Question

During a single crossover event, how many strands of DNA must break? (Recall that DNA is double-stranded)

Answer 1

Homologous recombination is exchange of two DNA sequence in a region that is identical or with shared homology. Homologus recombination occurs during prophase 1 of meiosis. Crossing over is the exchange of genetic material between non sister chromatid in meiosis.Crossover in homologus chromosomes are initiated by double stranded breaks in the DNA. Crossovers are tightly regulated processes.

A single crossover event occurs when there is alignment of homologous chromosomes. There is exchange of genetic material between different chromatids during recombination at a single point. Both the strands of DNA have to be broken for single crossover events to occur. Double stranded breaks during meiosis will occur only after DNA replication is over. A DNA nuclease creates double stranded breaks during recombination. In mitosis, however, either single stranded or double stranded breaks are created. Double stranded break is common during homologus recombination in all species, whether single crossover or double crossover. In a single crossover event, there is breakage of 2 strands of DNA in each sister chromatid. Hence, there will be breakage of total 4 strands of DNA, as each sister chromatid has double stranded DNA (there are two sister chromatids involved in single crossover event).