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Single crossovers in circular pieces of DNA do not seem to be a big topic, because if they happened, they would lead to a kind of combined chromosome with two inner strands and one large outer strand. (A bit like an 8)

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But do these single crossovers actually not occur, for example because the crossover machinery is physically not able to do it, or do we just not observe them because they will inevitably lead to cell death and/or the chromosome somehow breaking?

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this would be detected as DNA damage and normally be repaired I bet. The RNA polymerase running around the cross over region would ruin the nifty figure 8 shape for sure. –  shigeta Jun 27 '12 at 3:42
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1 Answer 1

Although crossover events can be observed in mitosis (mitotic recombination), they most frequently occur in prophase I of meiosis (crossing over in bivalents). Circular chromosomes are common in prokaryotes, but eukaryotes have linear chromosomes. Remember what meiosis accomplishes and that prokaryotes reproduce asexually. Prokaryotes don't have a need for -- or engage in -- meiotic division, so crossover events in circular chromosomes can really only be observed in instances of mitotic recombination, and I don't know the frequency of this in prokaryotes.

In prokaryote DNA replication, if any entangling of the two product circular chromosomes occurs, it is resolved by DNA topoisomerase. So perhaps a crossover would present a similar situation and the cell would resolve it by similar means.

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thanks for your answer! valid thought, though crossover does occur in prokaryotes frequently - think of plasmids. the phenomenon I'm wondering about is that crossover from plasmids always seems to be doubled, exchanging a fragment between the chromosome and plasmid, and never single :) –  Armatus Jan 19 '13 at 12:03
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good idea on topousomerase though! that might be –  Armatus Jan 19 '13 at 12:19
    
Changed the question to reflect this. –  Armatus Jan 24 '13 at 9:18
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