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MEIOSIS
Meiosis
 A type of nuclear division
 Meiosis is also called reduction division because it
results in the production of daughter cells with only *1
set of chromosomes (the parent cells has 2 sets).
Meiosis - Reduction Division
 Eukaryotic cells are diploid (have 2n chromosomes).
 Mitosis (asexual reproduction) produces diploid
daughter cells.
 Sexual reproduction involves the fusion of 2 gametes =
fertilization, to produce a zygote. If the gametes each
have 2n chromosomes then the zygote would have ?
Meiosis – Reduction Division
 Preservation of karyotype (number and physical
properties of chromosomes) is essential to survival of
organism.
 All of our cells are diploid, 2n chromosomes. Those
not destined to become gametes are called somatic
(body) cells, those destined to produce gametes are
called germ cells.
Meiosis
 Meiosis is similar to mitosis but differs in 4 ways:
 Meiosis has 1 round of duplication of DNA; 2 rounds of
nuclear division = Meiosis I and II.
 Homologous chromosomes are paired through
metaphase of meiosis I. They are physically joined =
synapsis.
Meiosis
 Differences from mitosis cont’d
 Homologous chromosomes exchange genetic material
(some of father’s DNA is exchanged with mother’s
DNA). This is called crossing over.
 Crossing over can result in recombination = production
of unique chromosomes, unlike those from the mother
or father.
Chromosome Replication, Synapsis, and
Crossing Over
A CLOSER LOOK AT THREE KEY EVENTS IN MEIOSIS
Centromere
Sister chromatids
Chromosomes
One homolog
Synaptonemal
complex
1. Replication,
during interphase.
Sister chromatids are held together by
proteins along the chromosome “arms”
and at the centromere. Shown: early
prophase of meiosis I, when chromosomes
have condensed.
2. Synapsis,
during prophase I.
Homologous chromosome are held
together by proteins in the synaptonemal
complex.
Second homolog
Non-sister
chromatids
during prophase I.
3. Crossing over,
Complex of proteins forms where
crossing over will occur. Chromosome
segments are swapped between
non-sister chromatids.
Protein complex
Crossing over usually occurs at least
once in each non-sister chromatid,
but is only shown on 1 pair here
Meiosis
 Sequence of Events.
 Chromosomes are duplicated – S phase
(Interphase)
 Prophase I



Homologous chromosomes pair and are physically
joined (synapsis) until Anaphase I.
*Crossing over can occur only in this phase.
Nuclear membrane disappears, spindle forms.
Meiosis
 Sequence of events cont’d
 Metaphase I


Homologous chromosome pairs line up at center of
spindle randomly.
Microtubules of the spindle attach to kinetochores.
 Anaphase I – homologous chromosome pairs
split and migrate to opposite poles of spindle.***
 Telophase I – spindle disappears and nuclear
membranes reappear.
 Cytokinesis – separation into 2 cells, may occur.
The Phases of Meiosis
PRIOR TO MEIOSIS
MEIOSIS I
Chromosomes replicate,
forming sister chromatids.
Homologous chromosomes separate.
Tetrad (4 chromatids from
homologous chromosomes)
Nuclear Chromatin
envelope
Non-sister
chromatids
Spindle apparatus
Chiasma
1. Interphase:
2. Early Prophase I:
3. Late Prophase I:
Chromosomes replicate
in parent cell, in
uncondensed state.
Chromosomes condense,
nuclear envelope breaks up,
spindle apparatus forms.
Synapsis of homologous
chromosomes.
Crossing over of
non-sister chromatids
(often multiple crossovers between the
same chromatids).
The Phases of Meiosis
MEIOSIS I
Homologous chromosomes separate.
4. Metaphase I:
5. Anaphase I:
Tetrads migrate to
metaphase plate.
Homologs separate
and begin moving to
opposite sides of cell.
6. Telophase I and
Cytokinesis:
Chromosomes move to
opposite sides of cell,
then cell divides.
 Sequence of events cont’d
 Brief interphase with no DNA duplication.
 Prophase II: nuclear membranes disappear,
spindles form in each cell.
 Metaphase II: chromosomes line up on spindles;
sister chromatids attached to opposite poles of
spindle.
 Anaphase II: sister chromatids split and move
to opposite poles of spindle.
 Telophase II: nuclear membranes reappear.
 Cytokinesis results in 4 haploid cells, with “n” number of
chromosomes.
The Phases of Meiosis: Meiosis II
MEIOSIS II
Sister chromatids separate.
7. Prophase II:
8. Metaphase II:
9. Anaphase II:
Spindle apparatus
forms.
Chromosomes line
up at middle of cell
(metaphase plate).
Sister chromatids
separate, begin
moving to opposite
sides of cell.
10. Telophase II and
Cytokinesis:
Chromosomes move to
opposite sides of cell,
then cell divides.
Meiosis KM
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Meiosis KM
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Meiosis KM
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