Replication of DNA :-
Watson and Crick had proposed a scheme for Replication of DNA.
The scheme suggested that the two strands would separate and as a template for the synthesis of new complementary strands. After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand.
This scheme was termed as
Semiconservative method of DNA Replication.
- Replication of DNA process is completed in following steps –
Unwinding of helix,
Formation of primer strands and
Elongation of new strands.
- For the synthesis of new strands, chemical substances like phosphate, nitrogenous base, sugar and hydrogen atoms obtained from cytoplasm.
- In living cells such as E. Coli, the process of replication requires a set of catalysts (enzymes). The main enzyme is referred to as DNA- dependent DNA polymerase. These enzymes are highly efficient enzymes as they have to catalyse polymerisation of a large number of nucleotides in very short time. Completes the process of replication within 18 minutes, that means the average rate of polymerisation has to be approximately 2000 bp per second. They also have to catalyser the reaction with high degree of accuracy.
- Any mistake during replication would result into mutation.
- The replication occur within a small opening of the DNA helix, referred to as replication fork.
The DNA-dependent DNA polymerases catalyse polymerisation only in one direction that is 5′ → 3′.
This creates some additional complications at the replicating fork, on one strand (the template with polarity 3′ → 5′), the replication is continuous while the template with polarity 5’→ 3′ it is discontinuous.
The discontinuously synthedised fragments are later joined by the enzyme DNA ligase.
- In eukaryotic, the replication of DNA takes place at s-phase of the cell cycle. The replication of DNA and cell division cycle should be highly coordinated.
Central dogma :-
Francis Crick proposed the central dogma in molecular biology.
Which states that the genetic information flows from DNA → RNA → Protein.
- In some viruses the flow of information is in reverse direction, that is from RNA to DNA.