Download DNA Replication and Repair

DNA Replication
and Repair
The Central Dogma of Molecular Biology
• genetic information is passed on
to the next generation
• semi-conservative
DNA
Replication
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Parent molecule
with two
complementary
molecules
Parental
strands
separate
Each parental
strand is a
template
Each daughter
DNA molecule
consists of one
parental and one
new strand
Overview of replication
Initiation
• DNA is unwound and stabilized
• Origins of replication: Replication bubble and replication fork
Priming
• RNA primers bind to sections of the DNA and initiate synthesis
Elongation
• Leading strand (5’  3’) synthesized continuously
• Lagging strand synthesized discontinuously then fragments are joined
• RNA primer replaced by DNA
Proofreading
• Mismatch repair by DNA polymerase
• Excision repair by nucleases
Review of DNA structure
• double helix
• each strand has a 5’
phosphate end and a 3’
hydroxyl end
• strands run antiparallel
to each other
• A-T pairs (2 H-bonds),
G-C pairs (3 H-bonds)
STEP 1
Initiation at origins
of replication
separation sites on DNA strands
• Depend on a specific AT-rich DNA sequence
– Prokaryotes – one site
– Eukaryotes – multiple sites
• Replication bubble
• Replication fork
• Proceeds in two directions from point of
origin
The proteins
of initiation
1. Helicase –
unwinds
double helix
2. Singlestrand
binding
proteins –
holds DNA
apart
STEP 2
Priming
initiation of DNA synthesis by RNA
RNA primers bind
to unwound
sections
through the
action of
primase
– leading strand –
only 1 primer
– lagging strand –
multiple
primers
– replaced by
DNA later
STEP 3
Elongation of a new DNA
strand
lengthening in the 5’ 3’ direction
DNA polymerase III can only add
nucleotides to the 3’ hydroxyl
end
Leading strand
- DNA pol III – adds nucleotides
towards the replication fork;
- DNA pol I - replaces RNA with
DNA
Lagging strand
- DNA pol III - adds Okazaki
fragments to free 3’ end away
from replication fork
- DNA pol I - replaces RNA with
DNA
- DNA ligase – joins Okazaki
fragments to create a
continuous strand
STEP 4
Proofreading
correcting errors in replication
Mismatch repair
• DNA pol III – proofreads
nucleotides against the
template strand
Excision repair
• nuclease – cuts damaged
segment
• DNA pol III and ligase – fill the
gap left
Telomeres at 5’ ends of lagging
strands
• no genes, only 100 – 1000
TTAGGG sequences to protect
genes
• telomerase catalyzes
lengthening of telomeres