8.2 Oogenesis
175
for activating cellular processes within phase intervals with the “right” combination
of chemicals, which can be easily shifted by manipulating the signaling waves. We
will return to modern implementations of these ideas in Sect. 8.3.
8.2 Oogenesis
The starting point of the development is meiosis, the first division of a diploid zygote,
an inseminated cell containing a double set of chromosomes. Meiosis differs from
common division of somatic cells (mitosis) by proceeding in two stages required
to separate two sets of chromosomes, maternal and paternal. Paradoxically, this essential proliferation step is dubbed by the term derived from the Greek word for
decreasing, lessening, which refers to the halving of the number of chromosomes
in one of the two cell divisions. Like in mitosis, replicated chromosomes are separated into pairs of identical (save mutations) sister chromatids (Fig. 8.5a). Crossing
over may happen at this stage, with genetic material exchanging between parental
chromatids, as indicated by the color changes in the picture.
Separation is facilitated in animal cells by the centrosome consisting of two
centrioles. When the cell starts to divide, microtubules are nucleated at the centrioles,
and, as they elongate, push the centrioles apart, forming a mitotic spindle (Fig. 8.5b),
which extends to move the centrioles to opposite poles of the cell. Chromatids, set
free by a dissolving nuclear membrane, attach to the strings of the spindle, and are
pulled apart, concentrating in the two hemispheres. A cleavage develops between
them, and an actomyosin ring formed in the equatorial plane constricts to pull the
Fig. 8.5 (a) First stage of cell division. (b) Mitotic spindle. The centriole pair is shown in red;
variously colored lines are microtubules, chromatids are dark green. (c) Second stage of meiotic
division (Severson et al, 2016)
175
for activating cellular processes within phase intervals with the “right” combination
of chemicals, which can be easily shifted by manipulating the signaling waves. We
will return to modern implementations of these ideas in Sect. 8.3.
8.2 Oogenesis
The starting point of the development is meiosis, the first division of a diploid zygote,
an inseminated cell containing a double set of chromosomes. Meiosis differs from
common division of somatic cells (mitosis) by proceeding in two stages required
to separate two sets of chromosomes, maternal and paternal. Paradoxically, this essential proliferation step is dubbed by the term derived from the Greek word for
decreasing, lessening, which refers to the halving of the number of chromosomes
in one of the two cell divisions. Like in mitosis, replicated chromosomes are separated into pairs of identical (save mutations) sister chromatids (Fig. 8.5a). Crossing
over may happen at this stage, with genetic material exchanging between parental
chromatids, as indicated by the color changes in the picture.
Separation is facilitated in animal cells by the centrosome consisting of two
centrioles. When the cell starts to divide, microtubules are nucleated at the centrioles,
and, as they elongate, push the centrioles apart, forming a mitotic spindle (Fig. 8.5b),
which extends to move the centrioles to opposite poles of the cell. Chromatids, set
free by a dissolving nuclear membrane, attach to the strings of the spindle, and are
pulled apart, concentrating in the two hemispheres. A cleavage develops between
them, and an actomyosin ring formed in the equatorial plane constricts to pull the
Fig. 8.5 (a) First stage of cell division. (b) Mitotic spindle. The centriole pair is shown in red;
variously colored lines are microtubules, chromatids are dark green. (c) Second stage of meiotic
division (Severson et al, 2016)