CBSE Class 12 Biology 2024-25: Chapter 5 Molecular Basis Of Inheritance Competency-Based Questions with Answers; Download Free PDF

Get ready for your CBSE Class 12 Biology exams with our easy guide on Chapter 5: Molecular Basis Of Inheritance. This chapter is very important, especially with the new exam pattern starting in the 2024-25 school year. The CBSE will include 50% more competency-based questions. These questions ask you to use what you've learned in real-life situations.

In this article, we focus on key questions from Chapter 5: Molecular Basis Of Inheritance. These questions help you think and solve problems. You’ll find different types of questions, such as multiple-choice, true-false, gap-filling, and short-answer questions. Each question has clear answers to help you understand the concepts better.

To help you study, you can download a free PDF that has all the competency-based questions from Chapter 5: Molecular Basis Of Inheritance along with their answer keys.

CBSE 12 Biology Chapter 5 : Molecular Basis Of Inheritance Competency-Based Questions

Q.1 Which of the following occurs as a result of DNA replication being semiconservative?

A. Chromosome number becomes half in the gametes.
B. Each chromosome has half DNA and half RNA.
C. Parental characters are found in each chromosome.
D. Each new DNA strand has half the number of nucleotides compared to the parent DNA strand"

Answer. C. Parental characters are found in each chromosome.

Q.2 An organism has 6 X 109 bp in its DNA. Which of the following is likely to be closest in height to the length of its DNA molecule?

- A wardrobe: 2 m
- A tall tree: 20 m
- An A4 sheet: 20 cm
- A eukaryotic nucleus: 10-10 m

A. A wardrobe
B. A tall tree
C. An A4 sheet
D. A eukaryotic nucleus

Answer. A. A wardrobe

Q.3 Which of the following is true about the structure of DNA and RNA?

P) The nitrogenous base is devoid of an OH group in DNA.
Q) The nitrogenous base of RNA gains the OH group lost by DNA.
R) The pentose sugar in RNA combines with an extra OH group.
S) The pentose sugar in DNA loses an OH group to form deoxyribose.

A. only P
B. only S
C. only R and S
D. only P and Q

Answer. B. only S

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Q.4 Keratin is a common protein found in the human body with its gene on chromosome no. 12. Centromeres are constricted regions on the chromosome that help during cell division. In which of the following regions will the keratin gene and centromere be found?

A. P
B. Q
C. R
D. S

Answer. C. R

Q.5 In Griffith's experiment, when he injected a mixture of heat-killed S strain (virulent) and live R bacteria (non-virulent), the mice died.

What type of colony/ies would he observe when the bacteria from the dead mice would be plated on a culture medium?

A. Only rough colonies would be observed.
B. Only smooth colonies would be observed.
C. Both rough and smooth colonies would be seen.
D. Each colony would have a mix of both rough and smooth textures.

Answer. C. Both rough and smooth colonies would be seen.

Q.6 Given below are some statements about DNA replication.
DNA replication is semiconservative.
One of the DNA strands is replicated in a discontinuous fashion.
Each strand of parental DNA acts as a template for the new strand.
DNA replication shows a low level of accuracy.
Based on the statements, pick the correct option.

A. P and S are true and S is the reason for P
B. P and Q are true and Q is the reason for P
C. P and R are true and R is the reason for P
D. S and Q are true and Q is the reason for S

Answer. C. P and R are true and R is the reason for P

Q.7 Two statements are given below - one is an Assertion (A) and the other is a Reason (R).

Assertion (A): DNA-dependent RNA polymerase catalyses polymerisation in the 5' to 3' direction.

Reason (R): The strand with 5' to 3' polarity is called the coding strand. Which of the following is correct?

A. Both A and R are true, and R is the correct explanation for A.
B. Both A and R are true, but R is not the correct explanation for A.
C. A is true, but R is false.
D. A is false, but R is true.

Answer. B. Both A and R are true, but R is not the correct explanation for A

Q.8 Which of the following statement/s are correct example/s of the degenerate feature of the genetic code?
UAA and UAG act as terminator codons. CUA codes for leucine in bacteria as well as humans.
Valine is coded for by GUU and GUC.

A. only Q
B. only R
C. only P and Q
D. only P and R

Answer. B. only R

Q.9 A DNA molecule is much longer than the length of nucleus of a cell.

Describe the organization of DNA inside a nucleus

Answer. 0.5 marks each for the following:

- Positively charged basic histone proteins combine to form a unit of 8molecules called the histone cluster.
- DNA is wrapped around the histone cluster to form a nucleosome.
- Many such nucleosomes repeat to form a bead-in-a-string structure called the chromatin which are thread like bodies.
- The chromatin is further packaged with the help of non-histone chromosomal proteins to fit into the nucleus.

[Key words mandatory in the complete answer are highlighted]

Q.10 One strand of a DNA segment is made up of repeats of adenine and cytosine - 5' ACACACAC 3'

Explain the formation of a dinucleotide (AC) and its base pair from nitrogenous bases.

Answer. 1 mark for each correct step such as:

- A nitrogenous base is linked to the OH of 1' C pentose sugar through a Nglycosidic linkage to form adenosine and cytidine nucleosides.
- Next, a phosphate group is linked to OH of 5' C of a nucleoside through a phosphoester linkage, forming adenine and cytosine nucleotides.
- The 3'C of adenine links to the 5'C of cytosine to form a 3'-5' phosphodiester linkage to form a dinucleotide.
- In a similar manner, a dinucleotide is formed between thymine and guanine.
- Finally, adenine forms two hydrogen bonds with thymine and cytosine forms three hydrogen bonds with guanine to form the dinucleotide base pairs.

Q.11 State TWO points of similarity between the process of replication and transcription in humans.

Describe the fate of the products of replication and transcription with respect to the following:

location of the product/s

lifespan of the product/s

In which phase/s of the cell cycle do these processes occur?

Answer. 1 mark each for the following:

Both processes involve the copying of a DNA molecule.
Both processes take place in the nucleus of the cell.
[Accept any other valid answer.]

1 mark each for the following:

location: DNA strands formed after replication continue to stay in the nucleus whereas mRNA formed aftertranscription moves/is exported to the cytoplasm.
lifespan: DNA strandsformed afterreplication continue to remain in the nucleus as long as the cell is living whereas mRNA is degraded after translation.

DNA replication: S phase
DNA transcription: throughout the cell cycle

Q.12 Why is the enzyme ligase not required in DNA amplification using PCR?

What will happen if DNA polymerase is used in PCR? Give a reason to support your answer.

State one advantage of PCR over DNA replication.

For any given sequence of nucleotides, DNA replication is faster than PCR. Justify.

Answer. Since no Okazaki fragments are formed in PCR amplification, ligase is not required in the process.

1 mark each for the following:

In PCR, heat is used to denature/separate the dsDNA.

DNA polymerase cannot withstand this high temperature and will denature.
PCR can be used to amplify any sequence of nucleotides - RNA or DNA whereas DNA replication is only for DNA sequences.

PCR can be used to amplify just one gene as well unlike DNA replication which involves duplication of the entire genome.

[Accept any other valid answer]

For a given sequence, DNA replication is much faster as all three steps (denaturation, annealing and replication) happen simultaneously whereas in PCR it happens one by one which takes longer.

[Accept any other valid answer]

Q.13 A DNA sequence consists of 35% cytosine nucleotides.

What would be the percentage of adenine nucleotides in the same DNA sequence? Justify your answer.

Answer. A possible complete answer: According to Chargaff's rule,

If, C = 35%, then G = 35%
If C and G make up 70% of the sequence then A and T will make up 30% So, A = T = 15%

[0.5 marks for mentioning the rule

0.5 marks for calculating the percentage of guanine nucleotides
0.5 marks for calculating the percentage of A + T nucleotides
0.5 marks for finding the percentage of adenine nucleotides]

Q.14 Although DNA replication by polymerases is an energetically expensive process, it still proves to be an efficient process. Justify the statement.

Answer. 1 mark for each correct point:

It is considered efficient because they catalyse polymerisation of a large number of nucleotides in a very short time.

They are highly accurate to avoid mutations.

Even though the process needs a lot of energy the deoxyribose triphosphates act both as substrate and energy providers for the reaction as the two terminal phosphates are high-energy phosphates.

Q.15 What are the four properties needed in a molecule to function as genetic material?

Compare these parameters between the DNA and RNA.

Answer. (a) 0.5 marks for each of the following:

can replicate
should be stable
allow for slow mutations
ability to express itself

(b) 0.5 marks for each comparison:
Both, DNA and RNA can replicate.

RNA is unstable when compared toDNA, due to the presence ofthe 2'-OH group at every nucleotide which makes it easily degradable.

Both, DNA and RNA can mutate.

Both can express themselves - RNA can do it directly by forming proteins whereas DNA can do it by forming an RNA first.

[Awardmarks even if only the comparison between RNA and DNA is mentioned]

Q.16 Justify:

RNA polymerase cannot transcribe a gene by itself.

The terminator sequence can be used to define the template strand.

Answer. RNA polymerase is only capable of catalysing the process of elongation. It associates transiently with the initiation factor and the termination factor to initiate and terminate the transcription, respectively.

The terminator sequence is located at the 3' end of the coding strand and can help identify the coding strand, the opposite of which is the template strand.

Q.17 Given below is a DNA sequence.

5' - TAACGATCGTACATGGAT - 3'

Identify the mRNA sequence that is transcribed from this DNA sequence.

Can this sequence be translated? Give a reason to support your answer.

[Note: Assume no post-transcriptional and post-translational modifications will take place.]

Answer. (a) 1 mark each for the following:

- Identify the complementary strand 3'
- ATTGCTAGCATGTACCTA - 5' - Find the mRNA sequence:

UAA CGA UCG UAC AUG GAU

[Marks to be awarded if only the mRNA sequence is mentioned]

(b) 0.5 marks each for the following:
No
The first codon of this sequence codes for a stop codon.

Q.18 What does a translational unit comprise?

If a codon CGA that codesfor arginine is present on the mRNA after codon AUG, describe how the translation process will be done step-wise.

Answer. A translation unit is the sequence of mRNA that codes for an amino acid sequence (polypeptide) with a start codon on one end and a stop codon on the other.

[1 mark for mentioning all three parts. No marks are to be awarded for missing any of the parts of a unit]

0.5 marks for each step:

Charging of tRNA: Arginine is charged in the presence of ATP and linked to its specific tRNA molecule.

The smallsubunit encounters the mRNA.

The large subunit, which hastwo sitesfortRNAsto bind, which have the initiator tRNA and the arginine tRNA bound to it.

Here, methionine and arginine form a peptide bond with each other in the presence of a catalyst before the ribosome moves ahead.

Q.19 The expressions of genes are regulated by metabolic, physiological and environmental factors/conditions.

With the help of TWO examples, justify thisstatement in the case of eukaryotes.

Answer. 1 mark each for the following:

Glucose in the body is used by cells to produce energy, which causes glucose levels in the blood to reduce and signal the pancreas to produce less insulin, thereby reducing the transcription and translation of genes coding for insulin.

The presence of mutagens in the environment impacts the expression of genes related to growth such that excessive uncontrolled growth begins in some cells, leading to cancer.

[Accept any other valid answer]

👉 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