Crossovers and Chiasmata
Crossovers
occur
at
diplotene of Meiosis I,
when each chromosome has replicated but remains attached
to a single
centromere: chromatids 1 & 2,
and 3 & 4,
are exact replicated copies of each other, and are
attached to the same
centromere. Crossovers can occur between either strand of
one
chromosome and either strand of the other: here between 1 & 3, and 2 & 3. (Other
possibilities are 1
& 4, or 2
& 4). Crossovers do not occur between chromatids of
the same chromosome
(between 1 & 2,
or 3
& 4). A crossover is seen cytologically as a
physical chiasma (pl.,
chiasmata),
an
X-shaped linkage
between
chromatids.