DNA Replication and Repair

Question 1

DNA Polymerase

Answer 1

1. The DNA identifies an RNA-primer (complementary and antiparallel)
2. DNA-polymrase read the template from 3’ to 5’
3. pauses and checks for mistakes through exonuclease activity (high fidelity)
   High fidelity is required to avoid permanent errors. If the incorrect nucleotide is not removed, then the DNA will have a mutation in it, that can be passed on to a progeny.

Question 2

RNA Polymerase

Answer 2

1. Primer not required
2. template reading identical to DNA-polymerase
3. Mispaired nucleotide are not removed (low fidelity)
   Low fidelity is permited because RNA has short half life and it will not be inherited (happening maybe in one gene in one cell)

Question 3

DNA-Replication: Telomerase

Answer 3

During DNA-replication, the DNA-Polymerase does not finish what it started. Consequently, the chromosomes would have been shorter in successive generation. The telomerase finishes the job. Telomerase works at the end of the chromosome (telomere region). Telomerase avoids chromosomes from shortening with increasing number of replications.

Question 4

Which cells have increase telomerase activity?

Answer 4

cancer cells (telomerase activity is not the cause, but is associated)

Question 5

DNA topoisomerase II in prokaryotes - what is its role in replication?

Answer 5

When DNA is replicated, the DNA overtightens and DNA topoisomerase relaxes the DNA. Drugs like Fluoroquinolones prevents the relaxation and tighted DNA are not able to replicate.

Question 6

Which drug targets DNA topoisomerase II in eurkaryotes ?

Answer 6

Etoposide

Question 7

DNA topoisomerase II is present in normal and cancer cells, then why give Etoposide?

Answer 7

Cancer cells are in G1 phase (always growing, therefore lot of topoisomerase II). But most normals cells are in G0 phase. However, normal cells in G1 phase (epithelial, nucleated red cells, hair follicles)

Question 8

DNA-Repair after UV radiation

Answer 8

Damage: thymine dimers on the strands are formed (G1 phase).
Recognition: Excision endonuclease cuts the dimers but does not fix the problem.
Repair enzymes: DNA polymerase and DNA ligase fixes the problem
Xeroderma pigmentosum: deficiency in excision endonuclease

Question 9

DNA-Repair after DNA replication errors

Answer 9

Damage: mismatched base in G2 phase, usually occur in regions of chromosomes, which have micosattelites
Recognition: two genes hMSH2/1 repair DNA mismatches.
Repair enzymes: DNA polymerase and DNA ligase
Mutation of hMSH initiates defective repair, resulting in hereditary nonpolyposis colorectal cancer

Question 10

Is there any repair in S-phase?

Answer 10

yes, through 3’-exonuclease

Question 11

Role of tumor suppressor genes in DNA repair

Answer 11

DNA repair may not occur properly when certain tumor supp genes have been inactivated through mutation or deletion

Question 12

Examples of tumor suppressor genes

Answer 12

p53, ATM, BRCA-1, Rb. They work the same but are in different location.

Question 13

Tumor suppressor genes: p53

Answer 13

p protein, 53 molecular weight
p53 prevents a cell with damaged DNA from entering the S-phase. p53 induces apoptosis. Inactivation or deletion of p53 is associated with Li Fraumeni syndrome and solid tumors