DNA Replication

DNA Replication

  • Occurs in the nucleus during the S phase of the cell cycle.
  • It is an autocatalytic function of DNA.
  • It occurs in a semi-conservative manner.


Sequence of base pairs in the genome where DNA replication begins. A Nick is created.


Helps in unwinding the two strands by breaking hydrogen bonds.


Helps in relieving tension caused due to unwinding.

Single stranded binding proteins

Prevents reannealing of the two strands.

Replication fork

Y-shaped fork formed due to the separation of the two strands.


forms the RNA primer at the ORI.

RNA primer

Short strand of RNA formed at ORI as DNA polymerase III requires a free OH group at 3′ end to add nucleotides.


The nucleotides are high energy compounds present in triphosphate form, they act as the substrate for the new strand and also provide energy to form hydrogen bonds between nitrogenous bases.


Addition of nucleotides and it always occurs in 5′→3′ direction.

DNA polymerase III

Needed to form the new strand. Shows polymerization in 5′→3′ direction and exonuclease activity in 3′→5′ direction.

Leading and Lagging strands

3′→5′ strand is the leading strand as the new strand starts forming as the two strands separate. 5′→3′ strand is lagging strands as the new strand starts forming only after a short part of DNA has opened up.

Continuous and Discontinuous

The new strand is formed in a continuous manner in the leading strand and in a discontinuous manner in the lagging strand.

Okazaki fragments

Short fragments formed on the lagging strand.

DNA polymerase I

Shows exonuclease and polymerization activity. Removes RNA primer and adds deoxyribonucleotides in its place.

DNA ligase

Helps in joining of Okazaki fragments.


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