Embed Size (px)
DESCRIPTION
DNA Replication
Citation preview
DNA Replication
ReplicationReplication• The duplication of DNA which occurs
during the S phase of Interphase.• 1 Strand 2 Complementary Strands• DNA Polymerase
•DNA has to be copied DNA has to be copied before before a cell dividesa cell divides
•DNA is copied during the DNA is copied during the SS or or synthesis phase of synthesis phase of interphaseinterphase
•New cells will need New cells will need identical identical DNA strandsDNA strands
Features of DNA Replication• DNA replication is semiconservative
– Each strand of template DNA is being copied.• DNA replication is bidirectional
– Bidirectional replication involves two replication forks, which move in opposite directions
• DNA replication is semidiscontinuous– The leading strand copies continuously– The lagging strand copies in segments (Okazaki
fragments) which must be joined
thymine
adenine
cytosine
guanine
phosphate
deoxyribose sugar
DNA nucleotide
Deoxyribose sugar
phosphate
Nitrogenous base (guanine)
RNA nucleotide
ribose sugar
phosphate
Nitrogenous base (uracil)
Sugar / phosphate “strand”Nitrogenous base “rung”
Hydrogen bond(H-bonds)
Step 1: Hydrogen bonds between complimentary bases break
DNA “unzips”
Step 2: DNA strandspull apart from each other
Step 3: DNA nucleotides in the cellmatch up with each side of the “unzipped” DNA
each “unzipped’ strands forms a template for a new strand
Step 4: Each “old’ strandforms a template for a “new” strand
two identical DNA molecules form
“old” (original) strand
“new” strand, identicalsequence to the original
(a) Hypothesis 1:Semi-conservative
replication
(b) Hypothesis 2:Conservative replication
Intermediate molecule
(c) Hypothesis 3:Dispersive replication
MODELS OF DNA REPLICATION
Origin
5’3’
3’5’
UNIDIRECTIONAL REPLICATION
Origin
5’3’
3’5’
BIDIRECTIONAL REPLICATION
Replication can be Uni- or Bidirectional
MECHANISM OF DNA REPLICATION
summary of DNA replication know structure, directionality, enzymes, functions
5’3’
3’5’
Proteins at the replication fork in E. coli
Rep protein (helicase)
Single-strandbinding protein (SSB)
BCG Primasome
pol I
pol III
pol III
DNA ligase
DNA gyrase - this is a topoisomerase II, whichbreaks and reseals double-stranded DNA to introducenegative supercoils ahead of the fork
Enzymes in DNA replication
Helicase unwinds parental double helix
Binding proteinsstabilize separatestrands
DNA polymerase binds nucleotides to form new strands
Ligase joins Okazaki fragments and seals other nicks in sugar-phosphate backbone
Primase adds short primer to template strand
Exonuclease removesRNA primer and inserts the correct bases
Binding proteins prevent single strands from rewinding.
Replication
Helicase protein binds to DNA sequences called origins and unwinds DNA strands.
5’ 3’
5’
3’
Primase protein makes a short segment of RNA complementary to the DNA, a primer.
3’ 5’
5’ 3’
ReplicationOverall direction
of replication 5’ 3’
5’
3’
5’
3’
3’ 5’
DNA polymerase enzyme adds DNA nucleotides to the RNA primer.
Replication
DNA polymerase enzyme adds DNA nucleotides to the RNA primer.
5’
5’
Overall directionof replication
5’
3’
5’ 3’
3’
3’
DNA polymerase proofreads bases added and replaces incorrect nucleotides.
Replication
5’
5’ 3’
5’ 3’
3’ 5’
3’Overall directionof replication
Leading strand synthesis continues in a 5’ to 3’ direction.
Replication
3’ 5’ 5’
5’ 3’
5’ 3’
3’ 5’
3’Overall directionof replication
Okazaki fragment
Leading strand synthesis continues in a 5’ to 3’ direction.
Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.
5’
Replication
5’
5’ 3’
5’ 3’
3’ 5’
3’Overall directionof replication
3’
Leading strand synthesis continues in a 5’ to 3’ direction.
Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.
Okazaki fragment
Replication
5’
5’ 3’ 5’
3’
3’
5’ 3’
3’
5’ 5’ 3’
Leading strand synthesis continues in a 5’ to 3’ direction.
Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.
Replication
3’ 5’
3’ 5’
5’ 3’
5’ 3’
3’
5’ 5’ 3’
Leading strand synthesis continues in a 5’ to 3’ direction.
Discontinuous synthesis produces 5’ to 3’ DNA segments called Okazaki fragments.
Replication
5’
5’
3’ 3’ 5’
3’
5’ 3’
5’ 3’
3’
5’
Exonuclease enzymes remove RNA primers.
Replication
Exonuclease enzymes remove RNA primers.
Ligase forms bonds between sugar-phosphate backbone.
3’ 5’
3’
5’ 3’
5’ 3’
3’
5’
Replication
5’ 3’5’
3’
5’
3’
3’ 5’
5’
5’ 3’5’ 3’ 3’
5’
3’
5’ 3’
5’
3’
3’ 5’
5’ 3’
3’ 5’ 5’
5’ 3’5’ 3’ 3’
5’
3’
5’
5’ 3’5’ 3’
3’
5’ 5’ 3’
5’
3’ 3’ 5’
3’
5’ 3’5’ 3’
3’
5’
3’ 5’
3’
5’ 3’5’ 3’
3’
5’
Components of the replication apparatus
dnaA binds to origin DNA sequencePrimasome dnaB helicase (unwinds DNA at origin) dnaC binds dnaB dnaG primase (synthesizes RNA primer)DNA gyrase introduces negative supercoils ahead
of the replication forkRep protein helicase (unwinds DNA at fork)SSB binds to single-stranded DNADNA pol III primary replicating polymeraseDNA pol I removes primer and fills gapDNA ligase seals gap by forming 3’, 5’-phosphodiester bond
