Recombination

What is genetic recombination?

Exchange of genetic material between two DNA molecules.

Why is recombination essential?

Resets stalled replication, restarts collapsed replication forks, repairs double strand breaks, generates genetic diversity, integrates viral genomes.

Who demonstrated genetic recombination in 1947?

Al Hershey and Max Delbrück.

What is the h locus in bacteriophage T2?

Determines if the bacteriophage is infectious.

What is the r locus in bacteriophage T2?

Determines lysis speed of the host cell.

What are the plaque appearances for h–r+?

Small, clear plaques.

What are the plaque appearances for h+r–?

Cloudy, large plaques.

What are the plaque appearances for h–r–?

Clear, large plaques.

What are the plaque appearances for h+r+?

Cloudy, small plaques.

Why is genetic recombination important for evolution?

Separates beneficial from detrimental mutations and allows allele testing in populations.

What are the three main types of recombination?

Homologous recombination, site-specific recombination, transposition.

What is a Holliday junction?

A four-way DNA structure formed during homologous recombination.

What is branch migration?

Movement of crossover points in a Holliday junction.

What happens if non-crossover strands are cleaved in a Holliday junction?

Recombinant heteroduplex forms.

What happens if crossover strands are cleaved in a Holliday junction?

Non-recombinant heteroduplex forms.

What initiates homologous recombination?

Double strand breaks (DSBs).

What enzyme converts double strand breaks for recombination?

Spo11.

What model describes double strand break repair in recombination?

Double strand break repair model (Szostak et al., 1983).

How do collapsed replication forks get repaired?

Require RecA, RuvAB, and RuvC proteins.

What are the two major pathways for DNA double-strand break repair?

Homologous recombination (HR) and non-homologous end joining (NHEJ).

What is site-specific recombination?

Recombination at specific DNA sequences, often mediated by enzymes.

What is transposition?

Movement of genetic elements within a genome.

What is unique about transposition compared to other recombination mechanisms?

Does not require sequence homology.

What are transposable elements?

Mobile genetic elements that can move within the genome.

Why are transposons important in evolution?

They contribute to genetic variation and chromosomal rearrangements.

What are insertion sequences (IS elements)?

Simple transposons with a transposase gene flanked by inverted repeats.

How do transposons generate direct repeats?

Through target site duplication during insertion.

What are composite transposons?

Transposons with two IS elements flanking a central gene region.

What is replicative transposition?

Copy of transposon inserts at a new site while the original remains.

What is the "cut-and-paste" transposition mechanism?

Transposon is excised and reinserted elsewhere.

How do transposons affect host DNA?

Cause inversions, deletions, and rearrangements.

What happens if transposons are in inverted orientation?

Recombination results in sequence inversion.

What happens if transposons are in the same orientation?

Recombination results in sequence deletion.

How do transposons contribute to antibiotic resistance?

They carry resistance genes and spread them across bacterial populations.

What was the significance of the 1968 Shingella epidemic in Guatemala?

Plasmid-borne antibiotic resistance led to 12,500 deaths.

How did fluoroquinolone resistance in E. coli develop between 1983-1993?

Increased antibiotic use led to resistant strains emerging.

What is MRSA?

Methicillin-resistant Staphylococcus aureus, a major cause of hospital infections.

What is vancomycin-resistant Staphylococcus aureus (VRSA)?

A strain of MRSA resistant to vancomycin.

What are carbapenems?

A class of last-resort antibiotics.

Where were carbapenem-resistant bacteria first found?

On a pig farm in the U.S.

Why is plasmid-borne antibiotic resistance dangerous?

Plasmids allow rapid spread of resistance genes.

What is colistin?

A last-resort antibiotic against multidrug-resistant bacteria.

Why is colistin resistance alarming?

Resistance genes are plasmid-borne and have spread globally.

What is a major contributor to antibiotic resistance spread?

Overuse of antibiotics in agriculture.

How do transposons impact bacterial infections?

They facilitate the spread of resistance genes, making infections harder to treat.