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Proofreading and DNA Repair
AP Biology
Unit 3
Random facts
• Replication occurs at a rate of
approximately 1000 nucleotides per second
• Error rate is 1/1,000,000 bases 
approximately 1000 mutations every time a
cell divides
Mismatch Repair
• Mismatch = when the wrong
nucleotides are paired
– ex. T with C
• Repair
– Special enzymes cut out the
mismatched DNA and replace
it with the appropriate base
pairs.
• Nuclease (cuts out DNA)
• DNA Polymerase (replaces
DNA)
• Ligase (seals it with rest of
DNA)
p53 protein
• Activated by DNA damage
• Prevents DNA replication
from happening by stopping
the cell cycle
• Activates several proteins to
repair the DNA
• May also activate “suicide”
genes that lead to cell death
• p53 gene mutated in at least
50% of cancer tumors
Image taken without permission http://en.wikipedia.org/wiki/P53
Telomeres
• Ends of DNA containing
noncoding, repeating
segments– “junk” at the end
• During replication, the
telomeres can’t be
replicated.
– Why? After the primer is
removed, there is nothing for
DNA Polymerase I to use to
build off of.
Telomeres
• Telomeric DNA prevents genes from being
worn away.
– How?
– Telomeres serve as a buffer– they are useless
segments that get worn away instead of genes.
Telomeres
• How does the length and amount of DNA
change as a person ages?
• It decreases due to telomeres being worn
away – DNA gets shorter.
• What serves the same role as telomeres in
RNA?
• 5’ G cap and Poly A tail
Telomerase
• An enzyme that lengthens the DNA in germ
cells (diploid) that become sperm and egg-adds the telomeres back on
• Why is this important to the sperm/egg?
• This allows the DNA in sex cells to have
the longest DNA possible to pass on to
offspring.
Telomerase
• Telomerase activity is usually low in
somatic (non-sex) cells
Cancer and Telomeres
• Cancer cells usually have short telomeres.
• Why?
– Cancer is when cells divide uncontrollably.
– Every time it divides, it also goes through DNA
replication  telomeres shortened.
– More divisions = shorter DNA
Cancer and Telomerase
• Cancer cells also show high telomerase
activity.
– Usually, cells self-destruct when telomeres get
too short.
– By extending the telomeres, cancer cells
preserve themselves from self-destruction.