Download - Molecular Events in DNA Replication
Molecular Events in DNA Replication
DNA ReplicationDNA replication is semi-conservative
= the mechanism of DNA replication in which each newly synthesized DNA molecule is composed of one strand from the original DNA molecule and one new strand
DNA ReplicationDNA replication starts at a number of sites along a
chromosome in eukaryotic cells – these are called the replication origin
In prokaryotic cells – generally only one site of replication
Complicated process – with dozens of proteins are involved and consists of 3 stages – initiation, elongation, and termination
Initiation – occurs at the origin of replication
1. DNA helicase – unwinds DNA to break its hydrogen bonds between complimentary bases
2. single-strand-binding proteins (SSBs) – bind to each of the single stranded pieces of DNA, stabilizing it
if there were nothing to keep the strands apart, they would reanneal (stick back together)
DNA Replication Animation
DNA Replication – Initiation cont.
Other Proteins / Enzymes of DNA Replication (Initiation stage)
as DNA is being unwound by helicase, DNA in front of the helicase gets bunched up
DNA gyrase and topoisomerase II – enzymes that loosens the tension in front of the replication fork
Replication Structures
Initiation creates an unwound, oval-shaped area called a replication bubble
Each Y-shaped region of the replication bubble is called a replication fork
Replication occurs in both directions from the origin of replication and as it proceeds, each replication fork moves in opposite directions along the DNA
Another look at a replication fork
ElongationPhase that synthesizes new DNA strands by
joining free nucleotides together
DNA polymerase III – enzyme which synthesizes nucleotide chains Adds nucleotides to the 3’ end of a growing
polynucleuotide strand in a continuous fashion towards the replication fork
The new strand generated from the parent strand that goes in the 5’ to 3’ direction is called the leading strand
ElongationDNA polymerase III – also synthesizes new DNA
from the parent strand that does not have a free 3’ hydroxyl end
This new strand is called the lagging strand
It is replicated away from the replication fork, in short segments and in a discontinuous manner
Lagging Strand Elongation1. Synthesis of lagging strand begins with the intial
binding of a short strand of RNA called an RNA primer to the parent strand
2. Primer is synthesized (made) by an enzyme called primase.
5’3’
5’ 3’
A
Lagging Strand Elongation3. With primer in place, DNA polymerase III
extends the strand by adding new nucleotides to the free 3’ hydroxyl end
These fragments extend away from the replicating forks and have to be repeated
Fragments called Okazaki fragments
AT
Leading Strand
5’
5’
3’
3’
helicase
SSBPs
gyrase
primase
5’
3’
DNA polymerase III
Lagging Strand
Okazaki fragment
Connecting Fragments on the Lagging Strands4. DNA polymerase I – removes the RNA primer and
fills gaps between fragments with DNA nucleotidesoccurs in the 5’ 3’ direction
5. DNA ligase – catalyses the joining of Okazaki fragments
How? connects the sugar-phosphate backbone of Okazaki fragments
Okazaki fragments are typically 1000 to 2000 nucleotides (NTs) in length
DNA Replication
Termination phase – occurs upon completion of the new DNA strands, and the two new DNA molecules separate from one another
Overview:DNA replication occurs:1. continuously on the leading strand2. discontinuously on the lagging strand
Replication Animation
Replication Overview - 1helicase unwinds the double stranded DNA structure
creating a replication fork
the single stranded region of the replication fork are maintained by SSBPs
gyrase relieves the tension ahead of the replication fork
Replication Overview - 2two original parent strands serve as templates for
the new daughter strands
daughter strands are produced in one of two methodsleading strand (continuous polymerization)lagging strand (discrete polymerization)
1000 – 2000 NT Okazaki fragments joined together
Replication Overview - 3primase begins each new daughter strand with a
short RNA primer
DNA polymerase III extends a DNA strand from the RNA primer
DNA polymerase I removes the RNA primer AND fills it in with DNA
DNA ligase joins the sugar-phosphate backbones of all adjacent DNA segments