DNA replication is the process by which a cell makes a copy of its DNA prior to cell division. It is a complex process that involves the coordinated action of many different proteins and enzymes. The basic steps of DNA replication can be summarized as follows:
- Initiation: The process of DNA replication begins at a specific site on the DNA molecule called the origin of replication. At this site, a group of proteins called the origin recognition complex (ORC) bind to the DNA and recruit other proteins and enzymes to the site.
- Unwinding: Once the proteins and enzymes are assembled at the origin of replication, they begin to unwind the DNA double helix. This is achieved by the action of an enzyme called helicase, which breaks the hydrogen bonds between the two strands of the double helix, allowing them to separate.
- Primer synthesis: Before DNA replication can begin, a short segment of RNA called a primer must be synthesized on each of the separated strands. This is accomplished by an enzyme called primase, which synthesizes a short RNA strand complementary to the DNA template.
- Elongation: Once the primers are in place, a group of enzymes called DNA polymerases begin to synthesize new DNA strands by adding nucleotides to the 3′ end of the primer. DNA polymerases can only add nucleotides in the 5′ to 3′ direction, so the two strands of the double helix are replicated in different ways.
On the leading strand, which is synthesized continuously in the same direction as the replication fork, DNA polymerase can add nucleotides continuously, moving along the template strand in the 3′ to 5′ direction. This process is relatively straightforward.
On the lagging strand, which is synthesized discontinuously in the opposite direction of the replication fork, DNA polymerase can only add nucleotides in short fragments called Okazaki fragments. These fragments are later joined together by another group of enzymes called DNA ligases.
- Termination: DNA replication continues until the polymerases reach the end of the DNA molecule or encounter another replication fork. At this point, the newly synthesized DNA strands are separated from each other and from the template strands, and the process is complete.
In summary, DNA replication is a complex process that involves the unwinding of the DNA double helix, synthesis of RNA primers, and elongation of new DNA strands by DNA polymerases. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in short fragments that are later joined together. The process terminates when the polymerases reach the end of the DNA molecule or encounter another replication fork.