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Transcript
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.