DNA Replication

• Nucleotides are made up of a sugar-phosphate backbone and the nitrogenous base.
• DNA is said to be replicated in a semi-conservative manner where a newly synthesized molecule contains an old strand from the parent DNA and a newly formed strand.
• Accuracy of replication is ensured because of specific base pairing and the ability of enzymes to proofread any errors that might happen.
• The process of DNA replication is divided into three subphases — initiation, elongation and termination.
• The initiation of replication is characterized by the separation of complementary strands of DNA into single strands that will serve as templates for the synthesis of new strands.
• The helix-unwinding protein or the helicase is the enzyme responsible for breaking the hydrogen bonds between the two complementary strands.

  The strands are then kept from getting back together by several single stranded binding proteins (SSBPs) that bind to the single strand.

  Topoisomerases on the other hand are responsible for relaxing the twisting tension formed during the unwinding of the DNA.
• Once the templates are exposed, short sequences of RNA are synthesized by the enzyme primase that will serve as primers for the elongation process
• The enzyme DNA Polymerase III adds nucleotide triphosphates complementary to the DNA template which starts from the 3’-OH end of the primer.
• The complement strand that is being synthesized using the 3’ to 5’ template elongates in the same direction as the unwinding fork, that is why it is called the leading strand. The elongation of this strand requires only one primer and proceeds continuously in the 5’ to 3’ direction.
• Since this template strand runs from 5’ to 3’, it is replicated also in the 5’ to 3’ direction but opposite the direction of the replication fork. This strand is called the lagging strand. The replication of this strand requires multiple primers, thus producing multiple discontinuous segments of complementary DNA called Okazaki fragments.
• Another enzyme called DNA Polymerase I acts for proofreading the DNA molecule. It also removes the RNA primers via its exonuclease activity then replaces it with DNA nucleotides.
• These nicks are then sealed by another enzyme called ligase.