Question

Diagram (draw) and describe the process of DNA replication. Include original molecule, the DNA molecule unzipping, the addition of new nucleotides, and the products of replication.  

Answer 1

Machinery of DNA replication

1) Origin of replication - Replication begins at a perticular region of DNA which is called origin of replication. It is because of the requirement of the origin of replication that a piece of DNA if need to be multiply in the host organism, require a vector with origin of replication.

2) Activation of deoxyribonucleotides - Four types of deoxyribonucleotides, namely, dAMP (adenine monophosphate), dGMP (guanosine monophosphate), dTMP (thymine monophosphate) and dCMP (cytosine monophosphate) are activated by phosphate, energy and enzyme phosphorylase into tri phosphate state.

3) Unwinding of helix - Unwinding of double stranted DNA molecule is brought about by enzyme helicase, which is ATP dependent. Unwinding of DNA molecule results in the formation of Y-shaped structure called, replication fork. Due to unwinding, a supercoiling gets developed on the end of DNA opposite to replication fork. This tension is released by enzyme topoisomerase.

4) Formation of primer strand - A new strand is now to be synthesised opposite to the parental strands. DNA polymerase ||| is the true replicase in E.coli, which is incapable of initiating DNA synthesis, i.e, it is unable to deposit the first nucleotide in a daughter strand. Another enzyme, known as primase, synthesizes a short primer strand of RNA. The primer strand then serves as a stepping stone to strat errorless replication. Once the initiation of DNA synthesis is completed, this primer RNA strand is then removed enzymatically.

5) Elongation of new strand - The enzyme DNA dependent DNA polymerase catalyse polymerisation only in one direction, that is 5' to 3'. Thus, replication is continuous on one templatestrand with polarity 3' to 5' and known as leading strand. The replication is discontinuous in the form of short strand called Okazaki fragments on the other strand with polarity 5' to 3' and known as lagging strand. The discontinuously synthesized fragments are latter joined by enzyme DNA ligase.