Final answer:
The correct option in the final answer is a. Supercoiling in DNA is caused by overwinding (positive supercoiling) or underwinding (negative supercoiling) of the DNA strands, which is managed by the action of topoisomerase during processes like DNA replication.
Step-by-step explanation:
Supercoiling arises due to the overwinding or underwinding of DNA strands during processes such as replication and transcription. Positive supercoiling occurs when the DNA is overwound, while negative supercoiling happens when DNA is underwound. An enzyme called topoisomerase is crucial for managing supercoils by cutting and rejoining DNA strands, thereby preventing excessive supercoiling and allowing proper DNA replication and transcription.
During DNA replication, helicase separates the DNA strands by breaking the hydrogen bonds between the nitrogenous base pairs which can induce supercoiling. Topoisomerase then acts to relax this supercoiling, allowing the replication machinery to function correctly. DNA is first unwound at the origin of replication, at which point replication forks are formed. Single-stranded binding proteins prevent the DNA from rewinding, and the topoisomerase prevents the development of supercoiling. Supercoiling must be regulated; if not, it could impede replication and other cellular processes. The correct option reflecting the difference between positive and negative supercoiling is Overwinding or underwinding of DNA strands (a).