Meiosis I
Prophase
I
Prophase I is one
of the most important stages of meiosis.
During this stage, many crucial events occur. First the DNA of the
chromosomes begin to twist and condense, making the DNA visible to the
microscope. Secondly, each chromosome actively seeks out its homologous pair
(which also has a sister chromatid). Since all somatic cells are diploid in
number (2n), each chromatid has an identical pair that also replicates during
interphase. The two replicated homologous pairs find each other and form a
synapse. The structure formed is referred to as a tetrad (four chromatids) or
bivalent. The X-shaped point at which the two non-sister chromatids intertwine
is called a chiasma (pl: chiasmata) A process known as crossing-over (Xing
over) occurs at this point. This is where two non-sister chromatids exchange
genetic material.
In this phase, the
nuclear envelope will disassemble, the nucleolus will disappear and meiotic
microtubules will form the spindle apparatus.
Crossing-over and synapsis MUST occur in
prophase I. This phase is
so critical that it takes up about 90% of the entire process of meiosis.
Metaphase
I
At metaphase, each
chromosome has reached its maximum density. The homologous pairs prepare for
separation. They interact with spindle fibers that form from either side of the
nuclear envelope of the cell. Another difference from mitosis is that the
kinetochore microtubules connect to only one side of each chromosome. The
kinetochores facing the inside of the tetrad are unavailable. And this is a
good thing! During metaphase, the tetrads are lined by the spindle fibers at
equatorial plate.
Anaphase
I
Anaphase I pulls
apart the tetrad, separating each homologue from the other. It is by random
chance that a certain chromosome of any tetrad is pulled to a certain pole.
This agrees with Mendel’s 2nd law (independent assortment).
Telophase
I and cytokinesis
This last stage of
meiosis I varies from species to species. Sometimes Telophase I is skipped and
meiosis starts its second division immediately. In general, however, two
nuclear envelopes begin to surround the separate chromosomes and cytokinesis
(splitting of the cytoplasm into two separate entities) will sometimes occur.
Then a phase
called interkinesis – very important
to understand the difference between interphase and interkinesis - will follow,
which essentially is a resting period from Telophase I to Prophase II. This
differs from mitosis because DNA
replication does not occur.
Meiosis II
At the beginning of
meiosis II, there are two daughter cells that contain an N number of duplicated
chromosomes. Although the number and type of chromosomes is correct, there is
too much DNA. Meiosis II corrects the DNA dosage by separating the chromatids
and packaging them into two separate daughter cells. In this manner, meiosis II
is very similar to mitosis.
Prophase
II
No synapsis occurs
in prophase II because no homologous pairs exist. Each dyad (1/2 a tetrad) is
composed of a pair of sister chromatids and they are connected by a centromere.
In prophase II, the nuclear envelope will fragment if formed during telophase
I, and meiotic microtubules will form the spindle apparatus.
Metaphase
II
Each chromosome
will be connected to a kinetochore microtubule on each side (compare this to
metaphase I). The chromosomes will line up randomly on the equatorial plate
during metaphase II.
Anaphase
II
Anaphase II
separates the dyads into individual chromatids as the kinetochore microtubules
shorten. Each sister chromatid ends up on one side of the cell.
Telophase
II and cytokinesis
At the end of
Telophase II, the nuclear envelopes forms around each set of DNA and the
cytoplasm divides once again. As a result, four haploid cells have formed from
one diploid cell (or 2N from a tetraploid cell, and so on). Cytokinesis at the
end of Telophase II will follow the same strategy as it does in mitosis; i.e. a
cell plate will form daughter plant cells or a cleavage furrow will form
between daughter animal cells.
Comparison
between mitosis and meiosis in animal cells
see:
http://www.accessexcellence.org/AB/GG/comparison.html