Meiosis
Advanced Animal Science
Meiosis
 Type
of cell division
 One germ cell makes 4 gametes
with ½ the # of chrom.
 Occurs only in germ cells in gonads
○ testes /ovaries
 Occurs in flowers
○ ovary and anther
Sperm
surrounding
an egg

This shows how only one single sperm gets to penetrate the
egg, releasing its nucleus of 23 chromosomes to merge with
the nucleus of the egg and its 23 chromosomes.
Homologous Chromosomes

Pair of chrom. similar in shape , size, and types of genes.
 Each locus (location of the gene) in same position on chrom.

Humans have 23 pairs of homologues





Housefly – 6 prs
Chicken – 39 prs
Apple – 17 prs
Dog – 39 prs
Cat – 19 prs
This is a karyotype
(an image of an organism’s
chromosomes)
This is a karyotype of a
normal human male
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
(from Dad)
Maternal
(from Mom)
Prophase I
Longest and most complex phase (90%).
 Chromosomes condense.
 Synapsis - a process: when hom. chrom.
come together, pair up, form a tetrad.

Prophase I - Synapsis
Nonsister chromatids
sister chromatids
Tetrad
sister chromatids
Prophase I - Crossing Over

Crossing over may
occur in the tetrad:
between nonsister
chromatids, ends
break and reattach
Crossing Over - Provides Variation
nonsister chromatids
Chiasma: site of
crossing over
Tetrad
variation
Metaphase I
Shortest phase; paired homologues align.
 INDEPENDENT ASSORTMENT occurs

 pairs of homologues line up independently of other pairs’
orientation toward the poles -- random. Adds variation.
Anaphase I

Homologous chromosomes separate
towards the poles (Tetrads separate)

Sister chromatids remain attached
Telophase I
Each pole now has haploid set of chromosomes
(however – still doubled).
 Cytokinesis occurs: two haploid daughter cells
formed.

Meiosis II
No interphase II
( no more DNA replication)
 Remember: Meiosis II is similar to mitosis

Prophase II / Metaphase II
Anaphase II
Telophase II
Same as telophase in mitosis.
 Nuclei form.
 Cytokinesis occurs (2nd time).
 Four haploid daughter cells produced (chromosomes now
back to single condition).
gametes ~ sperm or egg; ovule or pollen grain

Gamete Formation in Animals

Diff. bet. male and female gametes.
Male: spermatogenesis
 all 4 develop into sperm cells.
Female: oogenesis
 cytokinesis in meiosis is uneven.
 most of cytoplasm goes into 1 of the 4 meiotic products
(forms large egg cell)
 3 other cells are small “polar bodies”, break down (extra
chrom. lost).
Spermatogenesis
n=23
human
germ cell in
testes
sperm
n=23
Still doubled
chromosomes
n=23
2n = 46
haploid (n)
n=23
n=23
diploid (2n)
Still doubled
chromosomes
n=23
meiosis I
meiosis II
Oogenesis
human germ cell
in ovary
23
n=23
23
Still doubled
2n = 46
23
n=23
Still doubled
diploid (2n)
Ovum
n=23
meiosis I
meiosis II
Polar
Bodies
Mitosis vs Meiosis

Mitosis
 Body (somatic cells)
 2 daughter cells made





(identical)
Each w/ same # & kind
of chrom. as parent cell
1 division process
1 cytokinesis
No synapsis or
crossing over
Are diploid (2n)

Meiosis
 Germ cells of gonads
 4 gamete cells made





(all different)
Each w/ ½ chrom. # as
parent cell
2 divisions
2 cytokineses events
Synapsis &
crossing over occurs
in Prophase 1
Are haploid (n)
Chromosome numbers:

However many
“types” of
chromosomes an
organism has, that
number is the “n”
number of
chromosomes it has.

Body cells
(somatic cells)
therefore have a
“2n” # (diploid)

Gametes have
“n” # (haploid)

For humans,
2n = 46
n = 23
Nondisjunction

When the tetrad (in Anaphase I) or
the sister chromatids (in Anaphase II)
do not separate, creating an abnormal #
of chrom. to occur in the gametes.

Lethal most of the time
Karyotype… male
Karyotype… female