Download DNA REPLICATION Review of DNA Structure

DNA REPLICATION
Review of DNA Structure
• The chemical side groups of the nitrogen bases
form hydrogen bonds, connecting the two
strands.
– Adenine forms two
bonds
with thymine
hydrogen
– Guanine forms three
hydrogen bonds with
cytosine
Fig. 16.6
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Sugar-Phosphate
Backbone
• The phosphate
group of one
nucleotide is
attached to the sugar
of the next
nucleotide in line.
• The result is a
“backbone” of
alternating
phosphates and
sugars, from which
the bases project.
• The sugar-phosphate backbones of the double
helix strands run antiparallel to each other
– Each DNA strand has a 3’
end with a free hydroxyl
group
– and a 5’ end
with a free phosphate
group
– One strand runs 5’ to 3’
and the other runs 3’ to 5’
DNA Replication
• Before a cell divides it must replicate its DNA
• Semiconservative – the two parent strands serve as a
template for the synthesis of the new complementary
strands
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DNA Replication
Old Strand
New Strand
Replication Origin
• Replication begins at special sites called origins of
replication
– There may be hundreds or thousands of origin sites per
chromosome.
– Strands separate forming a replication “bubble” with replication
forks at each end.
– The replication bubbles elongate as the DNA is replicated and
eventually fuse.
Enzymes Involved
• Helicase – unwinds and unzips the DNA
helix at the replication forks
• Topoisomerases – relieves the torque from
the unwinding DNA
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Enzymes
• Primase – constructs
RNA primer
complementary to the
DNA templates
• After formation of the
primer, DNA
polymerase III –
elongates the new strand
by adding nucleotides to
the 3’end (~50 per
second)
• DNA polymerase I –
later replaces RNA
primers with DNA
nucleotides
Elongation
• When nucleotide triphosphates are linked to the
sugar-phosphate backbone it loses two of its
phosphates
• Replication always occurs in the 5’ to 3’ direction
Elongation
• Leading strand – elongates toward the replication fork,
continuous
• Lagging strand – elongates away from the replication fork
– Okazaki fragments: discontinuous short segments
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• Ligase – enzyme that joins fragments into a
single DNA strand
Proofreading and Repair
• Replication is extremely
accurate
– 1 error per billion
nucleotides
• DNA polymerase
proofreads each nucleotide
against the template and
fixes any mismatches
– Causes: replication errors,
physical and chemical
agents
When does DNA replicate?
• DNA replication only occurs when the cell
is preparing to divide
• When the cell is NOT dividing DNA is used
as a blueprint for protein synthesis
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