At meiosis prophase first synapsis makes chromosome pairs. Pairs can interchange chromosome segments {recombination, DNA}| {homologous recombination} {crossing-over}.
process
Both double helices unwind. Enzyme splits homologous strands at same positions. Ends can reattach to other homologous strand or repair themselves. Both double helices rewind.
crossing over: one cut
Enzymes can split homologous nucleic acids at the same position. Ends can reattach so halves exchange. Left end is from one nucleic acid, and right end is from other nucleic acid. Left end is from other nucleic acid, and right end is from one nucleic acid.
crossing over: two cuts
Enzymes can split homologous nucleic acids at same two positions. Ends can reattach so middle section exchanges. Left end is from one nucleic acid, middle is from other nucleic acid, and right end is from one nucleic acid. Left end is from other nucleic acid, middle is from one nucleic acid, and right end is from other nucleic acid.
recombination
Recombination makes strands with different allele sequences. Because recombination mixes alleles, meiosis increases variation. Only homologous chromosomes have recombination, because only homologous chromosomes pair and because enzymes can split them at same place. Yeast has high recombination.
Gene-middle recombination inactivates genes. Gene-end recombination allows recombined genes to replace original genes {transplacement}. Transplacement can replace normal genes with inactive genes, so cell loses gene function or product {gene knockout}. For experiments, knockout mice can have gene deactivation.
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Date Modified: 2022.0225