Download Ch. 12.2: Replication of DNA

Replication of DNA
• Before a cell can divide by mitosis or meiosis, it
must first make a copy of its chromosomes.
• The DNA in the chromosomes is copied in a
process called DNA replication.
• Without DNA replication, new cells would have
only half the DNA of their parents.
• DNA is copied during interphase prior to mitosis
and meiosis.
• It is important that the new copies are exactly like
the original molecules.
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Replication of DNA
• DNA replication depends on specific base pairing
• In DNA replication, the strands separate
– Enzymes use each strand as a template to
assemble the new strands
Nucleotides
Parental molecule
of DNA
Both parental strands serve
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as templates
Two identical daughter
molecules of DNA
Replication of DNA
Semiconservative replication:
• Parental strands of DNA separate, serve as templates,
and produce 2 molecules of DNA that have one
strand of parental DNA and one strand of ‘new’ DNA
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Copying DNA
• Matching bases allows DNA to
be easily copied
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Separating of DNA
• DNA Helicase: enzyme responsible for uncoiling
the double helix and unzipping the weak
hydrogen bonds between the base pairs
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DNA replication
DNA bases
in nucleus
• Enzyme
– DNA polymerase
– adds new bases
DNA
polymerase
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Replicating DNA
 Build daughter DNA
strand
use original parent
strand as “template”
 add new matching bases
 synthesis enzyme =
DNA polymerase

DNA
Polymerase
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Replicating DNA
• Adding bases
– only add
nucleotides to
3 end of a growing
DNA strand
• need a “starter”
nucleotide to
bond to
– strand only grows
53
5
3
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
3
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5
Leading & Lagging strands
• Leading strand - elongates as DNA unwinds
• Lagging strand - elongates in opposite
direction.
– Synthesized discontinuously into small
segments called Okazaki fragments.
• DNA ligase (enzyme) links these sections.
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Okazaki
Leading & Lagging strands
Limits of DNA polymerase III

can only build onto 3 end of an
existing DNA strand
5
3
5
5
5
3
3
5
5

Lagging strand
ligase
growing
3
replication fork
Leading strand

3
Lagging strand


3
Okazaki fragments
joined by ligase
 “spot welder” enzyme
5
3
DNA polymerase III
Leading strand
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
continuous synthesis
New copies of DNA
• Results in 2 exact copies of DNA to split between
reproduced cells
DNA
polymerase
DNA
polymerase
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