MEIOSIS
Meiosis involves two
successive divisions of a
diploid (2N) eukaryotic
cell of a sexually
reproducing organism
that result in four haploid
(N) sex cells, each with
half of the genetic
material of the original
cell. Through the
mechanisms by which
paternal and maternal
chromosomes segregate,
and the process of
When does meiosis occur?
• Meiosis occurs in diploid cells. The chromosomes
duplicate once, and through two successive
divisions, four haploid cells are produced, each with
half the chromosome number of the parental cell.
• Meiosis occurs only in sexually reproducing
organisms. Depending on the organism, it may
produce haploid gametes, which do not divide
further but instead fuse to produce a diploid zygote;
or it may produce haploid spores, which divide by
mitotic cell cycles and produce unicellular or
multicellular organisms.
• In animals, where the somatic (body) cells are
diploid, the products of meiosis are the gametes.
• In many fungi and some algae, meiosis occurs
immediately after two haploid cells fuse, and mitosis
then produces a haploid multicellular "adult"
organism (e.g., filamentous fungi, algae) or haploid
unicellular organisms (e.g., yeast, unicellular algae).
• Plants and some algae have both haploid and
diploid multicellular stages. The multicellular diploid
stage is the sporophyte. Meiosis in a sporophyte
produces haploid spores. These spores alone are
capable of generating a haploid multicellular stage
called a gametophyte. The gametophyte produces
gametes by mitotic cell cycles.
The process of meiosis:
Meiosis consists of two successive nuclear
divisions, meiosis I and meiosis II. Each
division consists of these stages: prophase,
metaphase, anaphase, and telophase.
The chromosomes duplicate
prior to meiosis
• Prior to meiosis,
all chromosomes
are duplicated in a
process similar to
chromosome
duplication prior to
mitosis.
• Outside the nucleus of animal cells are two
centrosomes, each containing a pair of
centrioles. The two centrosomes are
produced by the duplication of a single
centrosome during premeiotic interphase.
The centrosomes serve as microtubule
organizing centers (MTOCs). Microtubules
extend radially from centrosomes, forming an
aster.
• Plant cells do not have centrosomes.
Different kinds of microtubule organizing
centers serve as sites of spindle formation
MEIOSIS I
Prophase I
Chromosomes
become visible,
crossing-over
occurs, the
nucleolus
disappears, the
meiotic spindle
forms, and the
nuclear
envelope
disappears.
• At the start of prophase I, the chromosomes have
already duplicated. During prophase I, they coil and
become shorter and thicker and visible under the
light microscope.
• The duplicated homologous chromosomes pair,
and crossing-over (the physical exchange of
chromosome parts) occurs. Crossing-over is the
process that can give rise to genetic recombination.
At this point, each homologous chromosome pair is
visible as a bivalent (tetrad), a tight grouping of two
chromosomes, each consisting of two sister
chromatids. The sites of crossing-over are seen as
crisscrossed nonsister chromatids and are called
chiasmata (singular: chiasma).
• The nucleolus disappears during prophase I.
• In the cytoplasm, the meiotic spindle,
consisting of microtubules and other proteins,
forms between the two pairs of centrioles as
they migrate to opposite poles of the cell.
• The nuclear envelope disappears at the
end of prophase I, allowing the spindle to
enter the nucleus.
• Prophase I is the longest phase of meiosis,
typically consuming 90% of the time for the
two divisions.
Metaphase I
The pairs of
chromosomes
(bivalents) become
arranged on the
metaphase plate and
are attached to the
now fully formed
meiotic spindle.
• The centrioles are at opposite poles of the cell.
• The pairs of homologous chromosomes (the
bivalents), now as tightly coiled and condensed as
they will be in meiosis, become arranged on a plane
equidistant from the poles called the metaphase
plate.
• Spindle fibers from one pole of the cell attach to
one chromosome of each pair (seen as sister
chromatids), and spindle fibers from the opposite
pole attach to the homologous chromosome (again,
seen as sister chromatids).
Anaphase I
The two
chromosomes in
each bivalent
separate and
migrate toward
opposite poles.
• Anaphase I begins when the two
chromosomes of each bivalent (tetrad)
separate and start moving toward opposite
poles of the cell as a result of the action of
the spindle.
• Notice that in anaphase I the sister
chromatids remain attached at their
centromeres and move together toward the
poles. A key difference between mitosis and
meiosis is that sister chromatids remain joined
after metaphase in meiosis I, whereas in
mitosis they separate.
Telophase I
The homologous
chromosome pairs
reach the poles of
the cell, nuclear
envelopes form
around them, and
cytokinesis follows
to produce two
cells.
• The homologous chromosome pairs complete
their migration to the two poles as a result of the
action of the spindle. Now a haploid set of
chromosomes is at each pole, with each
chromosome still having two chromatids.
• A nuclear envelope reforms around each
chromosome set, the spindle disappears, and
cytokinesis follows. In animal cells, cytokinesis
involves the formation of a cleavage furrow,
resulting in the pinching of the cell into two cells.
After cytokinesis, each of the two progeny cells
has a nucleus with a haploid set of replicated
chromosomes.
• Many cells that undergo rapid meiosis do
not decondense the chromosomes at the
end of telophase I. Other cells do exhibit
chromosome decondensation at this time;
the chromosomes recondense in prophase
II.
MEIOSIS II
Prophase II
Meiosis II begins
without any
further replication
of the
chromosomes. In
prophase II, the
nuclear
envelope breaks
down and the
spindle
apparatus forms.
• While chromosome duplication took place
prior to meiosis I, no new chromosome
replication occurs before meiosis II.
• The centrioles duplicate. This occurs by
separation of the two members of the pair,
and then the formation of a daughter
centriole perpendicular to each original
centriole. The two pairs of centrioles separate
into two centrosomes.
• The nuclear envelope breaks down, and
the spindle apparatus forms.
Metaphase II
The chromosomes become arranged on the
metaphase plate, much as the chromosomes
do in mitosis, and are attached to the now
fully formed spindle.
• Each of the daughter cells completes the
formation of a spindle apparatus.
• Single chromosomes align on the
metaphase plate, much as chromosomes do
in mitosis. This is in contrast to metaphase I, in
which homologous pairs of chromosomes
align on the metaphase plate.
• For each chromosome, the kinetochores of
the sister chromatids face the opposite poles,
and each is attached to a kinetochore
microtubule coming from that pole.
Anaphase II
The centromeres separate and the sister
chromatids—now individual chromosomes—
move toward the opposite poles of the cell.
• The centromeres separate, and the two
chromatids of each chromosome move to
opposite poles on the spindle. The separated
chromatids are now called chromosomes in
their own right.
Telophase II
A nuclear envelope forms around each set of
chromosomes and cytokinesis occurs,
producing four daughter cells, each with a
haploid set of chromosomes.
• A nuclear envelope forms around each set
of chromosomes.
• Cytokinesis takes place, producing four
daughter cells (gametes, in animals), each
with a haploid set of chromosomes.
• Because of crossing-over, some
chromosomes are seen to have recombined
segments of the original parental
chromosomes