The formation of the new DNA strand using the parent stand as a template is known as DNA replication. It is semi-conservative in nature. The process of DNA replication we have described as follow: –
i) During the inter-phase between the two mitotic cycles the DNA replication takes place. The replication is semi-conservative mostly, but in case of repair, is non-and conservative in nature.
ii) The nucleotides of DNA such as, AMP, GMP, CMP, TMP are phosphorylated to form ATP, GTP, CTP and TTP with the help of enzyme phosphorylase.
iii) At the specific point the replication starts called the initiation point which is recognized by the respect initiation protein. It starts with a notch or incision made by endonuclease.
iv) The two strands of the DNA double helix unwind with the help of a DNA unwinding protein and an enzyme helicase denoted in bacteria named E. coli. This type of unwinding may be followed by intertwining because of the natural tension of the DNA. This is prevented by super helix relaxing protein. Moreover it may be noted that, the enzyme topoisomerase cut and rejoin the DNA double helix at several points resulting in the formation of replication bubbles. They finally end into Y-shaped DNA replication fork.
v) Up to 200 nuleotide long a RNA primer is synthesized by the DNA template close to the origin of replication with the help of enzyme RNA polymerase.
vi) The DNA nucleotides are then added to the end of the RNA primer by the enzyme DNA polymerase -III. The RNA primer is then degraded with the help of enzyme DNA Polymerase-I and replaced by a short DNA segment, which gets joint to the DNA strand by DNA ligase forming a complementary DNA strand called leading stand.
vii) The other parental strand synthesized small DNA fragments called Okazaki fragments. These fragments are attached to RNA primers and called the legging stand.
viii) The DNA replication can be unidirectional or bidirectional from the point of origin.
What is DNA polymerase?
The DNA polymerase is the key enzyme in. It exist in three different forms , those are DNA polymerase I, II, and III.
DNA polymerase –I: – it was isolated by Kornberg in 1957 and 10 is called Kornberg’s enzyme. It works in presence of four deoxy-ribonucleoside triphosphate (dNTPs) and bring about the DNA chain elongation for 5’ and 3’ end. The DNA thus produced are capable of supporting the metabolic activity of the cell and hence called biologically active DNA. It is proved in latter that DNA polymerase is the most stable enzyme helping in the removal of RNA primer and fill up or just join the gaps of the DNA created due to removal of the primer by synthesis of small DNA fragments.
DNA polymerase-II: – it is an enzyme capable of producing small DNA fragments and also capable of repairing that DNA strand damaged by external forces like ultraviolet ray.
DNA polymerase-III: – DNA polymerase-III is the enzyme causing polymerisation during DNA replication bringing about 3’ – 5’ proof reading activity. This holo-enzyme is a dimer with ten different polypeptide subunits. The largest subunit α brings about polymerisation while smaller submit ε bring about the 3’ – 5’ exonuclease activity. The β submit acts as a clamp and joins it to the template, the γ complex helps in loading. The holoenzyme complex along with DNA template and ribosome is called replisome.