Question

1) When DNA is damaged, a cell has a variety of repair pathways available to correct the problem. Discuss their timing, anything they are good at detecting, and explain their specific mechanisms.

• A) 3pts - Mismatch repair:

                 

• B) 3pts - Base excision [BER] repair:

• C) 3pts - Strand breakage repair – compare HRR [homologous recombination repair] and NREJ [non-recombination end joining]

2) 5pts - Discuss how an E. coli origin of replication is recognized [sequences?], opened [by dnaA], and helicase is added – include any important sequences and their roles in the story. How is this origin licensed?

Answer 1

Ans 1

(A) Mismatch repair: This system works if during DNA replication or recombination any error occurs due to the insertion or deletion of bases.

After recognising the error by 'mut' protein mismatch base is removed and new nucleotide is added.

DNA polymerase and DNA ligase works in this repair system

(B) BER(Base Excision Repair): This system works to remove any damaged bases.

Glycosylase detects the damaged region.

AP site is formed after detection which is broken down by AP endonuclease.

(C) SBR (Strand Breakage Repair): This system works if double stranded is broken due to various external factors such as high radiation or environmental factors.

Can be repaired with 2 pathway under this :

Non- homologous end joining- This system simply repair by losing or adding nucleotides and broken end are joined. This leads to mutation

Homologous recombination: At the broken region of chromosome , the sites get paired up with its homologue and damaged region get replaced by recombination. This does not lead to any mutation like non - homologous recombination can lead.