All living cells are capable of giving rise to a new generation of cells by undergoing
DNA replication and cell division. During cell division in eukaryotic cells, the replicated DNA is equally distributed between two daughter cells.
During replication process the hydrogen bonds between complimentary strands break and allow the DNA helix to unzip. Each strand of unwound DNA acts as template to build complimentary strand.
The process of DNA replication:
1. The first step in the process of DNA replication is to unwind the DNA. An enzyme known as helicase helps in breaking the hydrogen bonds and unwinding the DNA.
2. Even though the strands are separated they have a tendency of annealing back again due to complimentary nature of base pairs. Single stranded binding proteins (SSBs) bind to the exposed DNA and keep them apart by blocking the hydrogen bonding.
3. Another enzyme gyrase helps to release the tension in the separated strands by cutting and resealing them.
4. DNA is unwound at multiple locations forming bubbles known as replication bubbles. The junction where DNA is still attached is known as replication fork.
5. DNA polymerase III enzyme helps in synthesizing the complimentary strand using template strand as a guide.
6. DNA polymerase III can function only under certain conditions
i) it synthesizes DNA in 5’ to 3’ direction
ii) it require the presence of RNA primer to initiate complimentary strand
7. RNA primer is synthesized by primase enzyme.
8. Complimentary stand can be made continuously on one of the template strand known as leading strand. Other strand is known as lagging stand.
9. On leading strand one RNA primer is attached at 3’ end and complimentary stand is made uninterrupted.
10. On lagging stand multiple RNA primers are attached to template strand and synthesize small fragments of complimentary strands. These fragments are known as Okazaki fragments.
11. RNA primers are removed by DNA polymerase I and are replaced with appropriate deoxyribonucleotides.
12. Ligase enzyme joins all Okazaki fragments together by forming bonds between them.