NCERT Class 12 Biology: Chapter – 6 Molecular Basis of Inheritance Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 28:

In E. coli, the lac operon gets switched on when:

a. lactose is present and it binds to the repressor

b. repressor binds to operator

c. RNA polymerase binds to the operator

d. lactose is present and it binds to RNA polymerase

Answer: (a)

Very Short Answer Type Questions

Question 1:

What is the function of histones in DNA packaging?


DNA wraps itself around a protein called histone. Histones pack into structural units called nucleosomes.

Question 2:

Distinguish between heterochromatin and euchromatin. Which of the two is transcriptionally active?


Distinguish between Heterochromatin and Euchromatin
i. They are darkly staining and are scattered or accumulated near the nuclear envelope.i. Euchromatin is not readily stainable and is dispersed.
ii. These are transcriptionally less active or inactive.ii. These are transcriptionally active.

Question 3:

The enzyme DNA polymerase in E. coli is a DNA dependent polymerase and also has the ability to proof-read the DNA strand being synthesised. Explain. Discuss the dual polymerase.


1. In bacteria there are three types of DNA polymerases.

2. All the three DNA polymerases can add nucleotides in 5 ‘-3’ direction.

3. DNA III polymerases can proofread the newly formed strand and can sense the wrong base insertions.

4. It deletes wrong base and correct the mistakes by putting the right base in.

5. It cannot correct the substitution of uracil in place of thymine.

6. It can repair the damage done to DNA by UV rays exposure.

Question 4:

What is the cause of discontinuous synthesis of DNA on one of the parental strands of DNA? What happens to these short stretches of synthesised DNA?


1. In DNA both the strands are anti-parallel and complementary.

2. Both the strand acts as template.

3. As DNA synthesis can only take place in 5 ‘-3’ direction. From these two strands only one strand which is 3 ‘-5’ can synthesise the complementary strand in 5 ‘-3’ direction.

4. The left-out strand of the DNA which was 5 ‘-3’ has to be synthesised in opposite direction as short stretches in discontinuous manner.

5. These stretches are also known as ‘Okazaki fragments’ which are later joined together by DNA ligase enzyme.

Question 5:

Given below is the sequence of coding strand of DNA in a transcription unit

write the sequence of

a) Its complementary strand

b) The mRNA


a. Complementary strand:

Here, Adenine bonds with thymine & Guanine bonds with cytosine.

b. The mRNA:

In RNA Adenine bonds with uracil instead of thymine & Guanine bonds with cytosine.

Question 6:

What is DNA polymorphism? Why is it important to study it?


Difference in the nucleotide sequence between the individuals is called as DNA polymorphism. This difference can be in a single base pair, insertion or deletion of various base pairs.

Importance of DNA polymorphism:

1. It is important for genetic variation.

2. Used in criminal cases for identification of the culprit.

3. Used in genetic mapping.