CBSE Class 12 Pre-Board Exam 2024-25: Biology Important Questions with Answers

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The CBSE Class 12 pre-board exams for 2024-25 are an important step in your exam preparation. These exams help you check how well you’re prepared for the final board exams.

To help you study better, we’ve created a list of the most important Biology questions with answers. This Biology questions focuses on essential topics to help you perform well.

The CBSE Pre-board Exam Biology questions cover key topics. They include objective, short/long answer, and competency-based questions, all matching the latest exam pattern.

CBSE 12 Pre-board Exam 2024-25 Biology Most Important Questions

1. In which of the following plants are both male and female flowers born on the same plant and the mode of pollination can be geitonogamy or xenogamy ?

(A) Papaya

(B) Date Palm

(D) Spinach

Ans. (C) Maize

2. Which one of the following hormones is secreted by the human placenta that helps in the maintenance of pregnancy ?

(A) Relaxin

(B) Human Chorionic Gonadotropin

(D) Human Placental Lactogen

Ans. (B) Human Chorionic Gonadotropin

3. The periodic abstinence by a couple for family planning should be from :

(A) Day 5 to 10 of menstrual cycle

(B) Day 13 to 15 of menstrual cycle

(D) Day 16 to 20 of menstrual cycle

Ans. (C) Day 10 to 17 of menstrual cycle

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CBSE 12 Pre-board Exam 2024-25 Biology Most Important Questions	Click Here

4. Select the incorrect match from the following :

Human Karyotype Characters

(A) 45 + XX - Broad palm with characteristic palm crease

(B) 44 + XXY - Overall feminine development

(D) 44 + XY - Normal male

Ans. (B) 44 + XXY - Overall feminine development

5. You know that there are twenty different types of naturally occurring amino acids and four different types of bases in the DNA. A combination of 3 such bases code for a specific amino acid. If instead there are 96 different amino acids and 12 different bases in the DNA, then the minimum number of combination of bases required to form a codon is:

(A) 6

(B) 8

(D) 4

Ans. (C) 2

6. The type of bond represented by the dotted line in a schematic polynucleotide chain is :

(A) Hydrogen bond

(B) Peptide bond

(D) Phosphodiester bond

Ans. (C) N-glycosidic linkage

7. In which of the following conditions/diseases is there a substantial increase in the activity of mast cells observed in the human body ?

(A) Typhoid

(B) Allergy

(D) AIDS

Ans. (B) Allergy

8. Lactobacillus that sets milk into curd is categorised as :

(A) Cyanobacteria

(B) Archaebacteria

(D) Heterotrophic bacteria

Ans. (D) Heterotrophic bacteria

9. Which one of the following transgenic animals is being used to test the safety of the polio vaccine ?

(A) Sheep

(B) Goat

(D) Mice

Ans. (D) Mice

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CBSE Class 12 Biology Chapter 1 Sexual Reproduction in Flowering Plants: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 2 Human Reproduction: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 3 Reproductive Health: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 4 Principles Of Inheritance And Variation: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 5 Molecular Basis Of Inheritance: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 6 Evolution: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 7 Human Health And Disease: Important Competency-Based Questions 2024-25	Click Here
CBSE Class 12 Biology Chapter 8 Microbes In Human Welfare: Important Competency-Based Questions 2024-25	Click Here

10. Restriction Endonuclease Hind II always cuts DNA molecules at a particular point by recognising a specific sequence of :

(A) Six base pairs

(B) Four base pairs

(D) Three base pairs

Ans. (A) Six base pairs

11. The improved trait that is found in the genetically modified transgenic crop Golden rice is :

(A) High lysine content

(B) Insect resistant

(D) High vitamin-A content

Ans. (D) High vitamin-A content

12. The rate of formation of new organic matter by consumers, and the biomass available for consumption of herbivores as well as decomposers are referred to as :

(A) Gross primary productivity and Net primary productivity

(B) Net primary productivity and Secondary productivity

(D) Secondary productivity and Gross primary productivity

Ans. (C) Secondary productivity and Net primary productivity

For Questions number 13 to 16, two statements are given one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below.

(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is notthe correct explanation of the Assertion (A).

(C) Assertion (A) is true, but Reason (R) is false.

(D) Assertion (A) is false, but Reason (R) is true.

13. Assertion (A) : The laws of our country permit legal adoption and it is as yet, one of the best methods for childless couples looking for parenthood.

Reason (R) : Emotional, religious and social factors are no deterrents to the legal adoption of orphaned and destitute children in India.

Ans. (C) Assertion (A) is true, but Reason (R) is false.

14. Assertion (A) : Linked genes do not show dihybrid F2 ratio 9 : 3 : 3 : 1.

Reason (R) : Linked genes do not undergo independent assortment.

Ans. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

15. Assertion (A) : Agrobacterium tumefaciens is a pathogen of several monocot plants.

Reason (R) : It is able to deliver a piece of DNA known as T-DNA to transform normal plant cells into a tumor.

Ans. (D) Assertion (A) is false, but Reason (R) is true.

16. Assertion (A) : Indian Government has set up an organisation known as GEAC to decide the validity of GM research.

Reason (R) : Genetic modification of organisms has no effect when such organisms are introduced in the ecosystem.

Ans. (C) Assertion (A) is true, but Reason (R) is false.

SECTION B

17. Identify A, B, C and D in the table given below :

Terms	Part of the plant it represents
Pericarp	'A'
'B'	Cotyledon in seed of grass family
Embryonal axis	'C'
'D'	Remains of nucellus in a seed

Ans.

A – Wall of fruit / Wall of ripened ovary ,

B – Scutellum,

C – Radicle / Plumule / epicotyl / hypocotyl,

D – Perisperm,

18. Observe the picture given below. Name the naturalist and write the explanation given by him that evolution of life forms had occurred on the basis of this example.

Ans. Name of the Naturalist – Lamarck

- Evolution of life forms driven by use and disuse of organs.

- He said Giraffes in an attempt to forage leaves on tall trees had to adapt by elongation of their necks.

- They passed on this acquired character of elongated neck to their succeeding generations and slowly over the years all the future generations had long necks.

19. Write the basic steps followed in the Assisted Reproductive Technologies (ART) programme to help childless couples. Why is it also known as test tube baby programme ?

Ans.

• Ova from wife or female donor and sperm from husband or male donor are collected , induced to form a zygote under simulated conditions in the laboratory (In vitro) (outside body) , Zygote or embryo are transferred into the female body for development.

• Test tube baby programme – because initial process is carried out in the laboratory / in vitro

20. (a) A farmer while working on his farm was bitten by a poisonous snake. He was rushed to a nearby health centre where the doctor gave him an injection to save his life.

(i) What did the doctor inject and why ?

Ans.

- Readymade or preformed antibodies or antitoxins against the snake venom,

- A quick immune response is required in this case.

(ii) Name the kind of immunity provided by this injection.

Ans. Passive immunity

(b) Why do organic farmers not recommend complete eradication of insect pests ? Explain giving reason.

Ans. The organic farmers hold the view that complete eradication of pests is not only possible but also undesirable because without them beneficial predatory and parasitic insects which
depend upon them as food or hosts will not survive / One of the key belief of organic farmers is that biodiversity furthers health. Greater biodiversity leads to more sustainable ecosystem.

21. Study the diagram of a pyramid of biomass given below. Name the two standing crops that could be occupying level 'A' and level 'B' in it. Name this type of pyramid and the ecosystem in which it is found.

Ans.

A – Zooplankton,

B – Phytoplankton,

- Inverted pyramid of biomass,

- Sea Ecosystem/ Aquatic ecosystem

SECTION C

22. Explain the mode of action of contraceptive pills taken by human females. Mention the schedule to be followed for effective outcome.

Ans.

• - Pills contain progestogens or progestogen – estrogen combination.

- They inhibit ovulation , and implantation as well as, alter the quality of cervical mucus to prevent or retard the entry of sperms.

• - Pills have to be taken daily for a period of 21 days starting within first five days of menstrual cycle.

- After a gap of 7 days it has to be repeated in the same pattern till the female desires to prevent conception.

23. (a) Name and write two characteristics of the type of DNA that forms the basis of DNA fingerprinting technique.

Ans.

- Satellite DNA / Repetitive DNA/ VNTR

- Do not code for any protein , form a large portion of human genome, show high degree of polymorphism

(any two points)

(b) Mention any two applications of this technique.

Ans. useful in forensic applications , helps in determining population and genetic diversities ,forms the basis of paternity testing , to study evolution, to trace path of hereditary diseases.

(any two applications)

24. (a) Explain the significance of the experiment carried out by S.L. Miller. Name the scientists whose hypothesis prompted him to carry out this experiment.

Ans.

• Miller experimentally showed formation of amino acids, and this proved theory of chemical evolution of life / formation of organic molecules from inorganic molecules.

• Oparin , Haldane

(b) How does meteorite analysis favour this hypothesis ?

Ans. Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring in space.

25. Identify A, B, C, D, E and F in the table given below :

Name of Human Disease	Causative Organism	Symptoms
Pneumonia	Streptococcus	'A'
Typhoid	'B'	High fever, weakness, headache, stomach pain
Common Cold	Rhino virus	'C'
Ringworm	'D'	Dry scaly lesions on body parts, redness, itching
Ascariasis	Ascaris	'E'
'F'	Entamoeba histolytica	Constipation, cramps, stools with mucous and blood clots

Ans.

A – Fever / chills / cough / headache / greyish blue lips and nails / severe problem in Respiration,

B – Salmonella typhi,

C – Nasal congestion / discharge / sore throat / cough / hoarseness /tiredness,

D – Microsporum / Trichophyton / Epidermophyton,

E – Internal bleeding / fever / muscular pain / anaemia / blockage of intestinal passage,

F – Amoebiasis / Amoebic dysentery.

26. In a family, the father, the daughter and the son are colour blind, whereas the mother has normal vision. Do you think the son and the daughter have inherited the disease from their father ? Work out a cross to justify your answer.

Ans. No,

Son inherited disease from the mother and daughter inherited disease from both mother and father.

27. (a) (i) What are transgenic animals ?

Ans. Transgenic animals are those which have their DNA manipulated to possess and express a foreign gene.

(ii) Name the first transgenic cow and state its importance.

Ans. First transgenic Cow – Rosie,

- Produced human protein enriched milk (2.4 g/litre) / cow milk containing human alpha lactalbumin protein is nutritionally more balanced product for human babies than natural cow milk.

(b) (i) Explain the convention for naming EcoRI.

Ans. In EcoRI (comes from Escherichia coli RY13)

- E represent Genus Escherichia,

- co represent species coli,

- R represent RY 13 strain,

- I represent order in which the enzyme were isolated from that strain of bacteria

(ii) With the help of an illustration only, show the action of EcoRI on a DNA Polynucleotide.

Ans.

28. (a) Explain how it is ensured that the orchid Ophrys is pollinated by a specific species of bee.

Ans.

- Orchid Ophrys employs ‘Sexual Deceit’to get pollinated by a species of bee

- one petal of flower resembles female of bee in size, colour and markings

- Male bee attracted and pseudocopulates the flower and gets dusted with pollens.

- When same bee ‘pseudocopulates’ with other flower, it transfers the pollens to it.

(b) Describe co-evolution with the help of this example.

Ans. If female bee pattern changes during evolution the flower needs to co-evolve to resemble the female bee to get pollinated.

Questions No. 29 and 30 are case-based questions. Each question has 3 sub-questions with internal choice in one sub-question.

29. Read the following passage and answer the questions that follow. 4 total land area whereas its share of the global species diversity is an impressive 8·1 per cent ! However, in these estimates of species, prokaryotes do not figure anywhere.

Biologists are always keen on collecting data with respect to species diversity observed in different regions of the world. The data collected based on the survey conducted for species richness of groups of mammals in three different regions of the world is shown in the bar graph given below :

(a) Why is the species richness maximum in Region III in the bar graph ?

Ans. This region is less seasonal with constant and more predictable environment / More solar energy so higher productivity and higher diversity / it represent tropical lattitudes which remain relatively undisturbed for millions of years and had a long evolutionary time for species diversification .

(a) Why is the species richness minimum in Region I in the bar graph ?

Ans. Region I represent temperate region subjected to frequent glaciation and get lesser evolutionary time for species diversification / has more seasonal with less constant and less predictable environment which lead to lower specie diversification / have lower solar energy available which reduces productivity and inturn contributes to lesser diversity.

(b) Plants and animals do not have uniform diversity in the world but show rather uneven distribution. Mention what this kind of diversity is referred to as.

Ans. Latitudinal gradient in diversity

(c) Why is it that prokaryotes do not have an estimated number of their species diversity as seen in plants and animals ? Explain.

Ans. Conventional taxonomic methods are not suitable for identifying microbial species, and many species are not culturable under laboratory conditions.

30. Study the diagram given below that shows the steps involved in the procedure of selecting transformed bacteria and answer the questions that follow :

(a) Identify the colony that has got transformed. Justify your answer.

Ans. Colony 4 is transformed with plasmid containing recombinant DNA, as they will not show resistance towards tetracycline.

(b) What are the sites in a plasmid called where ampicillin and tetracycline resistance genes are inserted ? State their role in genetic engineering.

Ans. Award 2 marks to each student .

(c) Name two enzymes playing an important role in genetic engineering.

Ans. Restriction endonuclease / ligase / Taq DNA Polymerase

(c) State the role of β-galactosidase in insertional inactivation.

Ans. Insertional inactivation of gene encoding for ß-galactosidase will lead to colorless bacterial colonies (recombinant)

SECTION E

31. (a) (i) Explain the development of male gametophyte in an angiosperm.

Ans. Each microspore mother cell in sporogenous tissue undergoes meiosis, forming microspores tetrad by the process called microsporogenesis, microspores form cluster of four cells called microspore tetrad, as the anther matures and dehydrates, each microspore undergo assymetric division to produce a vegetative cell and a generative cell.

(ii) Draw a labelled diagram of a three-celled male gametophyte.

Ans.

(b) (i) Draw a diagrammatic sectional view of ovary of human female and label the following :

(1) Blood vessels

(2) Primary follicle

(3) Tertiary follicle

(4) Ovum

Ans.

(ii) At which stage of life are primary follicles formed in a human female ?

Ans. At embryonic stage

(iii) Explain the events (both hormonal and structural) that occur at the time of ovulation till the onset of the next menstrual cycle.

Ans. Both LH and FSH attain a peak level in mid of menstrual cycle rapid secretion of LH leading to maximum level is called LH surge , which induces rupture of graffian follicle and cause ovulation , ovulatory phase is followed by luteal phase during which remaining parts of graffian follicle transform as corpus luteum , it produces large amount of progesterone needed for maintaining endometrium , in absence of fertilization corpus luteum degenerates which causes disintegration of the endometrium leading to menstruation.

32. (a) Stability, as one of the properties of genetic material, was very evident in one of the very early experiments in genetics. Name the scientist and describe his experiment. State the conclusion he arrived at.

Ans.

- Frederick Griffith

Took two strains of Streptococcus pneumoniae bacteria and inject them into mice

- R strain – Rough and Non–virulent

- S strain – Smooth and virulent (with mucous coat)

Conclusion :

• R–strain bacteria had been transformed by heat killed S – strain.

• ‘Transforming principle’ transferred from heat killed S - Strain and enabled R – strain to synthesise a smooth polysaccharide coat. This must be due to the transfer of the genetic material.

• Heat which killed bacteria did not destroy some of the properties of genetic material which shows stability of genetic material.

(b) A tall pea plant bearing violet flowers with unknown genotype is given. Find the genotype by working out different crosses only by selfing the plants. Write the genotypic and phenotypic ratios of each cross shown by you.

Ans. Tall Pea plant with violet flowers can have 4 types of genotypes

TTVV , TtVV , TtVv , TTVv

Case I- TTVV X TTVV
↓

Genotype TTVV

Phenotype All will be tall and violet.

Case II-
Tt VV X Tt VV
↓

Gametes	TV	tV
TV	TTVV Tall Violet	TtVV Tall Violet
tV	TtVV Tall Violet	ttVV Dwarf Violet

Phenotypic ratio Tall Violet : Dwarf Violet

3 : 1

Genotypic ratio TTVV:TtVV : ttVV

1 : 2 : 1

Case III-

Tt Vv X Tt Vv

↓

Gametes	TV	Tv	tV	tv
TV	TTVV Tall violet	TTVv Tall Violet	TtVV Tall Violet	TtVv Tall Violet
Tv	TTVv Tall Violet	TTvv Tall white	TtVv Tall Violet	Ttvv Tall white
tV	TtVV Tall violet	TtVv Tall Violet	ttVV dwarf violet	ttVv dwarf Violet
Tv	TtVv Tall violet	Ttvv Tall white	ttVv dwarf violet	ttvv dwarf white

Phenotypic ratio - Tall Violet : Dwarf violet:Tall white: dwarf white

9 : 3 : 3 : 1

Genotypic ratio - TTVV: TTVv:TtVV:TtVv:TTvv:ttVV: ttVv: Ttvv: ttvv

1 : 2 : 2 : 4 : 1 : 1 : 2 : 2 : 1

Case IV

TTVv X TTVv
↓

Gametes	TV	Tv
TV	TTVV Tall Violet	TTVv Tall Violet
Tv	TTVv Tall Violet	TTvv Tall White

Phenotypic ratio - Tall violet Tall white

3 : 1

Genotypic ratio - TTVV : TTVv : TTvv

1 : 2 : 1

• (Any three self cross shown and explained with genotypic and phenotypic ratio)

33. (a) (i) Name and explain the property present in normal cells but is lost in cancer cells.

Ans.

- Contact Inhibition is present in normal cells but not in cancer cells,

- When normal cells come in contact with other cells it inhibits their uncontrolled growth.

(ii) All normal human cells have genes that may become cancerous under certain conditions. Name them and explain how.

Ans. Cellular oncogenes / Proto-oncogenes , when activated under certain conditions could lead to oncogenic transformation of the cells.

(iii) State the role of the following techniques in detection and diagnosis of cancer :

(1) Biopsy and Histopathology

(2) Magnetic Resonance Imaging

Ans.

(1) Biopsy and histopathology – A piece of suspected tissue cut into thin sections is stained, and examined under microscope by pathologist for increased cell counts.

(2) MRI – detects cancer of internal organs, uses strong magnetic fields and non–ionising radiations to detect pathological and physiological changes in living tissue

(b) Large quantities of sewage are generated every day in cities as well as in towns and are treated in Sewage Treatment Plants (STPs) to make them less polluting. Given below is the flow diagram of stages of STP.

Study the flow diagram and answer the questions that follow :

(i) (1) Why is primary effluent passed into large aeration tanks ?

(2) What is the sediment formed, referred to as ? Mention its significance.

(3) Explain the final step in the settling tank before the treated effluent is released into water bodies.

Ans.

(1) In aeration tanks there is growth of aerobic microbes and fungi (flocs) that consume major part of organic matter in effluent thus reducing BOD

(2)

• Activated sludge

• Used as inoculum in aeration tanks.

(3) Bacterial flocs are allowed to sediment. ( Activated sludge)

(ii) Name any two organisms commonly used as biofertilisers, belonging to different kingdoms. Write how each one acts as a biofertiliser.

Ans.

- Rhizobium (Bacteria), live symbiotically in nodules of roots of leguminous plants and fix atmospheric nitrogen into organic form and provide nitrogen to the plant.

- Glomus (fungi), live in symbiotic association with roots of higher plants and absorb phosphorus from the soil and passes it to plants.

- Cyanobacteria (Anabaena, Nostoc, Oscillatoria), Add organic matter to the soil and increase fertility (Paddy fields)

(Any two can be explained)

👉 CBSE Class 12 Study Materials

CBSE Class 12 Syllabus 2024-25	CBSE Class 12 Previous Year Papers
NCERT Books For Class 12 Books	NCERT Class 12 Solutions
CBSE Class 12 Full Study Material	CBSE Class 12 Sample Paper 2024-25