Question

Suppose a bacterium is found with a mutation in the topoisomerase used for DNA replication. Which of the following is the most likely effect of this mutation on DNA replication?

• A. During DNA replication, the DNA would become overwound ahead of the replication fork

• B. DNA replication would occur more slowly

• C. The newly synthesized DNA strand could not form a double helix

• D. Polymerase could not associate with the DNA strand and replication would not occur

• E. Following DNA replication, there would be breaks in the new DNA strand

Answer 1

Enzyme topoisomerase :-
• This enzyme controls supercoiling of DNA during unwinding of DNA strands by enzyme helicase.
• Helicase enzyme breaks hydrogen bonds between two strands of DNA and unwinds it.
• During this unwinding, supercolis are formed in DNA strand. Topoisomerase enzyme makes a nick on DNA strands and releases tension caused by supercoiling.
• Hence, both helicase and topoisomease enzymes convert double stranded DNA to single stranded and make these strands available for DNA replication process.

In absence of topoisomerase :-
• Supercoils of DNA will not be removed and DNA will become overwound.
• Now, further DNA strand will not available for DNA replication and this process will be stopped

Now in question,
Option (a) is correct answer.
• During DNA replication, the DNA would become overwound ahead of the replication fork.

Other options -
(b) The process will completely stopped due to absence of single stranded DNA or template DNA.
(c) Newly synthesized strands forms DNA double strand by hydrogen bonds and topoisomerase is not involved in this process.
(d) Polymerase associate with DNA strand with help of primer.
Given statement will be correct for - absence of primer.
(e) Breaks of new strand are filled by DNA polymerase and ligase enzymes not by topoisomerase.