The Process of Replication of DNA
¾ In Eukaryotes DNA replication takes place in S phase of cell.
¾ The process involves a number of enzymes/catalysts, of which the main enzyme is DNA-dependent DNA-polymerase, that catalyses of the polymerization of the deoxynucleotides at a rate of approximately 2000 bp per second.
¾ The process is also an energy-expensive process; deoxyribonucleoside triphosphates serve the dual purpose of
--------- (I) acting as substrate and
---------- (II) providing energy (from the two terminal phosphates).
¾ The intertwined strands of DNA separate from a particular point called origin of replication.
¾ Since the two strands cannot be separated in its entire length, replication occurs with small opening of the DNA- helix; the Y-shaped structure formed, is called replication fork.
¾ The DNA-dependent DNA-polymerases catalyse polymerisation of the nucleotides only in 5’-3’ direction.
¾ Consequently, one of the template strands (with 3’-5’polarity) the synthesis of DNA is continuous, while on the other template strand (with polarity 5’-3’), the synthesis of DNA is discontinuous, i.e., short stretches of DNA are synthesised.
¾ The discontinuously synthesisted strands are later joined together by the enzyme DNA-ligase.
¾ A few additional enzymes (and their functions), other than DNA polymerase and ligase, that are involved in the replication of DNA are:
i--helicases -- opens the helix
ii--topoisomerases -- removes the supercoiling of DNA
iii--Primase --synthesises RNA primer
iv--Telomerase--to synthesizes the DNA of telomeric end of chromosomes.