Can you please describe what occurs when a...

[Seth]

...molecule of DNA "unzips"?

[Angelic W]

Since the DNA molecule is composed of 2 complementary strands it is necessary for these two strands to separate from each other so that replication can take place. The separation of the two strands is known as "unzipping" because it looks like a zipper coming apart.

[Mr. Biology]

Replication begins at special sites along the DNA molecule called Origins of replication. It is at these sites that the helix opens and moves away from. This movement produces the replication fork. Here the new strands of DNA are added.
In order for the DNA strands to separate,3 types of enzymes are needed. Helicases, single-strand binding proteins and topoisomerase.
Priming: Once the replication fork is established a short segment of RNA,formed by an enzyme called primase, initiates the process of replication. With the primer in place, DNA polymerase catalyzes the synthesis of a new strand of DNA. The new nucleotides align themselves with the old DNA. These strands grow from the 5'--3' direction. The energy for this process comes from the 2 P molecules attached to the nucleotide as it is moved into place. There are 2 strands of DNA being formed at the same time. The Leading strand , which is completed in one piece, and moves toward the replication fork; and the lagging strand, which is produced in pieces called Okazaki fragments. These fragments are formed in the 5'--3' direction and linked together by the enzyme DNA ligase.
After the process has been completed the DNA molecules undergo proof reading. DNA polymerase is the enzyme that checks for these errors. If a repair is needed DNA polymerase and ligase will identify, cut, remove, and correct the mistake. This is called Excision repair.