Meiosis is a type of cell division that occurs in germ cells and produces gametes with half the normal number of chromosomes. It involves two rounds of division called Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and may exchange genetic material through crossing over. This provides genetic variation. Meiosis I reduces the chromosome number by half and separates homologous chromosomes. Meiosis II then separates sister chromatids to produce four haploid daughter cells, such as sperm or egg cells. Errors in chromosome separation during Meiosis can result in gametes with an abnormal number of chromosomes.

1. Meiosis
SUBMITTED BY: GROUP 2
SUBMITTED TO : MISS HINA NOOR
DEGREE NAME: BS. FORENSIC
SCIENCE

2. Meiosis
Type of cell division
 One germ cell makes 4 gametes
with ½ the number of chromosomes
 Divided in two phases:
 Meiosis 1
 Meiosis 2
 Occurs only in germ cells in gonads
○ testes /ovaries
 Occurs in flowers
○ ovary and anther

3. Homologous
Chromosomes
 Pair of chromosomes similar in shape , size, and types of
genes.
 Each locus (location of the gene) in same position on chromosomes
 Humans have 23 pairs of homologues chromosomes
 Housefly – 6 pairs
 Chicken – 39 pairs
 Apple – 17 pairs
 Dog – 39 pairs
 Cat – 19 pairs
This is a karyotype
(an image of an organism’s
chromosomes)
This is a karyotype of a
normal human male

4. Prophase I
 Longest and most complex phase (90%).
 Chromosomes condense during leptotene
 Synapsis - a process: when homologous
chromosomes
come together, pair up, form a tetrad during
zygotene

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

6. Prophase I - Crossing Over
 Crossing over may
occur in the tetrad:
between nonsister
chromatids, ends
break and reattach
during pachytene
 Pairs repel eachother
during diplotene

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

8. Metaphase I
 Shortest phase; paired homologues align.
 INDEPENDENT ASSORTMENT occurs
 pairs of homologues line up independently of other pairs’
orientation toward the poles

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

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

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

12. 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

15. 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

16. 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

17. ERRORS IN MEIOSIS
Nondisjunction
 When the tetrad (in Anaphase I) or
the sister chromatids (in Anaphase II)
do not separate, creating an abnormal
number of chromosomes to occur in the
gametes.
 Lethal most of the time