3B Scientific Cell Division II Chart, Meiosis - Manual (Page 6)

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Meiosis
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site cell poles. A spindle apparatus has developed and the nuclear membrane (2) dissolves. The chromo-
somes align at the equator level, forming the so-called metaphase plate. Viewed from the top, the chromo-
somes have a star-like shape (monaster or “mother“ star). The kinetochores (3) are protein complexes which
already have developed at the centromeres. A particularity of meiotic metaphase I is that the kinetochores
of each sister chromatid pair seem to have merged. The microtubules (4) of the central spindle, which now
have attached themselves precisely to the kinetochores of each sister chromatid pair (5), therefore all point
in the same direction. The chiasma bridges (6) are still intact. They play an important part in the correct
line-up of the homologous chromosomes at the equator level.
The endoplasmic reticulum (7) and the Golgi complex (8) have now been almost completely dissolved.
7. Anaphase I
In anaphase I of meiosis, the homologous chromosomes (1) disjoin and not, as in mitosis, the sister chro-
matids. In this process the chiasma bridges are dissolved, which so far held together the maternal and
paternal chromosomes.
Some mutant organisms, where meiotic crossing over occurs only on a limited level, have chromosome
pairs without chiasma bridges. These pairs are usually not fully disjoined (nondisjunction) and the resul-
ting daughter cells have one chromosome too few or too many. Such malformations are referred to as
numerical chromosome aberrations, which cause deformities.
Disjunction begins at the kinetochores (2), the place where the traction fibres of the central spindle are
attached. From here, the chromosomes are pulled slowly towards the centrioles (4) located at the cell
poles, moving along the microtubules (3) which create a traction effect as they become shorter. The micro-
tubules (5) that are not connected to chromosomes now become longer, thus increasing the distance bet-
ween the centrioles and elongating the cell. At the equator level, the beginning stage of a cleavage furrow
(6) becomes visible.
The process of crossing over during the prophase and the random distribution of the maternal and pater-
nal chromosomes to the cell poles result in a variation of the genetic information (ref. to introduction)
8. Telophase I, Cytokinesis I, Interkinesis, Prophase II and Metaphase II
Telophase I and Cytokinesis
In telophase I, the spindle disintegrates and a ring constriction (1) develops at the equator level. In additi-
on, a thin nuclear membrane develops (2). During the following phase of cytokinesis, the cell body is divi-
ded exactly at the middle, at the ring constriction between the two new daughter nuclei (3). The daughter
nuclei each contain the maternal or paternal chromosome complement slightly varied through the process
of crossing over, with the DNA already present in duplicate, i.e. one chromosome consisting of two sister
chromatids (4).
The endoplasmic reticulum (5) and the Golgi complex (6) both have returned to their initial shape and size.
At the end of cytokinesis, the first meiotic division is completed.
Interkinesis
The first and second meiotic divisions are divided by a short resting period (interphase). However, there is
no duplication of the chromosomes consisting of two chromatids (no S phase). Both sister chromatids of
each chromosome remain connected by the centromeres (7).