1. Proofreading and DNA Repair
AP Biology
Unit 3

2. 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

3. 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)

4. 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
Diagram shows p53 protein bound to DNA
Image taken without permission

5. 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.

7. 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.

8. 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

9. 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.

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

11. 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

12. 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.