We know that DNA is self-replicating structure and DNA replicates semi-conservatively. However, DNA replication is catalyzed by a set of enzymes. Let’s learn about machinery and enzymes involved in DNA replication.
In the process of DNA replication, the DNA makes multiple copies of itself. It is a biological polymerization which proceeds in the sequence of initiation, elongation, and termination. The whole process takes place with the help of enzymes where DNA-dependent DNA polymerase being the chief enzyme.
Initiation: DNA replication demands a high degree of accuracy because even a minute mistake would result in mutations. Thus, replication cannot initiate randomly at any point in DNA. For the replication to begin there is a particular region called origin of replication. This is the point where the replication originates. Replication begins with the spotting of this origin followed by the unwinding of the two DNA strands.
Unzipping of DNA strands in its entire length is unfeasible due to high energy input. Hence, first, a replication fork is created catalyzed by polymerases enzyme which is an opening in the DNA strand.
Elongation: As the strands are separated, the polymerase enzymes start synthesizing the complementary sequence in each of the strands. The parental strands will act as a template for newly synthesizing daughter strands. It is to be noted that elongation is unidirectional i.e. DNA is always polymerized only in the 5′ to 3′ direction. Therefore, in one strand (the template 3‘→5‘) it is continuous, hence called continuous replication while on the other strand (the template 5‘→3‘) it is discontinuous replication. They occur as fragments called Okazaki fragments. The enzyme called DNA ligase joins them later.
Termination: Termination of replication occurs in different ways in different organisms. In E.coli like organisms, chromosomes are circular. And this happens when the two replication forks between the two terminals meet each other.
DNA replication is highly enzyme dependent process. There are lots of enzymes involved in the replication which includes the enzymes DNA-dependent DNA polymerase, helicase, ligase, etc. Among them, DNA-dependent DNA polymerase is the main enzyme.
DNA-dependent DNA polymerase: It helps in the polymerization and catalyzes and regularises the whole process of DNA replication with the support of other enzymes. Deoxyribonucleoside triphosphates are the substrate as well as the energy provider for the replication process.
Helicase: Helicase is the enzyme which unzips the DNA strands by breaking the hydrogen bonds between them. Thus, it helps in the formation of the replication fork.
Ligase: Ligase is the enzyme which glues the discontinuous DNA strands.
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