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.