Final answer:
Topoisomerase is the enzyme that removes tension in the DNA helix during replication by cleaving and reannealing the strand, thus preventing supercoiling.
Step-by-step explanation:
The enzyme responsible for removing tension in the DNA helix downstream of the replication fork is topoisomerase. This enzyme acts by cleaving, unwinding, and reannealing the DNA strand to prevent the helix from becoming too supercoiled during replication. It works by making temporary cuts in the DNA molecule to relieve torsional strain caused by the unwinding and rewinding of the DNA strands. Although helicase is responsible for unwinding the DNA helix by pulling apart the two DNA strands at the replication fork, it is not responsible for reannealing or relieving downstream tension. That function is served by topoisomerase, which deals with the supercoils and knots that can form as a result of the replication process.