MEIOSIS
Meiosis
• The form of cell division by which gametes,
with half the number of chromosomes, are
produced.
• Diploid (2n)

haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Meiosis
• Sex cells divide to produce gametes (sperm
or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences.
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
• Centriole pairs also replicate.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling
the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes
Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
21 trisomy – Downs
Syndrome
Can you see the
extra 21st
chromosome?
Is this person
male or female?
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the
“Sex
Chromosomes”
are the 23rd set
XX chromosome - female
XY chromosome - male
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Chiasmata (chiasma) are the sites of crossing
over.
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
Sex Chromosomes
XX chromosome - female
XY chromosome - male
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Question:
• In terms of Independent Assortment how many different combinations of
sperm could a human male produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.
Anaphase I
Telophase I
• Each pole now has haploid set of
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.
Telophase I
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase plate
metaphase plate
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Meiosis
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
n=2
diploid (2n)
n=2
n=2
meiosis I
meiosis II
Variation
• Important to population as the raw
material for natural selection.
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Question:
• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes
Sources
• www.ursulinehs.org/powerpoint/meiosi
s.ppt
• www.biology4teachers.com/Cell%20Divi
sion/MEIOSIS.ppt
• http://www.iteachbio.com/Life%20Scien
ce/LifeFunctionsandTheCell/Meiosis.ppt