Question

How many of the following statements are correct ?

A. DNA dependent DNA polymerase requires DNA primer for initiating replication but DNA dependent RNA polymerase does not require any primer for transcription.

B. DNA dependent DNA polymerase requires RNA primer for initiating replication but DNA dependent RNA polymerase does not require any primer for transcription.

C. During replication, unwinding of DNA double helix forms a fork-like structure called replication fork.

D. During transcription, unwinding of the DNA double helix by DNA dependent RNA polymerase forms a transcription bubble.

E. During transcription, unwinding of DNA double helix by DNA dependent RNA polymerase forms a transcription fork.

• A. 2
• B. 3
• C. 4
• D. 5

Answer 1

The correct option is B 3

Statements B, C and D are correct, and statements A and E are incorrect.

Hence, three statements are correct.

DNA replication is copying of the entire genetic material that usually occurs in the S phase (synthesis phase) of the interphase of the cell cycle.

For this, the DNA functions as its own template. Since, both the strands of DNA act as a template to synthesise the new strands, unwinding of DNA strands takes place at a particular spot called origin of replication with the help of some enzymes like helicase, topoisomerase, gyrase etc.

The part of DNA strands that is unwound, depicts a ‘Y’ shape (fork-like structure) called replication fork.

To initiate replication in the 5'→3' direction, an RNA primer (small strand of RNA) is required that is formed at the 5’ end of a new strand of DNA with the help of a primase enzyme.

RNA primer is very much significant for the replication process because without RNA primer DNA dependent DNA polymerase would not be able to add any nucleotides.

Transcription is the process of copying genetic information from the template (3'→5’) strand of the DNA into RNA.

Here, only one strand of DNA acts as a template and for initiation of transcription, no RNA primer is needed.

DNA dependent RNA polymerase enzymes aid the entire process of transcription. This enzyme unwinds the template strand of DNA ahead of it and rewinds the DNA behind it. This region of unwinding is called a transcription bubble that is used for the elongation of RNA in the transcription process.

Therefore, it is not called a transcription fork as in case of replication.