DNA is an amazing substance which contains the genetic code of an organism. Since it is a double helix, either side can serve as a template that can be copied. Because of this feature, new cells can receive an entire blueprint replicated from a parental cell's DNA. In order for this to occur, the double-stranded DNA molecule must be "unwound" or each side separated from its complementary strand.
Replication of DNA occurs along the chromosome at a location known as the origin of replication. In prokaryotic cells, there is only one origin and in eukaryotic cells, there may be several. At the origin, enzymes separate the two strands of the double helix and form a bubble where replication will take place. A Y- shaped replication fork forms on each end and the DNA is copied from the parental strands in both directions until it is all replicated.
There are many proteins that are necessary to help assist the unwinding and replication of the parental DNA. Helicases are enzymes that help separate the two strands of DNA. The bonds that hold together the nitrogenous base pairs are known as hydrogen bonds and the action of helicase is to act on these bonds to separate the two complementary strands of DNA. Once they are separated, single-strand binding proteins attach to prevent the strands from reattaching to their partners while DNA replication proceeds.