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Proofreading and DNA RepairProofreading 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.