Page 115 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.4 Meiosis: Cell Divisions That Halve Chromosome Number 107

process known as synapsis. The “zipper” itself is an elabo- each chromosome contains only a single functional kineto-
rate protein structure called the synaptonemal complex chore. During metaphase I (see Fig. 4.15, meiosis I), it is the
that aligns the homologs with remarkable precision, juxta- kinetochores of homologous chromosomes that attach to mi-
posing the corresponding genetic regions of the chromo- crotubules from opposite spindle poles. As a result, in chro-
some pair (Fig. 4.16b). mosomes aligned at the metaphase plate, the kinetochores of
Pachytene (from the Greek for thick or fat) begins at maternally and paternally derived chromosomes are subject
the completion of synapsis when homologous chromo- to pulling forces from opposite spindle poles, balanced by the
somes are united along their length. Each synapsed chromo- physical connections between homologs at chiasmata. Each
some pair is known as a bivalent (because it encompasses bivalent’s alignment and hookup is independent of that of
two chromosomes), or a tetrad (because it contains four every other bivalent, so the chromosomes facing each pole
chromatids). On one side of the bivalent is a maternally de- are a random mix of maternal and paternal origin.
rived chromosome, on the other side a paternally derived
one. Because X and Y chromosomes are not identical, they
do not synapse completely. However, the pseudoautosomal Anaphase I: Homologs move to
regions previously shown in Fig. 4.8 provide small stretches opposite spindle poles
of similarity between the X and the Y chromosomes that At the onset of anaphase I, the chiasmata joining homolo-
allow them to pair with each other during meiosis I in males. gous chromosomes dissolve, which allows the maternal
During pachytene, structures called recombination and paternal homologs to begin to move toward opposite
nodules begin to appear along the synaptonemal complex, spindle poles (see Fig. 4.15, meiosis I). Note that in ana-
and an exchange of parts between nonsister (that is, be- phase of the first meiotic division, the sister centromeres
tween maternal and paternal) chromatids occurs at these do not separate as they do in mitosis. Thus, from each ho-
nodules (see Fig. 4.16c for details). Such an exchange is mologous pair, one chromosome consisting of two sister
known as crossing-over; it results in the recombination of chromatids joined at their centromeres segregates to each
genetic material. As a result of crossing-over, chromatids spindle pole.
may no longer be of purely maternal or paternal origin; Recombination through crossing-over plays an impor-
however, no genetic information is gained or lost, so all tant role in the proper segregation of homologous chromo-
chromatids retain their original size. somes during the first meiotic division. The chiasmata hold
Diplotene (from the Greek for twofold or double) is the homologs together and thus ensure that their kineto-
signaled by the gradual dissolution of the synaptonemal chores remain attached to opposite spindle poles through-
zipper complex and a slight separation of regions of the out metaphase. When recombination does not occur within
homologous chromosomes (see Fig. 4.16d). The aligned a bivalent, mistakes in hookup and conveyance may cause
homologous chromosomes of each bivalent nonetheless re- homologous chromosomes to move to the same pole, in-
main very tightly merged at intervals along their length stead of segregating to opposite poles. In some organisms,
called chiasmata (singular, chiasma), which represent the however, proper segregation of nonrecombinant chromo-
sites where crossing-over occurred. somes nonetheless occurs through other pairing mecha-
Diakinesis (from the Greek for double movement) is nisms. Investigators do not yet completely understand the
accompanied by further condensation of the chromatids. nature of these processes, and they are currently evaluating
Because of this chromatid thickening and shortening, it can several models to explain them.
now clearly be seen that each tetrad consists of four sepa-
rate chromatids, or viewed in another way, that the two
homologous chromosomes of a bivalent are each composed Telophase I: Nuclear envelopes re-form
of two sister chromatids held together at their centromeres The telophase of the first meiotic division, or telophase I,
(see Fig. 4.16e). Nonsister chromatids that have undergone takes place when nuclear membranes begin to form around
crossing-over remain closely associated at chiasmata. The the chromosomes that have moved to the poles. Each of the
end of diakinesis is analogous to the prometaphase of mito- incipient daughter nuclei contains one-half the number of
sis: The nuclear envelope breaks down, and the microtu- chromosomes in the original parent nucleus, but each chro-
bules of the spindle apparatus begin to form. mosome consists of two sister chromatids joined at their
centromeres (see Fig. 4.15, meiosis I). Because the number
of chromosomes is reduced to one-half the normal diploid
Metaphase I: Paired homologs attach number, meiosis I is often called a reductional division.
to spindle fibers from opposite poles In most species, cytokinesis follows telophase I, with
During mitosis, each sister chromatid has a kinetochore daughter nuclei becoming enclosed in separate daughter
that becomes attached to microtubules emanating from op- cells. A short interphase then ensues. During this time, the
posite spindle poles. During meiosis I, the situation is dif- chromosomes usually decondense, in which case they must
ferent. The kinetochores of sister chromatids fuse, so that recondense during the prophase of the subsequent second