Cell Division for Reproduction
Meiosis Terms to Know:
 Meiosis: Cell division that produces gametes (sex cells/sperm &
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egg). These cells are used for sexual reproduction. They have half
the number of chromosomes as the parent cell.
Diploid Number: the number of chromosomes found in the
body cells of an organism. These chromosomes occur in pairs.
Haploid Number: the number of chromosomes found in the
gametes (sex cells). This is half the number in the body cells.
Homologous Chromosomes: “look” “alike” chromosomes.
Pairs of chromosomes in a diploid cell. They are the same size
and carry similar genetic information.
Zygote: a fertilized egg cell (when sperm fertilizes the egg, it
forms a zygote)
More Information about
Meiosis:
 The diploid number for humans is 46, for dogs it’s 78,
for onion plants it’s 16. Notice the pattern? That’s
right—they are all even numbers. That’s
because chromosomes occur in pairs in
diploid cells.
 “Di”=2; Diploid cells have chromosomes
in 2’s
 “Ha”= ½; Haploid cells have half the number of
chromosomes as a normal diploid body cell. What is
the haploid number for humans? For dogs?
Homologous Chromosomes
Homologous chromosomes
have genes for the same traits
at the same locations. One of
each pair comes from the
mother, the other comes from
the father.
This karyotype shows the chromosomes
of a human male.
“C” Words:
 Chromatin-Strands of
DNA in a cell that is
not dividing—
unwound and
indistinct.
 Chromosomes-Strands
of DNA in a cell that is
dividing—wound up
and distinct.
Chromatin is unwound and
indistinct in the nucleus of the cell.
More “C” Words:
 Chromatids—one of 2
identical strands of
DNA in a chromosome
(called sister
chromatids)
 Centromere—the
structure that holds
together the 2 sister
chromatids.
Histones-protein “beads” that keep the
strand of DNA from tangling up.
More “C” Words
 Centrosome/CentrioleStructure found in
animal cells that
anchors the spindle
fibers
 Cytokinesis—the
division of the
cytoplasm following
mitosis (the dividing of
the nucleus)
Interphase:
Cell Membrane
This is a
normal
diploid
body cell
located in
the testis
(of a male)
or the ovary
(of a
female).
Nuclear
Membrane
Chromatin
Centrioles
DNA Strands
have
replicated.
Prophase I
When the DNA coils up in
Prophase I, homologous
chromosomes move together and
become intertwined.
Centrioles migrate to the
poles; Spindle forms
Nuclear membrane
disappears
Synapsis and Crossing Over
Synapsis: The pairing together of homologous chromosomes during
Prophase I. (Note: This does NOT happen in mitosis!)
Crossing Over: During Synapsis, homologous chromosomes can
exchange pieces of DNA with each other. This leads to greater
genetic diversity.
Metaphase I
Homologous pairs
of chromosomes
move to the
equator of the
cell.
Each chromosome pair
is attached to a spindle
fiber.
Homologous pairs
of chromosomes
are still intertwined.
Anaphase I
Homologous pairs
of chromosomes
separate. One
chromosome from
each pair moves
to one pole, the
other moves to
the opposite pole.
NOTE: Chromosomes do
not split in half here. Each
chromosome still has 2
sister chromatids.
Note the effects of
crossing over from
Prophase I—each
chromatid is slightly
different from the
other.
Telophase I
In Telophase I, the
chromosomes unwind to
form chromatin again. New
nuclear membranes form
and the cell divides into
two.
After Telophase I, the cells
rest. This is called
Interkinesis. (NOTE: this
is not Interphase—DNA
strands do not replicate
here!)
Meiosis I Animation
 http://www.phschool.co
m/science/biology_place
/biocoach/meiosis/mei1a
ni.html
Prophase II
Prophase II is very similar to Prophase in
Mitosis except that it is occurring in both cells
at the same time.
Chromatin winds up to form distinct
chromosomes.
Nuclear membranes disappear.
Centrioles migrate to the poles.
Metaphase II
In both cells, the chromosomes line up at the
equator.
Anaphase II
In Anaphase II, the chromosomes split in half at the
centromeres and move to opposite poles.
This is just like what happens in Anaphase of Mitosis.
Telophase II
In Telophase II, the chromosomes unwind to form chromatin
again. New nuclear membranes form. The cells each divide
into two cells—making four haploid cells.
Meiosis II Animation
 http://www.phschool.co
m/science/biology_place
/biocoach/meiosis/mei2
ani.html
Genetic Variability
One way meiosis generates genetic variability is through the different
ways in which maternal and paternal chromosomes are combined in
the daughter cells. For humans, with 23 chromosome pairs, there are
over 8 million metaphase arrangements!
Spermatogenesis vs. Oogenesis
Mitosis
vs.
 Produces body cells
Meiosis
 Produces sex cells/gametes
 One division producing  Two divisions producing 4
2 daughter cells
 Cells are genetically
identical
 Produces diploid cells
 These cells make up the
body of the organism
 No Synapsis and No
Crossing Over
cells
 Cells are genetically
different from each other
 Produces haploid cells
 These cells are used for
sexual reproduction
 Synapsis and Crossing
Over occurs
Mitosis vs. Meiosis
Nondisjunction
 Nondisjunction is an
error that occurs when
chromosomes fail to
separate correctly in
Meiosis.
 The result is that one
gamete receives too
many of one kind of
chromosome, and
another receives none of
that chromosome.
Nondisjunction can occur in
Meiosis I or Meiosis II:
Karyotypes:
Trisomy 21: Down’s SyndromeCauses mental retardation, heart
defects, short stature, and
characteristic facial features.
Trisomy 13: Patau Syndrome—
Causes serious eye, brain and
circulatory defects.
Other Syndromes:
XXY—Klinefelter’s Syndrome:
Causes infertility, males show
some feminine characteristics.
XO—Turner’s Syndrome:
Causes sterile females who
possess sex organs that fail to
mature at puberty; generally
very short stature.