

Mitosis= reproduction of body cells
(SOMATIC CELLS)
Meiosis= reproduction of sex cells
(sperm and eggs)
 Sex Cells (GAMETES) = reproductive cells
produced in sex organs (testes/ovaries)
 Reduces chromosome number from DIPLOID (2n)
down to HAPLOID (n)
46
Body Cell

Why do sex cells have only half the genetic
information of body cells?
46
46
92!
MITOSIS






Body Cells
One Division
Create 2 identical cells
Copies have the exact
same number of
chromosomes
46 chromosomes in every
body cell
Paired chromosomes
MEIOSIS
Sex Cells (egg and sperm)
 Two Divisions
 Creates 4 cells
 Copies has half the number
of chromosomes
 23 chromosomes in a
sperm or egg cell
 NOT in pairs


Meiosis Overview Video
Chromosomes
Chromatids
• All male sperm is genetically different from one another
• All female eggs are genetically different from one another
• When they fuse in fertilization, the chromatids combine to
make a new individual!
HOMOLOGOUS (“same”)
pairs of chromosomes =
same sized chromosomes
that carry the same genes

PROPHASE I
Chromosomes coil up and become
visible
 Nuclear membrane and the
nucleolus disappear
 Spindle fibers form
 Centrioles begin to move to
opposite sides
 HOMOLOGOUS CHROMOSOMES
pair up and CROSSING OVER
occurs.

CROSSING OVER



Chromosomal material is
exchanged between sister
chromatids
Allows for genetic variation!
Then, paired chromosomes
line up in the center for
metaphase I
ANAPHASE I
Homologous chromosomes
split and move to opposite
ends
This is where the chromosome
number is divided in half
(2n  n)
TELOPHASE I
Cleavage furrow forms
 Cytokinesis occurs
 Nuclear membrane DOES
NOT reform
 Chromosomes DO NOT go
back to chromatin
 2 daughter cells have been
created with the haploid
number of chromosomes
 Meiosis I Animation








Centrioles move apart
Spindle forms between centrioles
Sister chromatids line up in the
middle of cell
Fibers pull sister chromatids apart
Nuclear membrane and nucleolus
reform
Cell membrane pinches in and
divides in the cytoplasm
End Result: 4 sex cells
(different than what started out with!!)

Keeps the number of
chromosomes the same from
one generation to the next
 Human sperm cell + Human egg
cell= zygote
▪ 23 chromosomes + 23 Chromosomes=
46 chromosomes

Provides genetic variation
 Why you don’t look exactly like your
siblings



http://www.cellsalive.com/meiosis.htm
http://www.youtube.com/watch?v=D1_mQS_FZ0&feature=related
http://www.youtube.com/watch?v=MyXvx5F
kfTE&feature=related

Can you find the
homologous pairs?

How many
homologous pairs
do you see?

Males – Meiosis is called
SPERMATOGENESIS
 4 equally sized sperm
produced
 Cytoplasm divides evenly
 Ongoing in testes throughout
a male’s life
 Human males produce
approximately 1000 sperm per
second (30 billion/year).

Females – Meiosis is called
Oogenesis
 4 cells, but only 1 becomes
an egg
 Uneven cytokinesis – 1 cell
receives most of the
cytoplasm = EGG (needs
extra cytoplasm to
accommodate fertilization)
 Other 3 cells receive very
little cytoplasm and die off
= POLAR BODIES
 Meiosis in human females begins before person is born
but stops in prophase I and does not resume until after
puberty.
 Each month, approximately 1000 primary oocytes will
mature but most will die.
 Ovulation occurs approximately once every 28 days.
Females ovulate approximately 400 times during their
lifetime
 Women are born with all of the primary oocytes that
they will ever have (2 million). At puberty, there are
approximately 400,000 left.


Compare Mitosis and Meiosis
http://www.pbs.org/wgbh/nova/miracle/divi_fla
sh.html