Objectives of DNA recombination. The different processes of DNA recombination: homologous recombination, site-specific recombination, transposition, illegitimate recombination, etc.
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FIGURE 15.17: RecBCD unwinding and cleavage
FIGURE 15.18: RecA strand invasion
FIGURE 03: Recombination occurs at specific stages of meiosis
Chromosomes must synapse (pair) in order for chiasmata to form where crossing-over occurs.
The stages of meiosis can be correlated with the molecular events at the DNA level.
Double-strand breaks that initiate recombination occur before the synaptonemal complex forms.
If recombination is blocked, the synaptonemal complex cannot form.
Meiotic recombination involves two phases: one that results in gene conversion without crossover, and one that results in crossover products.
a pair of parental alleles is detected in the meiotic products.
Site-specific Recombination: Bacteriophage lambda integration in E. coli
A site-specific recombination reaction (eg. catalyzed by Int of bacteriophage lambda)
FIGURE 15.38: Cre/lox system for gene knockouts
Adapted from H. Lodish, et al. Molecular Cell Biology, Fifth edition. W. H. Freeman & Company, 2003.
Duplication of the DNA sequence at a target site when a transposon is inserted
by a cointegrate intermediate.
FIGURE 16.13: Glycosylases remove bases
FIGURE 16.14: Base removal triggers excision repair
Direct Repair:Reversal of O6 methyl G to G by methyltransferase
Effect of DNA damage on replication: (i) coding lesions won’t interfere with replication but may produce mutation, (ii) non-coding lesions will interfere with replication and may lead to formation of daughter-strand gaps (DSG) or double-strand breaks (DSB).
DSG and DSB may be repaired by recombination process, to be discussed in the following section.
Model for nonhomologous end-joining