Chapter 17 Recombination Recombination is the term originally
Chapter 17 Recombination
• Recombination is the term originally used by geneticists to describe the outcome of crossing-over between pairs of homologous chromosomes during meiosis. • In the 1960 s, models were proposed for the molecular events that underlie crossing-over, and it was realized that a key part of molecular recombination is the breakage and subsequent rejoining of DNA molecules. • Biologists now use “recombination” to refer a variety of processes that involve the breakage and reunion of polynucleotides. • These include: homologous recombination, site-specific recombination, and transposition.
17. 1 Homologous recombination
17. 1. 1 Models for homologous recombination The Holliday model
The double-strand break model for homologous recombination • The Holliday model could not explain gene conversion. • The double-strand break (DSB) model. • During meiosis, high rate of DSB (100 -1000 times than. . ). • Homologous recombination is involved in DNA repair, and specifically is responsible for repairing double-strand breaks that occur as aberrations in the replication process.
17. 1. 2 The biochemistry of homologous recombination First in E. coli
5’-(A/T)TT(G/C)-3’ at the stop
Other homologous recombination pathways in E. coli • Mutants of E. coli that lack components of the Rec. BCD system are still able to carry out homologous recombination, albeit with lowered efficiency. • Two others are Rec. E and Rec. F.
Homologous recombination pathways in eukaryotes • Equivalent proteins exist. • Lower frequency of sequences equivalent to E. coli chi sites, one per 40 kb in yeast genome compared with one per 6 kb for E. coli.
17. 1. 3 Homologous recombination and DNA repair • Today we believe that the principal function of homologous recombination is in postreplicative repair, its role in crossing-over being of secondary importance in most cells.
In the cases that the damages cannot be bypassed.
When the template has a single-strand nick.
17. 2 Site-specific recombination
17. 2. 1 Integration of DNA into the E. coli genome
17. 2. 2 Site-specific recombination is an aid in genetic engineering
17. 3 Transposition • Transposition is not a type of recombination but a process that utilizes recombination, the end result being the transfer of a segment of DNA from one position in the genome to another. • A characteristic feature of transposition is that the transferred segment is flanked by a pair of short direct repeats.
17. 3. 1 Replicative and conservative transposition of DNA transposons
17. 3. 2 Transposition of retroelements
17. 3. 3 How do cells minimize the harmful effect of transposition? • Hypermethylation, 90% of the methylated cytosines in human genome are located in the interspersed repeat sequences.
- Slides: 31