interference (RNAi).
• RNAi takes place in all eukaryotic organisms as a method of cellular defense.
• This method involves silencing of a specific mRNA due to a complementary dsRNA molecule that binds to and prevents translation of the mRNA (silencing).
• The source of this complementary RNA could be from infection by viruses having RNA genomes or mobile genetic elements (transposons) that replicate via an RNA intermediate.

Additional Information

Bt Cotton- 

• Some strains of Bt produce proteins that kill certain insects such as lepidopterans, coleopterans, and dipterans.
• Bt forms protein crystals during a particular phase of their growth.
• These crystals contain a toxic insecticidal protein.

Golden rice-

• Rice is a staple food in many countries.
• By inserting two genes from daffodil and one gene from a bacterial species into the rice.
• Scientists produced rice capable of synthesizing β - carotene.

Tomato-

• Transgenic variety of Tomato - Flavr Savr.
• Due to inhibition of polygalacturonase enzyme which degrades pectin.
• So tomato variety remains fresh.

Concept:

• Members of the different populations (species) often interact with each other, in various ways in their environment.
• These interactions may be Positive, Negative, or Neutral.​​
• Positive interaction: Both the species are benefited. For example- Mutualism, Protocooperation.
• Negative interaction: One species is harmed. For example- Ammensalism, Parasitism, predation.
• Neutral interaction: Neither species benefits nor suffers. For example-  Lactobacillus and Streptococcus.

​Explanation:

• Rhizobium forms a symbiotic relationship with the leguminous plants
• The term symbiosis means "living together'' (G. syn = together, bios = life).
• Symbiosis includes all interactions in which two species actually live together without regard to benefit or harm to the participants
• Plants cannot take the atmospheric nitrogen directly, they require it in the soluble forms
• The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a soluble form.
• But Rhizobium cannot make its own food. So it lives in the roots of a gram, peas, moong, beans, and other legumes and provides them with nitrogen.
• Most of the pulses (dals) are obtained from leguminous plants.
• In return, the plants provide food and shelter to the bacteria. They, thus, have a symbiotic relationship.

Additional Information

Key Points

• Ecology studies the interactions of organisms between each other and with the environment.
• The environment faces seasonal and annual variations that helps in forming biomes.
• The localized variations in a biome further leads to the formation of different habitats.
• The habitat comprises of various abiotic and biotic components that also act as factors for causing the variations.
• The abiotic factors include - temperature, water, light and soil.
• The biotic factors include the organisms present in that habitat and their interactions.

Important Points

• Temperature is an important abiotic component because it affects the enzyme kinetics in organisms and hence regulate various metabolic activities.
• Organisms are divided into 2 groups based on temperature component:
  • Eurythermal - Organisms that can tolerate and thrive in a wide range of temperatures. E.g. - Dog, Cat, Man.
  • Stenothermal - Organisms that are restricted to a narrow range of temperatures. E.g. - Penguin, Crocodile.
• Homeostasis is the regulation of internal environment relative to the external environment such that the physiological processes can continue at optimum and constant rate.
• Organisms can also be categorized according to their response towards such abiotic factors like temperature.
  • Homeothermal - These are the warm-blooded organisms that maintain their body temperature at an constant optimum range at which physiological processes can continue even when the environment changes. For example, when external temperature is more than internal temperature, humans sweat to decrease the body temperature by evaporative cooling. On the other hand, when external temperature is less than body temperature, we shiver to generate heat and thus increase our internal temperature.
  • Poikilothermal - These are the cold-blooded organisms that cannot maintain homeostasis and their body temperature changes with environment temperature. Example - All reptiles.

Additional Information

• Water is also an important abiotic factor affecting organisms with its pH and chemical composition.
• Water bodies mainly differ in their salinity levels.
• Organisms are divided into:
  • Euryhaline - Organisms that can tolerate a wide range of salinities. E.g. - Molly fish.
  • Stenohaline - Organisms that are restricted to a narrow range of salinities. E.g. - Goldfish.

Key Points

• Ecosystem is defined as a functional system comprising organisms of a given area interacting with the physical environment and also among themselves.
• This interaction allows flow of energy, leading to a clearly defined trophic structure, biodiversity and nutrient cycles.
• The various components of an ecosystem can be broadly divided into:
  • Abiotic components - include the non-living components of the environment present in that ecosystem. They can be climatic conditions as well as chemical substances present in the environment. 
  • Biotic components - include all the living forms in that ecosystem. They can be autotrophic or heterotrophic in their mode of nutrition. 

Important Points

• Ecosystems can be broadly classified into 3 types:
  • Natural - These ecosystems are formed gradually due to natural forces in the environment. They are usually complete with producers, consumers and decomposers. It is the most commonly found ecosystem.
  • Artificial - They are man-made ecosystems where the components are introduced and controlled by us as per our need. Example - Crop fields, aquarium.
  • Incomplete - This is a special type of ecosystem where producers are absent. Example - Caves and depths of the ocean.
• Natural ecosystems are further classified into:
  • Terrestrial - This includes all land ecosystems like grasslands, deserts and forests.
  • Aquatic - This includes marine ecosystems like sea and ocean. It also includes freshwater ecosystems like ponds, lakes and rivers.

Additional Information

• Succession -
  • Ecological succession is the gradual change in the species composition of a given area caused due to successive colonization of different plant communities.
  • Every ecosystem we see is actually in one or the other succession stages called seral stages.
  • Whether it starts from a dry (xeric) area or from a wet (hydric) area, succession always leads to a forest community.
  • The species that first colonizes an area is called pioneer species and the last stage of succession is called climax community, which shows great biodiversity.
• Ecotone -
  • It is the transitional area between two different ecosystems caused due to sudden changes in the environment.
  • For example, mangroves are an ecotone between terrestrial and marine ecosystems.
  • They include species that are highly adaptive to colonize the transitional area.
  • They may also include species that are common to both the ecosystems surrounding the ecotone.
• Community -
  • An ecological community is defined as an assemblage of two or more species populations in a particular area.
  • These communities interact with the environment, which keeps constantly changing.
  • As a result, the environment causes change in the communities as well as gets modified by the communities.
  • This interaction is the basis for ecological succession.

Key Points

• Ecological pyramids are graphical representations of trophic systems in an ecosystem.
• Trophic system refers to the system in which energy flows from one trophic level to another.
• Trophic level is the specific place in the food chain that is occupied by an organism based on their source of food.
• In an ecosystem, we find unidirectional flow of energy from Sun to producers to consumers.
• Ecosystems need a constant supply of energy to synthesize the molecules required in order to counteract the universal tendency of increasing disorderliness.
• In a grazing food chain, the energy trapped by the producer is passed on to the consumer or leads to death, which starts the detritus food chain and the energy is passed on to the decomposer.
• according to Lindeman's 10% Law, only 10% of the energy in a trophic level is transferred to the next, so that there can only be a limited number of trophic levels in each ecological pyramid.
• Shape of pyramid can be upright (base broader than top), inverted (top broader than base) or variable.

Important PointsEcological pyramids are mainly of 3 types:

• Pyramid of Number -
  • It is based on the number of individuals in each trophic level.
  • It is upright in grassland ecosystem, while inverted in parasitic ecosystem and variable in forest ecosystem.
• Pyramid of Biomass - 
  • It is based on the quantitative biomass of organisms present in unit area (standing crop), for each trophic level.
  • It is usually upright, but can be inverted in pond ecosystem.
• Pyramid of Energy -
  • It is always upright because of the 10% Law, meaning energy keeps decreasing at successive trophic levels.

Concept:

• The sequential, gradual, and predictable changes in the species composition in an area are called succession or ecological succession.
• The entire sequence of communities that successively changes in a given area is called sere(s).
• The individual transitional communities are termed as seral stages or seral communities.
• The community that is in near equilibrium with the environment is called a climax community.
• The species that invade a bare area are called pioneer species.​

Explanation:

• Lichen is the symbiotic association of fungi and algae
• Lichens are ecologically very important as they are the pioneer species on a bare area or Xerarch condition.
• The lichen secretes some acids to dissolve rock and help in weathering and soil formation.
• Later, some small bryophytes invade and hold a small amount of soil.
• The bryophytes are succeded by herbs, shrubs, and ultimately big trees.
• At last, a stable climax forest is formed.
• The xerophytic habitat gets converted into a mesophytic one

Important Points 

• Lichens are good indicators of air pollution. 
• Lichens are sensitive to SO2 (sulphur dioxide). They die in more concentration of SO2. Therefore they are not found in those areas (polluted areas) where the concentration of SO2 is high

Additional Information

• Mycorrhizae is a symbiotic association between a fungus and roots of higher plants. The term mycorrhiza refers to the role of the fungus in the plant’s rhizosphere, its root system.
• Mycorrhizal fungi allow plants to draw more nutrients and water from the soil. They also increase plant tolerance to different environmental stresses. They play a major role in the soil aggregation process and stimulate microbial activity. 

Concept:

• Biodiversity conservation refers to the protection, scientific management, and upliftment of the natural flora and fauna. It involves the conservation of various threatened plant and animal species.
• Biodiversity conservation can be done in two modes i.e. In-Situ and  Ex-Situ

Explanation:

• Ex-Situ Conservation: The mode of conservation includes the protective maintenance of threatened species outside the areas where they naturally occur.
• Examples of Ex-Situ conservation are botanical gardens, zoological gardens, gene banks, etc.
• In-Situ Conservation: The conservation method to protect the entire ecosystem in which the species are protected in their natural habitat.
• Examples of In-Situ conservation are national parks, wildlife sanctuaries, biosphere reserves &Sacred groves.

​Additional Information

• There are more than 100 national parks in India covering an area of 40,564.00 km2. Some of the important National parks of India are Ranthambore National Park, Jim Corbett national park, Kaziranga national park
• There are about 553 Wildlife sanctuaries present in India. 
• Sacred groves are the patches of protected forest areas that are home to different tribal people. Examples -Khasi and Jaintia Hills in Meghalaya and Aravalli Hills of Rajasthan

Key Points

• Species diversity is not uniform throughout the world.
• Diversity decreases as we move from the equator to the poles and this is called the latitudinal gradient in diversity.
• Tropics (latitudinal range of 23.5° N to 23.5° S) harbour more species than temperate or polar areas.
• More solar power results in greater productivity and species diversity.
• Solar energy received by the tropical region is higher than in subtropical and temperate regions.
• The temperate region receives greater solar energy than the polar.
• So tropics have greater species diversity, followed by sub-tropical, temperate, and lastly polar.
• Stability in environmental conditions throughout the year promotes niche specialization and higher species richness.
• Tropical regions have fewer seasonal variations and have a more or less constant environment.
• Sub-tropical and temperate regions have seasonal fluctuations. The polar region has extreme environmental conditions.
• This is the second reason why the tropics have more diversity as compared to the sub-tropical, temperate, and polar regions.
• Temperate regions were subjected to glaciations during the ice age, while tropical regions remained undisturbed.
• It takes thousands of years for any region to become species diverse. So, constant conditions over a long period of time promote diversity.
• This is the third reason why the tropics have greater diversity than any other region.

Therefore, diversity and species abundance:

Tropics>Sub-tropics>Temperate>Poles

Concept:

• Ozone is a gas that is present naturally within Earth’s atmosphere. It is formed of three oxygen atoms (O3).
• Ozone can be formed from diatomic oxygen. The UV radiations split apart molecular oxygen into free oxygen (O) atoms. These oxygen atoms combine with molecular oxygen to form ozone

Explanation:

• The ozone layer is important for mankind as it acts as a shield absorbing ultraviolet radiation from the sun.
• UV rays are highly injurious to living organisms. Hence it is important to maintain the ozone layer, but some of the ozone-depleting substances continuously degrade ozone due to which a hole is formed in the ozone called the ozone hole.
• Recognising the deleterious effects of ozone depletion, an international treaty, known as the Montreal Protocol, was signed in Montreal (Canada) in 1987 (effective in 1989) to control the emission of ozone-depleting substances.
• Subsequently, many more efforts have been made and protocols have laid down definite roadmaps, separately for developed and developing countries, for reducing the emission of CFCs and other ozone-depleting chemicals.

Explanation-

Biomagnification

• Biomagnification refers to an increase in the concentration of the toxicant at successive trophic levels.
• This happens because a toxic substance accumulated by an organism cannot be metabolized or excreted, and is thus passed on to the next higher trophic level.
• This phenomenon is well known for mercury and DDT. 

​

Eutrophication

• Eutrophication is the natural ageing of a lake by nutrient enrichment of its water. In a young lake, the water is cold and clear, supporting little life.
• With time, streams draining into the lake introduce nutrients such as nitrogen and phosphorus, which encourage the growth of aquatic organisms. 
• During the past century, lakes in many parts of the earth have been severely eutrophied by sewage and agricultural and industrial wastes.
• The prime contaminants are nitrates and phosphates, which act as plant nutrients.

Biochemical Oxygen Demand (BOD)

• It is possible to estimate the amount of biodegradable organic matter in sewage water by measuring Biochemical Oxygen Demand (BOD).
• Micro-organisms involved in the biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result, there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge.
• This causes the mortality of fish and other aquatic creatures.

UV radiation

• UV radiation of wavelengths shorter than UV-B is almost completely absorbed by Earth’s atmosphere, given that the ozone layer is intact.
• But, UV-B damages DNA and mutation may occur. It causes ageing of the skin, damage to skin cells, and various types of skin cancers.
• In the human eye, the cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of the cornea, called snow-blindness, cataract, etc.
• Such exposure may permanently damage the cornea.

Key Points

• A greenhouse is a glass building that provides optimum temperature for plants to grow in cold climates. The glass traps the heat inside to keep it warm.
• Carbon dioxide acts in a very similar way for our atmosphere in trapping the heat and thus known as a greenhouse gas.
• The CO₂ present in the atmosphere absorbs the infrared radiation and re-emits the heat back to the surface.
• 50% of the heat is lost to space and rest 50% comes back to the Earth's surface.
• This keeps the average temperature of Earth inhabitable.
• This phenomenon is called greenhouse effect.

Important Points

• The primary greenhouse gases include water vapour, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).
• Various human activities like burning of fossil fuels has led to a significant increase of greenhouse gases in the atmosphere over the past two centuries.
• This has in turn led to the global warming which is a global concern for life on Earth.
• Global warming causes increase in global average temperature, melting of polar ice, increase of sea-level, etc.

Additional Information

• SO₂ - Sulphur dioxide is a toxic gas produced by burning of sulphur-containing materials and naturally from volcanic eruptions.
• NO₂ - Nitrogen dioxide is also a by-product of burning of fuels and also production of fertilizers.
• O₃ - Ozone present in the atmosphere protects us from UV radiation by absorbing it. It can also act as a secondary greenhouse gas.