Ans. To prepare DNA for replication, a series of proteins aid in the unwinding
and separation of the double-stranded DNA molecule. These proteins are required because DNA must be single-stranded before replication can proceed.
1. DNA Helicases:-- These proteins bind to the double stranded DNA and stimulate the separation of the two strands.
2. DNA single-stranded binding proteins (SSBS): ---- These proteins bind to the
DNA as a tetramer and stabilize the single-stranded structure that is generated by
the action of the helicases. Replication is 100 times faster when these proteins are attached to the single-stranded DNA.
3. Topoisomerases: --- These enzymes that change the degree of supercoiling in DNA by cutting one or both strands.
4. DNA Polymerase:--- Enzymes involved in template-directed syn- thesis ofnDNA from deoxyribonucleotide triphosphates.
I, II and III are known in E. coli; III appears to be most important in genome replication and I is important for its ability to edit out unpaired bases at the end of growing strands. All the three DNA polymerases function in 5'3" direction
only for DNA polymerization.
5. Primase:--- Primase helps in the formation of short RNA strands on single-stranded DNA templates, thereby generating the hybrid duplexes required for the initiation of synthesis by DNA polymerases.
6. DNA Ligase:---- Nicks occur in the developing molecule because the RNA primer is removed and synthesis proceeds in a discontinuous manner on the lagging strand. The final replication product does not have any nicks because DNA ligase forms a covalent phosphodiester linkage between 3'-hydroxyl and 5'-phosphate groups.
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