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DNA repair
Lecture 13
Pp 209-215
DNA repair
• Damaged DNA must be repaired
• If the damage is passed on to subsequent
generations, then we use the evolutionary
term - mutation. It must take place in the
germ cells - the gametes - eggs and sperm
• If damage is to somatic cells (all other cells
of the body bar germ cells) then just that
one individual is affected.
Damage from where?
• Consequences of DNA replication errors
• Chemical agents acting on the DNA
• UV light imparting energy into DNA
molecule
• Spontaneous changes to the DNA
Why repair DNA?
• DNA pol does a great job, but not good enough
• Introduces errors in about 1 in 10E7 nucleotides
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added, which it does not correct
Other mechanisms exist (as we will see) to
correct many of the errors left by the replication
system
Most mistakes and damage corrected (99% leaving just a few - only 1 in 10E9 errors are left)
Mutations are permanent changes left in the
DNA
Why repair DNA?
• Repair of non-replication related damage to the
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DNA must also be a priority for the cell.
These defects also will prevent translation and
duplication of the DNA
Cell will die.
Again, any errors or changes to the DNA become
Mutations - which are permanent changes left in
the DNA
Sickle Cell Disease
• This is a very good illustration of the devastating
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effects of even tiny changes to the DNA
Red Blood Cells
Hemoglobin  Has a large protein component
 2 beta globin chains
 A single base change -substitution causes the disease
06_19_sickle_cell.jpg
Spontaneous Mutations
• Involves thermal energy
• Due to random molecular collisions
between molecules and DNA in the cell
• Cannot be prevented
• Parts of the DNA molecule are stripped off
and alterations introduced
• Many outcomes…
Direct DNA Damage
• Some agents damage DNA directly
• Chemicals and light
 Chemicals - alkylating agents
 Methy and ethyl groups added to DNA bases
• This type of damage can be repaired by direct
reversal involving special enzymes
 They remove the offending atoms and restore the base
DNA Damage
• Just a few types of damage is repaired via
simple reversal of the chemical change 
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UV light induced dimers
Methylation of bases
Ethylation of bases
Large chemical groups added to the DNA
• Most other damage require other
systems…
Random photons of ultraviolet (UV) light induce aberrant bonding
between neighbouring pyrimidines (thymine & cytosine) bases on
the same strand of DNA. The will prevent the replication machine
from duplicating the DNA. The cell will die!
06_24_radiation.jpg
This type of defect can be readily reversed by a process called
photoreactivation. Visible light energy is used to reverse the defect (in
bacteria, yeasts, protists, some plants, and some animals but NOT in
humans)
Other forms of DNA damage
• Deamination - An amino group of Cytosine
is removed and the base becomes Uracil
• Deamination - An amino group of Adenine
is removed and the base becomes
Hypoxanthine
• Deamination - An amino group of Guanine
is removed and the base becomes
Hypoxanthine
And…
• Depurination - the base is simply ripped
out of the DNA molecule leaving a gap (like
a missing tooth)…
Molecular level viewRemember these are random events
06_23_Depurination.jpg
DNA level view of the same two events as last slide
06_25_mutations.jpg
Which is which?
• The cell has a big problem to overcome…
• How does it tell which strand carried the
correct information?
• We think we know…
The cell has to pick the right strand to fix or else…
06_21_Errors corrected.jpg
The cell has a mechanism of identifying new strand synthesis by
leaving nicks that DNA. There are enzymes which scan these new regions
looking for errors
06_22_DNA mismatch.jpg
Correction mechanisms
• Direct reversal of damage Photoreactivation (bacteria, yeast,
some vertebrates - not humans) Two
thymines connected together by UV
light.
• Excision Repair - removal of defective
DNA. There are three distinct types
 1) base-excision
 2) nucleotide-excision
 3) mismatch repair
base-excision
• Presence of the Uracil in DNA is a great
example of this type
• Special enzymes replace just the defective
base
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1 snip out the defective base
2 cut the DNA strand
3 Add fresh nucleotide
4 Ligate gap
nucleotide-excision
• Same as previous except that
 It recognizes more varieties of damage
 Remove larger segments of DNA (10 -100s of
bases)
mismatch repair
• Special enzymes scan the DNA for bulky
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alterations in the DNA double helix
These are normally caused by mismatched
bases
AG
AC
CT
These are excised and the DNA repaired
Basic mechanism is
the same for all
three types
1) Remove damaged
region
2) Resynthesis DNA
3) Ligate
06_26_three steps.jpg
Consider…
• Sunlight - sunbathing or daily exposure
 Impact of ozone depletion
 Impact on different skin tones
• Environmental degradation
Evolution acts on mutations
• If we did not have mutation then we would
all be the same!
• Any changes in the environment would be
deleterious to all members of the
population equally
• = There would be no evolution!!!!
• But mutation does exist and it is supported
by comparison of related organisms…
06_27_humans_whales.jpg