Exam 2 - Recombination & Repair

L10

What role does RecA protein play in homologous recombination?

Recognizes related DNA sequences.

Which genetic exchange processes utilize homologous recombination to incorporate new DNA into the recipient’s genome?

1. Transformation

2. Transduction

3. Hfr conjugation

How do restriction enzymes protect cells from foreign DNA? How does the host protect its own genome?

They cleave unprotected alien DNA. The host methylates target sites in its own DNA to distinguish.

How do CRISPR systems recognize foreign DNA? How does the CRISPR system “learn” from exposure to foreign DNA?

Contains a catalogue of DNA sequences from previous invaders and uses it as recognition sequences to selectively attack in foreign DNA. New DNA can be added to the system for future reference.

What base changes occur in transition and transversion point mutations?

• Transition: pyrine → purine or pyrimidine → pyrimidine

• Transversion: purine ↔ pyrimidine

How do missense, nonsense, and frameshift mutation in a gene affect the translation of mRNA from the mutant gene?

• Missense: exchanges one amino acid for another, sequence is changed

  • May have small or large effect.

• Nonsense: changes codon for stop codon, creates shorted protein

  • Often renders inactive

• Frameshift: shifts entire reading frame, changing sequence

  • Catastrophic

What class of mutation is describes as restoring function to an initial mutant? Can it involve changes in a second gene?

• Reversion mutation

• Yes, but this type of reversion is not true-revertant and is a suppressor mutation

How do spontaneous mutation arise in cytosine residues? What repair system is used to correct this mutation?

• Spontaneous deamination to U

• Base excision repair, doesn’t distort helix enough for other repair systems to recognize

How does UV radiation affect dsDNA? What repair systems can correct this legion?

It causes incorrect replication in the form of dimers between pyrimidines. Photoreactivation and nucleotide excision repair can be used.

What are examples of chemical agents that act as mutagens?

Base analogs that mimic nucleotides, nitrous acid, ethidium bromide.

What repair mechanisms provide error-proof repair? (5)

1. Photoreactivation

2. Methyl mismatch

3. Base excision

4. Nucleotide excision

5. Recombinational

What lesion is repairs by photolyase? What activator is needed by this enzyme?

Pyrimidine dimers, require light >300 nm

How does the methyl mismatch repair system distinguish the parental DNA strand from newly synthesized DNA in repairing legions?

The parent strand has methylated bases.

How does the base excision repair system initiate the repair process?

An abasic site is created to be cleaved and a replacement is synthesized.

How does RecA help in repair of thymine dimers at the replication fork? Why are other repair enzymes requires to complete the process?

• RecA cuts and pastes undamaged daughter strand segment onto damaged strand gap

• This system allow replication to bypass the error, it does not fix the dimer.

Which repair systems utilize DNAP 1 for synthesis?

All except for photoreactivation and error-prone pathways.

What is the function of LexA protein in the SOS response?

When LexA is bound to SOS genes, it represses them and the SOS system is off.

What role does DNAP IV play in SOS repair?

DNAP IV is expressed from a SOS gene and performs inaccurate replication. It lacks the ability to proofread.