DNA Replication and Repair
WHY DOES DNA MAKE COPIES OF ITSELF?
• Later in this course we will
be talking about mitosis.
• When cells reproduce (to
replace skin cells, etc) they
undergo mitosis. In
mitosis, cells divide in two
and make copies of
genetic information, so that
each cell has a complete
copy of the DNA (46
chromosomes).
How does DNA MAKE COPIES OF ITSELF?
DNA Replication
• When cells divide they replicate the DNA.
• DNA replication is semiconservative
(DNA molecule is composed of one parent
strand and one newly synthesized strand)
DNA Orientation
DNA has a direction
because of the 5′ and 3′
ends
 5' end - terminal
phosphate group
 3‘end - terminal hydroxyl
group
 Note DNA is always
made in the 5’ to 3’
direction
 (Page 215 – simulation)
The Process
• In eukaryotes (animals, plants, etc. that
have cells enclosed in membranes and
are organized into higher structures) DNA
cannot be simply pulled apart because of
the hydrogen bonds.
• First, the two parent DNA strands must be
unraveled and kept separate.
• Enzymes work together to expose DNA
template strands.
• DNA helicase unwinds the double helix by
breaking hydrogen bonds between the
complementary base pairs, which results in
an unzipped helix.
• DNA gyrase is an enzyme that relieves
tension produced by the unwinding of DNA
during replication.
"Pssst...Bob, you're unzipped!"
• Bases (A, T, C, G) want to pair (anneal
/stick) with their compliment.
• To prevent this, single-stranded binding
proteins (SSBs) keep separated strands
of DNA apart. They bind to exposed DNA
and block hydrogen bonding.
• The leading strand is made easily
because it can be made in one continuous
piece from the 5’ to 3’ direction.
• DNA polymerase III brings in the
nucleotides to make new DNA.
• The lagging strand (goes in 3’ to 5’
direction) composed of short segments of
DNA, known as Okazaki fragments, is
built discontinuously away from the
replication fork.
• First, the enzyme primase lays down RNA
primers that will be used by DNA polymerase III
as a starting point to build the new
complementary strands.
• DNA polymerase III makes a polynucleotide in
the 5’ to 3’ direction using the RNA primers as
starting points.
• Next, DNA polymerase I removes the
RNA primers and replaces them with the
appropriate DNA nucleotides.
• Then, DNA ligase joins the Okazaki
fragments together by catalyzing the
formation of a bond between the 3’
hydroxyl group and a 5’ phosphate group
(phosphodiester bond).
• Once the new DNA has been made, DNA
polymerase I and DNA polymerase III
proofread by taking out incorrectly paired
nucleotides at the end of the complementary
strand and adding the correct nucleotides.
More about DNA Replication …
• Replication begins in two different
directions from the origins.
• DNA cannot completely unwind because it
would be too big in the cell, so it is
replicated in sections.
• As the two strands of DNA are disrupted,
the junction where they are still joined is
called the replication fork.
• DNA replication goes towards the fork on
one strand and away from the fork on the
other.
• Eukaryotes may have more than one
replication fork and if they get near each
other, a replication bubble forms.
• http://www.johnkyrk.com/DNAreplication.ht
ml
• http://www.youtube.com/watch?v=teV62zr
m2P0
• Questions on DNA
• Worksheet