# What exactly happens during the crossover [closed]

My knowledge of biology is rather limited, but I think I have a grasp of some basic concepts. For me (as a person close to math) a chromosome is a sequence of elements from the set {A,C,T,G} of some (arbitrary?) length. In humans, there are 23 pairs of chromosomes, and during meiosis each of these pairs produces a singe "mixed" chromosome.

Q1 In each pair, do chromosomes have to be of the same length? If yes, does it mean that this length is the same for all humans? Do chromosomes in different pairs have different lenghts?

Now, we have two chromosomes from the first pair and they get mixed during the meiosis.

Q2 In which exactly place do they get mixed?

• if chromosomes are same length, I expect they exchange pieces of the same length only as well; however, if they are of different length, do they exchange pieces of different lengths? e.g. suppose first chromosome in a pair has length 100, second is 105 (I do understand that in reality the numbers are much bigger). Can that happen that the new chromosome is created using the first 10 elements of the first, and the last 80 of the second (so the first 10 of the first replace the first 25 of the second), with the total length of 90?

• how many exchanges may happen in a single meiosis, namely can that happen that chromosomes exchange several separated pieces from the middle?

I guess, that this process may not be understood completely yet, so I'm rather looking for an answer either "No, it can't happen at all, we know that for sure", or "this we don't know yet, perhaps it may happen". Thanks.

Q1 In each pair, do chromosomes have to be of the same length?

Yes, approximatively.

If yes, does it mean that this length is the same for all humans?

Yes, approximatively.

Do chromosomes in different pairs have different lenghts?

Yes. E.g., in humans, chromosome 1 is 294 millions nucleotides long while the chromosome 22 is only 50 millions nucleotide long.

Q2 In which exactly place do they get mixed?

These crossover can appear pretty much anywhere. The probability of a crossover to happen at a given locus (locus=location on a chromosome) is not a constant throughout the chromosome though. In any case, the crossover always (almost always) happen at the same locus for each pair of the chromosome. In other words, if ~3 millions nucleotides goes from the paternally inherited to the maternally inherited chromosome, then ~3 millions nucleotides goes from the maternally inherited to the paternally inherited chromosome.

• A good answer until the last paragraph. "the rate of crossover is limited to exactly 1 per chromosome" is wrong: the number per chromosome averages the same as the length in Morgans of the chromosome (higher in females), so more for chr. 1 than chr. 22. "the crossover happens at the location at which spindle attach during meiosis" is wrong: spindle attaches to the centromere, not the crossing-over chiasma. Jul 11 '18 at 18:59
• Yes, you're right. Thank you. I deleted this last paragraph.... but then I don't fully answer the question anymore... Jul 11 '18 at 22:05
• Thanks for the answer, pretty much comprehensive. The last paragraph is confusing though: you're saying that the crossover can happen at any location of the sequence (with non-uniform probability, fine), but "in other words" you're saying about the length of the exchanged sequences - those two facts are important, but to me they are quite different. Could you elaborate? Also, rephrasing the last part of my question was: during a single meiosis, can crossover happen at two separate locuses?
– Ilya
Jul 11 '18 at 22:25
• @Ilya, the process can be thought of as having the two chromosomes laid out parallel to one another and aligned, at a random locus (same in both) both are cut and the loose ends are reattached to the other chromosome. Hence the pieces exchanged are (approximately) the same size. And yes, multiple crossovers can occur on a chromosome in one meiosis. The longest chr., Chr. #1, averages 4 and a quarter crossovers in a human female meiosis (males average 2.9 crossovers on Chr. #1). Jul 12 '18 at 5:04