MEIOSIS
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Two successive nuclear divisions occur, Meiosis I
(Reduction) and Meiosis II (Division).
Meiosis produces 4 haploid cells.
***Mitosis produces 2 diploid cells.
Meiosis I reduces 2n to n (reduction) while Meiosis II
divides the remaining set of chromosomes in a mitosislike process (division).
Most of the differences between the processes occur
during Meiosis I.
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Meiosis is a series of two nuclear divisions
called meiosis I and meiosis II.
These two divisions are each divided into
further phases:
-Prophase
Metaphase
 Anaphase
 Telophase
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Meiosis I
Meiosis I encompasses four stages:
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Prophase I
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Metaphase I
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Anaphase I
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Telophase I
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The stages are similar to the stages in
mitosis but the largest differences occur in
prophase I.
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In most cases, at the end of meiosis I, two
daughter cells are produced.
Meiosis II
Meiosis II also encompasses four stages:
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Prophase II
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Metaphase II
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Anaphase II
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Telophase II
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At the end of meiosis II, four daughter cells
are produced.
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Each of these resulting daughter cells is
haploid. (n)
Interphase
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Before a dividing cell enters meiosis, it
undergoes a period of growth called
Interphase.
G1 (first gap) phase: The cell increases in
mass in preparation for cell div.
S phase: during which DNA is synthesized.
G2 (second gap) phase: the cell synthesizes
proteins & continues to increase in size.
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In the latter part of interphase, the cell still
has nucleoli present.
Nucleus is bound by nuclear envelope and
the cell’s chromosome have duplicated but
are in the form of Chromatin.
2 pairs of centrioles formed from the
replication.
MEIOSIS-1
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the largest differences between mitosis and
meiosis occur in prophase I.
Prophase I is usually longer in duration
usually much more complex.
It can take days for prophase I to complete.
It is estimated that prophase I accounts for
some 85 - 95 percent of the total time for
meiosis.
PROPHASE-1
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Chromosomes condense and attach to the nuclear
envelope.
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Synapsis occurs (a pair of homologous chromosomes
lines up closely together) and a tetrad is formed. Each
tetrad is composed of four chromatids.
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Synapsis- the pairing of homologous chromosomes
during prophase I of meiosis.
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Crossing over may occur.
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Chromosomes thicken and detach from the nuclear
envelope.
Synapsis
PROPHASE-1
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Similar to mitosis, the centrioles migrate away
from one another and both the nuclear envelope
and nucleoli break down.
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Likewise, the chromosomes begin their migration
to the metaphase plate.
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Chiasma-The places where pairs of homologous
chromatids remain in contact during late prophase
to anaphase of the first meiotic division.
Metaphase I
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Metaphase I is of much shorter duration and
complexity when compared to prophase I.
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The following changes occur:
Tetrads align at the metaphase plate.
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Note that the centromeres of homologous
chromosomes are oriented toward the opposite cell
poles.
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Metaphase I
Metaphase I
Anaphase I
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Anaphase I is very similar to anaphase in mitosis.
The following changes occur:
Chromosomes move to the opposite cell poles.
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Unlike in mitosis, the homologous chromosomes
move to opposite poles yet the sister chromatids
remain together.
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Anaphase I
Anaphase I
Telophase I
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The following changes occur:
The spindles continue to move the homologous
chromosomes to the poles.
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Once movement is complete, each pole has a haploid
number of chromosomes.
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In most cases, cytokinesis occurs at the same time as
telophase I.
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At the end of telophase I and cytokinesis, two daughter
cells are produced, each with one half the number of
chromosomes of the original parent cell.
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Telophase I
Telophase I