BIOL 300 Summer Discussion: DNA Recombination and Repair

Recombination

Breaking and rejoining of DNA molecules to increase genetic diversity, ensure proper chromosome segregation, and repair DNA damage.

The reason siblings look different

Because during meiosis, DNA from both parents mixes through homologous recombination.

Homologous Chromosomes

Chromosomes that are similar in size, shape, and centromere position but may have different alleles

Sister Chromatids

Identical copies of a chromosome with the same genes, DNA, and alleles

Non-Sister Chromatids

Chromatids from homologous chromosomes with the same genes but different DNA sequences and alleles.

Homologous Recombination

DNA swapping between similar/identical sequences

Non-Homologous Recombination

DNA swapping between totally different sequences

Site-Specific Recombination

Swapping DNA at very specific short sequences typically 14-50 nucleotides long

Double-Stranded Break Model (DBSB)

Model involving DNA break, strand invasion, formation of 2 Holliday junctions, and patch or splice recombinants.

Holiday Junction

A 4-stranded DNA structure formed during recombination. (Think of road intersection)

SDSA Model

Synthesis Dependent Strand Annealing; DNA fixes itself without making a Holliday junction or 2nd end captures. Only makes "patch" outcomes.

Purpose of RecBCD

It digests DNA until it hits a chi site; acts as a helicase and exonuclease.

Purpose of RecA

Forms a filament on DNA and promotes strand invasion.

Purpose of RuvA

Recognizes and binds to the Holliday junction.

Purpose of RuvB

Binds and helps move the DNA strands through the junction.

Purpose of RuvC

An endonuclease that cleaves the Holliday junction to resolve it.

Enzyme used in site-specific recombination in phage λ

Integrase

What sites does Integrase recognize?

attB (bacterial DNA) and attP (phage DNA).

What are the products of site-specific recombination in phage λ?

attL (BOP') and attR (POB').

How is the phage genome excised from DNA?

Integrase and Xis recognize attL and attR and cut/rejoin DNA to separate phage from bacterial DNA.

What is base excision repair?

DNA repair that flips out a damaged base and replaces it.

What is nucleotide excision repair?

Removes bulky lesions like thymine dimers by cutting out a DNA segment.

What is mismatch repair?

Fixes base mispairings that escape proofreading in DNA replication.

When does Recombination occur?

prophase 1 of meiosis 1

Chiasmata

The point of crossing over

Bivalent Chromosomes (Tetrad)

a pair of homologous chromosomes held together

Synapsis

Association between two homologous chromosomes

Synaptonemal Complex

A higher-order structure that forms after synapsis

Replicated Chromosome

Chromosomes that have a pair

of sister chromatids

Joint Molecule

Two DNA strands that have mixed together and are holding onto each other.

Recombinaiton Joint

The exact spot where DNA strands swap places between two different DNA molecules.

Heteroduplex Joint

A stretch of DNA made of one old strand and one newly added (invading) strand.

Branch Migration

The spot where DNA strands are joined can move forward or backward along the DNA.