- Following statements have been made about removal of supercoiling produced by DNA unwinding at the replication fork:
- Accumulation of supercoils is the result of DNA helicase activity during unwinding of DNA
- Problem of DNA supercoiling is valid only for circular chromosomes of bacteria and not for the linear chromosomes
C. Supercoiling of DNA is removed by topoisomerases by breaking either one or both strands of DNA on the unreplicated DNA in front of replication fork.
- Both topoisomerase I and topoisomerase can remove positive super-coiling during replication
Which one of the following options has all correct statements?
(1) A, B and C (2) A B and D
(3) A, C and D (4) B, C and D
Analysis of Each Statement
A. Accumulation of supercoils is the result of DNA helicase activity during unwinding of DNA
-
True. DNA helicase unwinds the double helix at the replication fork, creating positive supercoiling ahead of the fork due to overwinding of the DNA duplex14.
B. Problem of DNA supercoiling is valid only for circular chromosomes of bacteria and not for the linear chromosomes
-
False. Although supercoiling is particularly problematic in circular bacterial chromosomes, linear chromosomes also experience supercoiling ahead of replication forks. The problem is not exclusive to circular DNA.
C. Supercoiling of DNA is removed by topoisomerases by breaking either one or both strands of DNA on the unreplicated DNA in front of replication fork
-
True. Topoisomerase I creates transient single-strand breaks, while topoisomerase II creates transient double-strand breaks to relieve supercoiling ahead of the fork.
D. Both topoisomerase I and topoisomerase II can remove positive supercoiling during replication
-
True. Both types of topoisomerases can relax positive supercoils generated during replication, although type II topoisomerases (like DNA gyrase in bacteria) are especially important for this function.
Summary Table
| Statement | Correctness | Explanation |
|---|---|---|
| A | Correct | Helicase unwinding causes positive supercoiling ahead of fork |
| B | Incorrect | Supercoiling occurs in both circular and linear chromosomes |
| C | Correct | Topoisomerases break one or both strands to remove supercoils |
| D | Correct | Both topoisomerase I and II remove positive supercoiling |
Final Answer
(3) A, C and D
Keywords
DNA supercoiling, DNA helicase, replication fork, topoisomerase I, topoisomerase II, positive supercoiling, DNA replication, bacterial chromosomes, linear chromosomes, DNA topology
Conclusion
DNA helicase activity during replication causes positive supercoiling ahead of the replication fork, which is resolved by topoisomerases. Both type I and type II topoisomerases relieve this supercoiling by transiently breaking one or both DNA strands. This problem is not limited to circular bacterial chromosomes but is also relevant to linear chromosomes. Therefore, statements A, C, and D are correct, while B is incorrect.
Correct option:
(3) A, C and D
29 Comments
Priya khandal
July 27, 2025Yes sir
Priya dhakad
July 29, 2025The correct answer is 3
Because Helicase unwinding causes positive supercoiling ahead of fork, Topoisomerase break both strands to remove supercoils and remove positive supercoiling.
Mitali Saini
July 29, 2025Final answer is (3) A, C and D
Priya dhakad
July 29, 2025The correct answer is 3
Because Helicase unwinding causes positive supercoiling ahead of fork, Topoisomerase break both strands to remove supercoils and remove positive supercoiling.
Soniya Shekhawat
July 29, 2025Too easy sir and correct option is A,Cand D
Kajal
July 29, 2025Correct answer is A,C,D as we studied
Anisha jakhar
July 29, 2025Correct statements are A,C,D.
Ankita pareek
July 29, 2025Option 3rd is correct
Alec Joseph
July 29, 2025Correct ans is 3.
Monika jangid
July 29, 2025Yes A C D option is correct bcoz helicase unwind dna causes supercoiling , topoisomerase break both strands to remove supercoiling, topoisomerase I and II remove positive supercoil
Dipti Sharma
July 29, 2025Correct answer: 3 A,C,D
Because:
1. Helicase cause +ve supercoiling
2. Both topoisomerase can remove +ve supercoiling during replication
3. Supercoiling occurs in both circular and linear chromosomes.
Ankita pareek
July 30, 2025Topoisomerases remove supercoiling ,helicase unwinding leads to positive supercoiling
Juber Khan
July 30, 2025Option 3 right bcz the statement B is wrong (problem of supercoiling valid not only for circular chromosome,,, supercoiling occurs on both circular and linear chromosome
Santosh Saini
July 30, 2025Statement a,c and d are correct, topoisomerase remove supercoiling by braking one or both strands
Khushi Vaishnav
July 30, 2025Supercoiling occurs in both circular and linear chromosomes. So the b statement is incorrect
And a c and d are correct statement.
HIMANI FAUJDAR
July 30, 2025Correct answer is A,C,and D
Mansukh Kapoor
July 30, 2025The correct answer is option 3rd because that supercoiling is not specific for circular DNA it can be both in circular and linear
Niti Tanwar
July 30, 2025Answer is A,C,D
Mohini
July 31, 2025A, C ,D is correct because supercoiling can occur in linear as well as circular DNA and it is removed by topoisomerase.
Deepshikha choudhary
July 31, 2025option 3 is correct
Vanshika Sharma
July 31, 2025Opt 3 is correct
Dharmpal Swami
August 1, 2025Topo.1 cut ssdna
Topo.2 cut dsdna
Positive supercoiling are remove
Manisha choudhary
August 1, 2025Helicase jab DNA unwinding krta h tb DNA m supercoiling hoti h
Supercoiling plasmid and linear DNA dono m hoti h DNA double stranded nhi hota double helix hota isliye unwinding k time supercoiling hoti h
Topoisomerase -1 ek strand ko transient break kr k and topoisomerase -2(gyrase) dono strand ko transient break kr supercoiled DNA ko relax krte h then ligate kr deta h
Shivani
August 1, 2025Helicase unwinding cause the supercoiling
Both topoisomerase can remove positive supercoiling
Supercoiling occurs in both circular and liner chromosomes.
So statement A,C,D are correct .
Varsha tatla
August 2, 2025Final answer a c d option
Anjani sharma
August 5, 2025Answer A,C,D are correct
1 Helicase unwinding cause the supercoiling
2 topoisomerase remove supercoiling by braking one or both strands
3 Supercoiling occurs in both circular and linear chromosomes.
Nilofar Khan
August 19, 2025Correct answer is (c) A,C,D
Aafreen Khan
August 23, 2025A, C and D are correct answer
Deepika Sheoran
November 7, 2025Option 3 rd is correct
Topoisomerases 1st cut single stranded DNA
Topoisomerases 2 nd cut double standard
DNA
Positive super coiling are remove.