Chapter 27: Recombination

Define: 

• Recombination: Exchange of genetic information between two chromosomes or other molecules of DNA

• Crossover: Structure formed when the strands of two DNA molecules are broken and joined to each other

• Heteroduplex: A DNA double helix composed of single strands from two different DNA molecules

• Chi site: Specific sequences on the DNA of prokaryotes where crossovers form

• Patch recombinants: DNA double helix with a short patch of heteroduplex due to transient formation of a crossover

• Resolvase: Enzyme that carries out resolution of DNA

• Site-specific recombination: Recombination between two lengths of DNA that are largely unrelated. Proteins, known as integrases, recognize specific sequences and form crossovers between them. Same as nonhomologous recombination

• Gene conversion: Recombination and repair of DNA during meiosis that leads to replacement of one allele by another. This may result in a non-Mendelian ratio among the progeny of a genetic cross

Describe the differences between homologous and nonhomologous recombination

• Homologous: Recombination between two lengths of DNA that are identical, or nearly so, in sequence regions of the DNA.

• Nonhomologous: Recombination between two lengths of DNA that are largely unrelated. It involves specific proteins that recognize particular sequences and form crossovers between them. Same as site-specific recombination.

Describe a Holliday junction and the possible results from the resolution of a Holliday junction

• Holliday Junction: DNA structure formed during recombination and found at the crossover point where the two molecules of DNA are joined

• Resolution: Cleavage of the junction where two DNA molecules are fused together so releasing two separate DNA molecules. Refers to the breakdown both of crossovers formed during recombination and of co-integrates formed by transposition.

Describe Chi sites and how are they recognized

• Chi sites are short DNA sequences that signal the RecBCD enzyme to repair DNA double-strand breaks

• recognized as single-stranded DNA passes through the tunnel in the RecC subunit

Describe the role of RecA in single-strand invasion

• binds to the single strand with the free 3’ end and inserts it into another DNA double helix to give a temporary triple helix and stabilizes the single-stranded region of DNA

Describe how lambda phage integrates into the E. coli chromosome

• Bacterial DNA and λ-phage DNA are aligned at the “O” region of the attachment sites. Int protein induces two double-stranded breaks that are resolved, giving a crossover. Since the two recognition sites are altered in their flanking regions, λ cannot be excised by Int alone but needs another protein, known as excisionase or Xis, in addition

Identify the eukaryotic cell cycle when recombination take place and explain why it is confined to this stage

• prophase I stage of meiosis

• meiosis is the specific cell division process where homologous chromosomes pair up, allowing for the exchange of genetic material through crossing over, which is the mechanism behind recombination.

Explain how gene conversion happens

• Recombination and repair of DNA during meiosis that leads to replacement of one allele by another. This may result in a non-Mendelian ratio among the progeny of a genetic cross.