MEIOSIS:
-the process used by sexually reproducing
organisms to create cells responsible for producing
offspring.
-like does not beget like in meiosis!!
-cells have the haploid number of chromos
(23 in humans; n=23) (autosomes,
produced by mitosis, are identical diploid #
= 46 in humans; 2n = 46)
-since they have half the chromosomes they are
not identical to the parent cell
-unlike mitosis, meiosis happens only in cells
concerned with reproduction
-these cells are called the GAMETES (sperm and egg)
-the end result of meiosis are the sperm cells in
male organisms and egg cell in female organisms
http://scienceline.org/wp-content/uploads/2008/06/ashford_sperms_color.jpg
http://powerstates.com/wp-content/uploads/egg-sperm.jpg
Meiosis requires 2 nuclear
divisions (it goes through
prophase, metaphase,
anaphase and telophase 2
times)
Creates cells with a haploid
number of chromosomes.
Interphase in meiosis is
identical to interphase in
mitosis
Metaphase, anaphase and
telophase are also the
same.
Interphase
Prophase 1 with
crossing over and
tetrads
http://mtwow.org/meiosis.gif
Meiosis 2 with
no interphpase
to double
chromos
Gametes with
haploid
chromosome
number
Meiosis differs from mitosis in prophase I.
-2 pairs of sister chromatids will join at the
centromere forming a cluster of 4 sister chromatids
called a tetrad.
When these chromatids
lie close together, they
will exchange pieces of
chromosome (crossing
over).
This creates 4 unique
chromatids.
tetrad
Meiosis = 2 nuclear
divisions, mitosis =
one nuclear division.
Meiosis cells are all
genetically dissimilar,
mitosis cells are all
genetically identical.
Meiosis produces
haploid cells. Mitosis
produces diploid cells.
Meiosis has crossing
over. Mitosis has no
crossing over.
Meiosis = reproduction
Mitosis = growth, replacement and repair of worn out cells.
What is so important about meiosis???
-First, it creates genetically dissimilar chromatids
during crossing over..this allows for variability within
species. This can be the difference between survival
and extinction.
…weeds that can resist
herbicides and bacteria
that won’t respond to
Antibiotics are great
examples of this variability!!
http://www.niaid.nih.gov/SiteCollectionImages/topics/antimicrobialresistance/1
whatIsDrugResistance.gif
-Second, it ensures that the diploid chromosome
number in species stays constant during
reproduction by creating haploid cells that fertilize each
other.
23 + 23 = 46 vs. 46 + 46 = 92
-chromosome number is very important in determining species of
organism and function of that organism.
http://www.toonpool.com/user/997/files/chromosomes_kid_draw_on_wall_901815.jpg
Human Chromosomes before
being separated and placed in
formal karyotype form. Can
you count 23 pairs?
Normal human karyotype. Is
this a male or female?
www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm
homepages.uel.ac.uk/V.K.Sieber/human.htm
Sex
chromosomes
are located
on the 23rd
pair . XX is a
female and
XY is a male
Meiosis can also be a
determining factor in
chromosomal disorders.
If non-disjunction (or failure of
the sister chromatids to separate
during anaphase of either meiosis I or II) occurs, then
the resulting cell formed after fertilization will contain
more or less chromosomes than the normal diploid
number.
In some cases, this can result in abnormal conditions
for the organism.
Some examples of Chromosomal Disorders are:
-Down’s Syndrome—also called Trisomy 21
happens when non-disjunction occurs on the
21st pair of chromosomes resulting in 3
chromosomes being here.
-affects about 1/700 children born…most common
birth defect in the US.
www.ucl.ac.uk/%7Eucbhjow/bmsi/bmsi_7.htm
Non-disjunction can also occur in the sex chromosomes.
--Klinefelters syndrome affects 1/2,000 males
XXY results in males with female sex
characteristics
--Turners Syndrome affects 1/5,000 females
XO results in females who never develop
secondary sex characteristics
Turner’s Syndrome
Klinefelter’s Syndrome