Question

Briefly explain the steps that are involved in DNA replication. And Briefly describe how errors in DNA replication are corrected.

Answer 1

The process of DNA replication:

During DNA replication, the double helix unwinds and the parental strands act as templates for the synthesis of new daughter strands. During this process. One strand is oriented in 5' to 3' direction (leading strand) and another is oriented in 3' to 5' direction (lagging strand) but the DNA polymerase enzyme (the prime enzyme of DNA replication) works only in 5' to 3' direction. So, the first strand oriented in 5' to 3' direction adds the nucleotides continuously one by one in the direction of the replication fork, which is called as leading strand.

DNA replication in the lagging strand:

The lagging strand receives several RNA primers. Replication process in the lagging strand is more complicated and occurs in multiple fragments due to its orientation (its direction of replication is opposite to that of growing replication fork).

The DNA template of lagging strand is read by a primase, which synthesises complementary RNA primer. The primed segments are extended by DNA polymerase forming the Okazaki fragments, now the RNA primers are removed. The synthesis of lagging strand occurs using short DNA segments called Okazaki fragments each consisting of 100 to 200 nucleotides in humans (1000 to 2000 nucleotides in bacteria). The replication occurs in 5 to 3 direction, but away from the replication fork.  DNA ligase connects the Okazaki fragments by catalyzing the formation of the phosphodiester bond between the Okazaki fragments of the complementary strand.

In our body, we have three repair mechanisms namely proofreading, mismatch repair, and excision repair mechanisms.

Proofreading mechanism is performed by DNA polymerase that ensures accurate DNA replication. If it recognizes any mismatched pairs, remove it and try to repair with the new base pair.

In mismatch repair, the replicated DNA is surveyed by a set of proteins, and the mismatched pair is replaced with the correct base pair.

In base excision repair mechanism, the damaged DNA is spliced out, the new DNA base pairs are synthesized and ligated with the help if DNA ligase.