Lecture 6 - DNA Repair and Replication

Question 1

DNA replication

Answer 1

Multiple origins of replication, one centromere, and two telomeres - replicates in interphase. SEMI-CONSERVATIVE, 2 template strands. DNA always synthesized in 5’>3’

Question 2

What other models are possible for DNA replication?

Answer 2

Conservative (make a brand new copy of DNA), dispersive (chunks of new and old combined)

Question 3

How was semiconservative nature of DNA replication proven?

Answer 3

Meselson-Stahl experiements with heavy nitrogen.

Question 4

When does DNA replication occur?

Answer 4

In S phase of interphase, only once per cell cycle.

Question 5

Describe the replication process

Answer 5

Question 6

RNaseH

Answer 6

removes primers from lagging strands

Question 7

FEN-1

Answer 7

The protein encoded by this gene removes 5’ overhanging flaps in DNA repair and processes the 5’ ends of Okazaki fragments in lagging strand DNA synthesis

Question 8

Ligase

Answer 8

Enzyme that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond.

Question 9

Describe the editing capability of DNA polymerase

Answer 9

DNA pols have 3’-5’ exonuclease activity, contributes significantly to fidelity of overall process. Chews from outside.

Question 10

DNA mismatch repair

Answer 10

Complex of mismatch repair proteins scans newly replicated DNA for nucleotide mismatches, removes a section of strand containing wrong base, and resynthesizes the strand. (Mutations in these genes can cause colon cancer)

Question 11

Describe error-prone DNA pols

Answer 11

Damaged DNA templates might require specialized low-fidelity pols to replicate. Lesions can be avoided by replication restart copying first off lagging strange then reestablishing replication fork. OR Translesion synthesis and reestablishing replication fork.

Question 12

Describe the replication stress response

Answer 12

Replication forks are vulnerable to various “replication stresses.” Cells have evolved mechanisms called checkpoints to deal with these events to preserve the replication fork. If these fail, the fork fails and could lead to bad shit with DNA

Question 13

What protein is responsible for replication stress response?

Answer 13

ATR - makes sure the replication stops while the cell checks for damage

Question 14

What are examples of diseases caused by replication defects?

Answer 14

Ataxia telangiectasia - lymphomas, dilation of blood vessels, immune dysfunction - ATM protein mutated, important for replication stress response

Bloom syndrome - carcinomas, leukemias, etc. caused by mutation to BLM, another protein involved in replication fork stress response

Question 15

Describe some cancer drugs and their effect on replication

Answer 15

AZT - chain terminator, interupts the formation of HIV DNA

5-FU - affects pyrimidine synthesis

Camptothecin - targets topoisomerase

Hydroxyurea - depletes cancer cells of deoxyribonucleotides

Question 16

How can DNA damage arise?

Answer 16

Spontaneous events, exposure to environmental mutagens, DNA rep errors or stress

Question 17

Examples of DNA damage

Answer 17

Mismatched base pair, depurination, pyrimidine dimer (UV light is cause), bulky adduct, alkylation, deamination

Question 18

How is DNA damage fixed?

Answer 18

Some enxymes exist to directly repair some mutations, while other times bases are excised (via single strand or double strand breaks) and repaired

Question 19

If one intact DNA strand exists, how is DNA repaired?

Answer 19

1. Altered DNA is removed by enzymes called DNA repair nucleases
2. DNA pol fills in the gap
3. DNA ligase completes process

Base excision, nucleotide excision, and mismatch repair only differ by the enzymes used in step 1

Question 20

If both strands of DNA are damaged, how is it repaired?

Answer 20

Non-homologous end joining - wrong bases are excised, DNA pol puts right ones in, DNA ligase patches everything up

Question 21

What kinds of diseases happen when there are defects in DNA repair mechanisms?

Answer 21

Xeroderma pigmentosum - severe predisposition to skin cancers

etc.