Do single crossovers occur in circular polynucleotides?

Question

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?

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.