Meiosis
Occurs in two major divisions:
• Meiosis I
• Meiosis II
Each of these divisions occurs in 4 stages:
• Prophase
• Metaphase
• Anaphase
• Telophase
1
 Meiosis is a special type
of cell division that
occurs in the gonads.
 Its purpose is to create
reproductive cells known
as gametes.
 These gametes are
haploid as they only
carry one copy of each
type of chromosome.
 Like mitosis, the gonad cells go through
Interphase prior to meiosis.
 This means that the chromosome number
prior to meiosis doubles.
 As with mitosis, meiosis only deals with the
separation of the nuclear material, whereas
cytokinesis is still needed to split up the
organelles and cytoplasm.
 Unlike mitosis, meiosis produces 4 non-identical
haploid cells.
 Their purpose is for reproduction only.
 Also meiosis has the same stages as mitosis,
however they occur twice in order to reduce the
chromosome number from diploid to haploid
(reduction division).
The first half of meiosis
(Reduction Division)
MEIOSIS I
Prophase I
 Nuclear membrane dissolves
 Chromosomes are attached to their copy by a
centromere.
 Centrioles appear and move to the poles.
 All the chromosomes of the same
chromosome number (i.e. all chromosome #1
and their copies), known as homologous
chromosomes, join together to form a tetrad
or bundle of four chromosomes (2 from each
parent).
 Homologous
chromosomes pair up
– this is called synapsis
 Once paired together,
the non-sister
chromatids exchange
segments – this is
called crossing over
 This is one of two
reasons for the
UNIQUENESS of the
offspring!
Crossing Over
Metaphase I
 The tetrads align themselves at the equator with
the maternal pair of chromosomes facing one
pole and the paternal chromosome pair facing
the other pole.
 All tetrads align independent of one another.
 This is called independent assortment and is the
other reason for the variation seen in offspring.
 Once aligned, the tetrads are held in place by
spindle fibres released from the centrioles at the
poles.
 The spindle fibres attach to the centromeres of
the double stranded chromosome (where some
are different now!).
Anaphase I
 The spindle fibres contract and
pull apart the tetrad such that one
pair of sister chromatids goes to
one pole and the other pair goes
to the other pole.
 the centromere remains INTACT ;
It still holds the chromosome
copies (sister chromatids)
together.
Telophase I and Cytokinesis
 A nuclear membrane reforms around each new
nucleus and the cytoplasm is divided by
cytokinesis creating 2 non-identical cells.
 These two cells immediately proceed into the
next round of meiosis as there is no second round
of interphase.
 In some cell types, telophase I does not occur and
the cell proceeds directly to the second round of
meiosis.
The second half of meiosis
Meiosis II
 The stages of meiosis II are all exactly identical to
the happenings from mitosis.
 The only difference is that they are occurring in two
cells at the same time.
 The end product is four, non-identical, haploid cells.
 The process of meiosis occurs somewhat differently
in males and females.
Prophase II
 Nuclear membrane dissolves
 Double stranded chromosomes
appear
 Centrioles move to opp poles
Metaphase II
 Chromosomes line up along
equator
 Spindle fibers attach to
centromeres
Anaphase II
 The spindle fibres contract,
breaking the centromere, and
pull apart the sister
chromatids.
 Single stranded chrosomes
form “v” and move
Telophase II &
cytokinesis
 Nuclear membrane reforms
 Cleavage furrow starts to form.
 The end product is four, non-
identical, haploid cells.
Homework
 Page 174 #1, 2, 10
Meiosis Video
http://highered.mcgraw-hill.com/sites/dl/free/0072495855/291136/meiosis.swf