1030
UNIT 5
Continuity
27
MEIOSIS I
MEIOSIS II
Prophase I
Prophase events occur, as in mitosis. Additionally, synapsis
occurs: Homologous chromosomes come together along their
length to form tetrads. During synapsis, the “arms” of
homologous chromatids wrap around each other, forming
several crossovers. The nonsister chromatids trade segments
at points of crossover. Crossover is followed through the
diagrams below.
Anaphase I
Unlike anaphase of mitosis, the centromeres do not separate
during anaphase I of meiosis, so the sister chromatids (dyads)
remain firmly attached. However, the homologous
chromosomes do separate from each other and the dyads
move toward opposite poles of the cell.
Telophase I
The nuclear membranes re-form around the chromosomal
masses, the spindle breaks down, and the chromatin
reappears as telophase and cytokinesis end. The 2 daughter
cells (now haploid) enter a second interphase-like period,
called interkinesis, before meiosis II occurs. There is no
second replication of DNA before meiosis II.
Meiosis II begins with the
products of meiosis I (2
haploid daughter cells) and
undergoes a mitosis-like
nuclear division process
referred to as the equational
division of meiosis.
Metaphase I
The tetrads align randomly on the spindle equator
in preparation for anaphase.
Prophase II
After progressing through the
phases of meiosis and
cytokinesis, the product is 4
haploid cells, each genetically
different from the original
mother cell. (During human
spermatogenesis, the daughter
cells remain interconnected by
cytoplasmic extensions during
the meiotic phases.)
Metaphase II
Anaphase II
Telophase II
and cytokinesis
Products of
meiosis:
haploid
daughter cells
Crossover
Interphase cell
Centriole
pairs
Nuclear
envelope
Chromatin
2
n
=
4
Spindle
Sister
chromatids
Nuclear envelope
fragments late
in prophase I
Tetrad
Dyad
Chromosomes
uncoil
Nuclear
envelopes
re-form
Cleavage
furrow
Centromere
Interphase events
As in mitosis, meiosis is preceded
by DNA replication and other
preparations for cell division.
Figure 27.7
Meiosis.
The series of
events in meiotic cell division for an
animal cell with a diploid number (2
n
)
of 4. The behavior of the chromo-
somes is emphasized.
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