Does crossing over always occur in meiosis?

Is it inevitable that crossing over will occur no matter what in prophase 1 of meiosis? Can I assume it will definitely happen? Also my teacher said that crossing over can occur twice on the same pair of homologous chromosomes, and that this can happen at the same spot where it happened the first time, meaning that the final result is no net crossing over. Is this true? Also why is the highest possible recombination frequency 50%?

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  • 8 years ago
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    Crossing over is not rare, though it varies from species to species. In humans, it occurs on average once per chromosome arm per meiotic division. In other words, during one meiosis, you'll get around 46 crossovers in a human stem cell. But nonrecombinant chromosome pairs - those that don't undergo recombination - are not rare. They happen all the time. In fact, my PhD project is studying exactly how that works.

    To your second question, it is technically true, but very uncommon. Far more common is for a pair of chromosomes to start undergoing recombination, and then back out and go back to being separate. Also, depending on how the Holliday junction is resolved, only a tiny bit of DNA may actually be transferred from one chromosome to another.

    Recombination frequency is a measure of how often you see two pieces of DNA go to different spindle poles. If two pieces are completely independent, they assort randomly. Because there are two poles, half the time they go together and half the time they separate. Thus, 50% recombination frequency. If you have two pieces of DNA on the same chromosome that have a 50% recombination rate, that means that they're so far apart from each other that basically just about every time a cell goes through meiosis, there will be recombination between them.

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  • 8 years ago

    No, crossing over doesn't always happen. It's actually sort of rare. Crossing over can occur more than once on the same homologues, however, it is extremely unlikely to occur in the exact same spot. So it is likely that the net crossing over would not be zero.

    For some reason recombination frequencies are escaping me now. But I'm pretty sure it has something to do with the fact that you're working with two chromosomes.

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  • Anonymous
    4 years ago

    convinced, there is continually crossing over in meiosis, this is in prophase one this is to make certain genetic variety edit* to the man who suggested it doesn't continually happen, that could want to probly be an exception

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