DNA Replication

The process of DNA replication results in two replicas of the DNA molecule.  The number of chromosomes do not actually change rather when the DNA replicates each chromosome becomes double stranded consisting of two identical chromatids (known as sister chromatids) which are held together at the centromere. This process is extremely important in ensuring that each cell ends up with the designated amount of DNA after nuclear division, this occurs during the S phase of the cell cycle.

The process of DNA replication

  • The doubled stranded DNA must uncoil, this occurs through the breaking of the hydrogen bonds holding together the nucleotides through an enzyme known as DNA helicase. The breaking of the bonds results in unpaired bases that now act as a template forming what is called the replication fork.
  • Short RNA primers laid down by RNA primase bind to the DNA and are used to initiate replication.
  • DNA polymerase attaches a complementary DNA nucleotides to the unpaired chains following the base pairing rules (Adenine binds with Thymine and Cytosine binds with Guanine).
  • The pairing of complementary nucleotides is fine for what is known as the leading strand (the strand that allows replication in the 5’ to 3’ direction) as continuous replication can occur.
  • However, the other strand known as the lagging strand (which does not allow the DNA polymerase to add nucleotides forming in the 5’ to 3’) requires another mechanism for adding bases. The lagging strand is formed through adding RNA primers by RNA polymerases and constructing small fragments of DNA at the RNA primers known as Okazaki fragments. Once the replication has finished the RNA primers are removed, this leaves gaps in the lagging strands complement, so new DNA nucleotides are added in the empty spaces and joined together by DNA ligase.
  • This replication is known as semiconservative replication as the each DNA molecule contains one strand of the DNA and a newly formed DNA strand.

See also

Cell cycle