Final answer:
An enzyme that relaxes supercoiled DNA molecules by creating a transient break in the strands to allow for rotation and relaxation is known as topoisomerase. DNA gyrase, a type of topoisomerase II, functions ahead of the replication fork to prevent DNA from becoming overly coiled during replication.
Step-by-step explanation:
An enzyme that creates a transient break in one duplex strand and then allows the intact, complementary strand to undergo a controlled rotation, relaxing the supercoiled molecule is called a topoisomerase. Specifically, DNA gyrase, which is a type of topoisomerase II found in bacteria, has this role. It is essential in the process of DNA replication as it relieves the torsional strain that results from unwinding the DNA double helix. Topoisomerases function by breaking and then resealing the DNA, thereby preventing the DNA from becoming excessively coiled and tangled ahead of the replication fork.
While helicase is responsible for unwinding and opening up the DNA helix by breaking the hydrogen bonds between nucleotides, topoisomerase deals with the supercoils that result from this unwinding. Topoisomerase acts ahead of the replication fork, cutting the phosphate backbone of the DNA strand to allow controlled rotation and subsequent re-ligation, thus preventing supercoiling from hindering the replication process.