Question

B4 (a). Explain the role of following proteins in the replication of E. coli chromosome (10 points) 1) RNase H 2) Gyrase 3) Primase 4) Dna C

Answer 1

Answer : Many proteins and enzymes wokr in a co-ordinated manner during the prokaryotic replication process. Activity of one protein/enzyme facilitates the activity of other proteins or enzymes. The role of few molecules involved are explained below.

RNase H : RNase H is involved in the removal of primer sequences in the okazaki fragment during the prokaryotic replication.

Primase Enzyme : An enzyme that facilitates the replication of DNA by synthesizing short RNA molecules called Primers which has exposed 3' OH group. To this exposed 3' OH group, DNA polymerases keep adding complimentary nucleotides  to the bases present in the parent strand. In prokaryotes, primer synthesis is carried out by a protein called Dna G and in eukaryotes, it is DNA polymerase alpha.  

Gyrase : Gyrase is a type II topoisomerase which relevives tortional strain genererated ahead of replication fork due to the movement of helicase enzyme. this is achieved by removing positive supercoils or introducing negative supercoils. When helicase (Dna B protein) starts denaturing double stranded DNA molecules in to single stranded molecules, there generates tortional strain ahead of helicase movement. This prevents replication of DNA by DNA polymerase. This strain is releived by gyrase enzyme which is present only in prokaryotes.  

Dna C protein : Dna C is responsible for loading of Dna B to replication origin during prokaryotic replication initiation. During initiation of replication in prokaryotes, Dna A protein recognises origin sites which in turn bound by Dna B protein has helicase activity which unwinds DNA. Binding of this Dna B protein to the origin site requires Dna C protein.