DNA replication – Steps – Process

Hydrogen bonds between the two DNA strands of DNA double helix break and this unwinds the two antiparallel strands. This process is aided by enzyme ‘helicase‘.

The splitting happens in places of the chains which are rich in A-T, as only two hydrogen bonds in between two, while there are three hydrogen bonds between Cytosine and Guanine. The initiation point where the splitting starts is called origin of replication.

Single-strand binding proteins (SSBs), do not allow the two strands to join back together.

This creates replication bubble. Replication bubbles form at multiple sites along the length of DNA, this speeds up the replication process. The structure which is created is known as Replication Fork.

RNA primase binds at the initiation point of the 3′-5′ parent chain. RNA primase synthesize first nucleotide of the new strand. It attracts RNA nucleotides which bind to the DNA nucleotides of the 3′-5′ strand with hydrogen bonds between the bases.

Here, free 3′ OH group of previous nucleotide get attached with the one phosphate group. Energy is released when the bond of that phosphate group with other two is broken; this energy is used for polymerisation i.e. process of formation of new strands.

The elongation process is different for the 5′-3′ and 3′-5′ template.

5′-3′ Template – The daughter strand proceeding in 3′-5′ direction, uses a 5′-3′ template. This strand is called as leading strand because DNA polymerase delta can read the template and continuously adds complementary nucleotides (like Adenine opposite to Thymine).

3′-5’TemplateDNA polymerase delta can not read 3′-5′ template. The daughter strand using this template for elongation is called as lagging strand. It grows discontinuously away from the replication fork. In the lagging strand the DNA polymerase epsilon reads the template. First RNA primase adds a section of RNA primer; the DNA polymerase I (Exonuclease) reads the fragments and removes the RNA Primers. For more lagging strand building, helix must continue to unwind. Thus, lagging strand is built discontinuously. The gap between two RNA primers is called as Okazaki Fragments. The gaps are closed with the action of DNA polymerase by adding complementary nucleotides to the gaps and DNA Ligase by addition of phosphate in the remaining gaps of the phosphate – sugar backbone.

When the DNA polymerase reaches to an end of the strands termination occurs. The RNA primer is removed; DNA polymerase could not to seal the gap as there is no primer. So, the end of the parental strand where the last primer binds is not replicated. This end of linear DNA consists of non-coding DNA that contains repeat sequences. It is called as telomere. As a result, a part of the telomere is removed in every cycle of DNA replication.

Mechanism of repair fixes possible errors caused during the replication process. Enzymes like nucleases remove the wrong nucleotides and the DNA polymerase fills the gaps.

In the entire replication bubble, leading and lagging strands work in opposite directions.

Same happens in the other replication bubbles. The process of formation of leading and lagging strands continues in both the directions until the entire DNA molecule has been replicated.

As we know, there are many replication bubbles along the length of DNA, these replication bubbles continue to grow until they join together.

Each newly synthesized daughter double helix is made up of one old and one new chain. So the replication is called as semi-conservative replication.