Replication in Prokaryotes Chapter 6 part II

DNA replication DNA replication is semiconservative The two strands of DNA unwind with the help of DNA helicase Stabilizing protein or single stranded binding protein Primase DNA polymerase III DNA ligase RNAse or polymerase I (Rnase)

DNA replication There are many replication bubbles along the DNA (in eukaryotes) so that more than one region of DNA is replicated at a time In prokaryotes, there is bidirectional synthesis from an origin of replication on a circular chromosome: two replication forks work in opposite directions If generation time is shorter than replication time, then they can both be completed in time if there is bidirectional synthesis

Replication Fork DNA synthesis begins at the origin of replication in prokaryotes Replication fork: zone of unwound DNA where replication occurs DNA helicase unwinds the DNA Extension of DNA –Occurs continuously on the leading strand –Discontinuously on the lagging strand Okazaki fragments are on lagging strand

Events at the DNA replication fork

Bi-directional replication of both strands at same time in opposite directions from 0rigin

DNA replication Different DNA polymerases involved DNA polymerase I –Excises RNA primer and fills in gaps that result DNA polymerase III –Main polymerizing enzyme for replication

Replication of the leading strand The leading strand grows towards the replication fork in the 5’  3’direction It is copied continuously as the replication fork opens The RNA primer is replaced by DNA by the enzyme DNA polymerase I

Replication of the lagging strand The lagging strand grows in short discontinuous segments (Okazaki fragments) away from the replication fork in the 5’  3’direction. DNA polymerase III synthesizes the DNA A short RNA primer is used to prime each of the short segments DNA polymerase I excises the RNA primer DNA ligase joins the segments together

Summary of Replication 1.Topoisomerase I relieves supercoiling 2.Helicase separates parental strands 3.SSBs bind to each strand to stabilize replication fork and prevents two strands from reannealing 4.Primase adds RNA primer in 5’  3’ direction on both strands

Summary of DNA Replication 5. DNA polymerase (III) binds to both strands –Always adds new base to 3’ end (3’ hydroxyl) Made in the 5’  3’ direction! –Leading strand--3’ end toward fork continuous –Lagging strand--3’ end away from fork Okazaki fragments about 1,000bp long 3’ 5’

Correction of errors by DNA polymerase III