Process of DNA replication
Replication cannot be initiated in any random part of DNA. Region in a DNA where replication initiates is termed as ‘Origin of Replication’.
Step 1.
Enzyme Helicase breaks hydrogen bonds, thus separating the two strands of DNA. Replication fork structure is formed.
Step 2.
- Continuous synthesis takes place in the Leading strand. In this strand, DNA is synthesized in the same direction as the growing replication fork. Observe the direction of movement of Helicase & DNA polymerase.
- Discontinuous synthesis takes place in the Lagging strand. Synthesis in this strand is more complicated than the Leading strand. DNA polymerase can add new free nucleotides to the 3’ end of the new strand. In the lagging strand, no free 3’-OH end is available. Therefore, DNA polymerase is unable to initiate the process. Enzyme Primase initiate the process by creating a small RNA fragment called Primer. DNA polymerase then extends the primed segments adding free nucleotides. RNA primers are replaced with DNA. Thus, we have DNA fragments. Observe the direction of synthesis in Lagging strand (It is opposite to the direction of growing replication fork). DNA Ligase now joins the DNA fragments and forms a complete DNA.
These DNA fragments are termed as ‘Okazaki fragments’ after the name of the scientist who first described the process of Discontinuous synthesis on Lagging strand.
This entire process of DNA replication occurs during S-phase of cell cycle in eukaryotes. Research is still going on for more detail on the replication process.
