Download FIRST ANAPHASE Our previous view of the first meiotic division

pp. 20-21
FIRST ANAPHASE
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Our previous view of the first meiotic division as a reduction division that separates two homologous
chromosomes into daughter cells must now be modified in accordance with the exchange of
chromosomal material manifested by chiasmata. That is, if no exchange occurs between homologous
chromosomes in meiosis, their separation is purely reductional because a chromosome in one daughter
cell does not contain any material from its homologue in the other cell (Fig. 2-12a). However, if genetic
exchange occurs, each separating dyad in anaphase carries part of its homologue, resulting in an equal
(equational division of the exchanged chromosome material to both daughter cells (Fig. 2-12b). If we
consider that each of homologous pair of chromosomes is contributed by a different parent, chiasma
exchange in meiosis will redistribute chromosome material from both parents to the daughter cells.
Gametes formed from such daughter cells may, therefore, carry a wide mixture of chromosome material,
part from one parent and part from another.
Figure 2-12
Distinction between reductional and equational
division for a specific section of chromosome
based on whether genetic exchange has or has not
taken place during the first meiotic division. (a)
reductional division two daughter cells differ in
the sense that one contains one member of a pair
of homologous chromosomes or specified section
of such a chromosome (outlined area) that the
other does not. (b) In equational division the two
daughter cells have the same content in respect to
a designated chromosome or chromosome section.
(Note that separation between the centromeres of
two homologous chromosomes is always expected
to be reductional in the first meiotic division in
present examples, because each centromere on a
homologue functions as a single unit during this
division, although there is evidence that it has
already replicated. In the second meiotic division
centromere distribution becomes equational since
the centromere finally splits and each of the two
second-division daughter cells receives a copy the
same parental centromere.)