Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the number of chromosomes as the original parent cell. It involves two rounds of division and results in four daughter cells each with only one copy of each chromosome type. This ensures genetic variation between gametes and allows for recombination of genes from both parents during fertilization. Key events in meiosis include homologous chromosome pairing, crossing over, and independent assortment of homologs, which contribute to genetic diversity in offspring.

1. Meiosis
Advanced Animal Science

2. 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

3. Sperm
surrounding
an egg

4.  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.

5. 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

6. Homologous
Chromosomes
Paternal
(from Dad)
Maternal
(from Mom)
eye color
locus
eye color
locus
hair color
locus
hair color
locus

7. Prophase I
 Longest and most complex phase (90%).
 Chromosomes condense.
 Synapsis - a process: when hom. chrom.
come together, pair up, form a tetrad.

8. Prophase I - Synapsis
Nonsister chromatids
sister chromatids sister chromatids
Tetrad

9. Prophase I - Crossing Over
 Crossing over may
occur in the tetrad:
between nonsister
chromatids, ends
break and reattach

10. Crossing Over - Provides Variation
nonsister chromatids
Chiasma: site of
crossing over
variation
Tetrad

11. 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.

12. Anaphase I
 Homologous chromosomes separate
towards the poles (Tetrads separate)
 Sister chromatids remain attached

13. Telophase I
 Each pole now has haploid set of chromosomes
(however – still doubled).
 Cytokinesis occurs: two haploid daughter cells
formed.

14. Meiosis II
 No interphase II
( no more DNA replication)
 Remember: Meiosis II is similar to mitosis
Prophase II / Metaphase II
Anaphase II

15. 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

18. 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).

19. Spermatogenesis
2n = 46
human
germ cell in
testes
diploid (2n)
n=23
Still doubled
chromosomes
n=23
Still doubled
chromosomes
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II

20. Oogenesis
2n = 46
human germ cell
in ovary
diploid (2n)
n=23
Still doubled
n=23
Still doubled
meiosis I
23
23
23
Ovum
n=23
Polar
Bodies
meiosis II

21. 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)

22. 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

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

28. Karyotype… male

29. Karyotype… female