Meiosis
Be fruitful and increase in number…
Genesis 1: 22; 9:1; 35:11
Meiosis is a two-part cell division process
in organisms that sexually reproduce.
The outcome is the production of gametes
– sex cells.
Why is meiosis
necessary?
Meiosis has to take place
at some point in a sexual
reproductive cycle because
it halves the chromosome
number.
(Otherwise, every time
fertilization took place,
the chromosome number
would be doubled!)
46…92…184…368…736…& so on!!
During this phase of mitosis, the
nuclear membrane disintegrates.
The chromosomes become visible as
they shorten, coil, and thicken
(condense).
The spindle begins to extend outward
from the two centrioles/centrosomes,
which move to the opposite poles of
the cell.
Each chromosome is composed of
two sister chromatids containing
identical genetic information.
During most of meiosis (through to the
end of metaphase II) the sister
chromatids remain attached to each
other at the centromere.
Notice there
are two pair
of pairs!
equator
The chromosomes move towards the
equator/metaphase plate — halfway
between the two poles of the cell.
Next, the spindle fibres attach to the
centromeres of each chromosome
(made up of the two pairs of sister
chromatids).
This is a major difference between
meiosis and mitosis.
It causes the two members of each
chromosome pair to be separated
from each other during the next stage
of meiosis, anaphase I.
homologs
During this stage of meiosis, the cell
starts to lengthen. The two
homologs of each chromosome
pair separate and move toward
opposite poles, drawn by the
spindle.
This contrasts with mitosis, where
the sister chromatids of
each homolog separate and move
toward opposite poles.
Here, the chromatids remain
together as one complete,
replicated chromosome.
Cleavage furrow
At each pole, now, there is a
complete haploid set (half of the
original amount) of chromosomes
- but each chromosome still has
two sister chromatids.
A cleavage furrow appears and
by the end of this stage the
parent cell has divided into two
daughter cells.
This separation of the cytoplasm
is called cytokinesis.
These daughter cells begin
immediately to prepare for the
second meiotic division.
As in prophase I, the chromosomes
are condensed.
The centrioles/centrosomes have
replicated and are moving toward
the poles.
Again, the chromosomes are not
yet attached to the spindle, which
is growing outward from the
centrosomes.
In metaphase II , in each of the
two daughter cells produced by
the first meiotic division, the
spindle again draws
the chromosomes to the
equator/metaphase plate. This
time, unlike metaphase I, the
centromere will split in two - to
draw the sister chromatids
to opposite poles (as in mitotic
metaphase).
During anaphase II the sister
chromatids of each chromosome
separate and move toward opposite
poles.
Once they are no longer connected,
the former chromatids are called
chromosomes.
As the chromosomes are dragged
along by the spindle, their arms can
be seen dragging along behind so
that the chromosomes form V-shapes.
The poles themselves move further
apart as cytokinesis begins and the
cell lengthens (not shown in figure).
During telophase II, the
chromosomes reach opposite
poles, cytokinesis occurs, the two
cells, produced by meiosis I,
divide to form four
haploid daughter cells,
and nuclear membranes form.
Meiosis is now complete.
Mitosis
Meiosis
 process of cell division
 process of cell division
 2 Daughter cells
 4 daughter cells
 Daughter cells contain a
complete set of
chromosomes:
diploid - 46 for humans
 Daughter cells contain
half of the chromosomes
of the parent cell:
haploid – 23 for humans
 One set of phases
ipmat
 Two sets of phases
ipmat pmat
 Produces new body cells
 Produces sex cells
(gametes)
References
• http://biology.about.com/od/meiosis/ss/meiosis_diagrams.htm
• http://en.wikipedia.org/wiki/File:Meiosis_diagram.jpg
• http://www.macroevolution.net/metaphaseii.html#.T8BtQ7A7gl8
• http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBoo
kmeiosis.html