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two cell divisions (I and II) to produce daughter cells with the haploid set of
chromosomes.
Meiosis I is the reductional division , in which the number of chromosomes is
reduced from 2n to n. Prophase of meiosis I can take a long time, and it has
been divided into substages ( Figure 3.4 ). During prophase I, the chromosomes
condense and become visible. Homologous chromosomes pair and become
closely associated along their length. Each homologous chromosome consists of
two sister chromatids joined at the centromere; thus, the pairing of homologous
chromosomes produces a four-stranded structure. During prophase I, the paired
chromosomes are able to exchange genetic information by crossing over ; this
crossing over results in a shuffling of the genetic information in the gametes.
The number of locations where genetic information was exchanged by crossing
over often is indicated by the formation of chiasmata . These chiasmata are vis-
ible under the microscope during prophase I. Chiasmata result from the physical
exchange of nucleotides between chromatids of the homologous chromosomes.
During metaphase I, the two homologous chromosomes are located on oppo-
site sides of the metaphase plate ( Figure 3.4 ). The orientation of each chromo-
some pair relative to the two poles is random and thus which member of each
pair of chromosomes (one set was originally derived from the mother and the
other set was originally derived from the father) will move to a particular pole
is random. This random alignment of chromosomes on the metaphase plate is
the basis of Mendel's Law of Independent Assortment . Thus, chromosomes orig-
inally derived from the individual's mother and father will end up assigned to
daughter cells in a random manner.
During anaphase I, the homologous chromosomes separate from each other
and move to opposite poles. This physical separation of homologous chromosomes
during anaphase I is the physical basis of Mendel's Law of Segregation . After ana-
phase I, a haploid set of chromosomes consisting of one homolog from each pair is
located near each pole of the spindle. During telophase I, the spindle breaks down
( Figure 3.4 ). Chromosomes may pass directly from telophase I to prophase II of
meiosis II. Alternatively, there may be a pause between the two meiotic divisions.
Chromosome duplication does not occur between meiosis I and II, however.
Meiosis II is similar to a mitotic division, with each daughter cell from meiosis
I being replicated, resulting in the production of four haploid cells ( Figure 3.4 ).
Meiosis II is different from mitosis, however, because the chromatids of a chro-
mosome usually are not identical along their entire length. This is because cross-
ing over could have occurred during prophase of meiosis I and resulted in an
exchange of genetic information between the chromatids.
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