XY Crossovers and Unequal Crossovers

The forty-six chromosomes of the human diploid genome are composed of twenty-two pairs of autosomes, plus the X and Y chromosomes that determine sex. The X and Y chromosomes are very different from each other in their genetic composition but nonetheless pair up and even cross over during meiosis. These two chromosomes do have similar sequences over a small portion of their length, termed the pseudoautosomal region, at the far end of the short arm on each one.

PROPOSED MECHANISM FOR CROSSING OVER

Double-stranded break

Nuclease creates single-stranded tails

One of the single-stranded tails inserts itself between the double helix of the nonsister chromatid "2" (red), separating its strands.

DNA synthesis fills both gaps (broken blue lines) using both strands of chromatid "2" (red) as the templates.

Strands are cut, swapped, and rejoined

Cut, swap, join

Cut, swap, join

1"

The exact mechanism for crossing over is unknown. It is believed to involve cutting of one chromatid, removal of nucleotides from both strands, insertion of one strand into the matching region of the nonsister chromatid, and further cutting, swapping of segments, and rejoining of the strands. Adapted from <http://www.ultranet .com/~jkimball/Biology Pages/c/crossingover .html>.

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