Proofreading and DNA Repair

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Proofreading and DNA Repair
AP Biology Unit 3
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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

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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)

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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
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Telomeres

• Ends of DNA containing noncoding, repeating
  segments junk at the end
• During replication, the telomeres cant be
  replicated.
• Why? After the primer is removed, there is
  nothing for DNA Polymerase I to use to build off
  of.

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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.

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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

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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.

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Telomerase

• Telomerase activity is usually low in somatic
  (non-sex) cells

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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

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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.