This document summarizes the process of meiosis. It begins by defining meiosis as the type of cell division that occurs in sex cells and results in four daughter cells each with half the number of chromosomes as the parent cell. It then describes the two divisions of meiosis - meiosis I and meiosis II. The rest of the document delves into the specific stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II. It also explains how genetic recombination occurs through crossing over in prophase I. Finally, it provides overviews of spermatogenesis and oogenesis, the processes by which sperm and eggs are formed through meiosis in

1. Meiosis
By
Numan Sharif

2. Meiosis
Reductional division
• Takes place in Sex cells
• One parent cell divides into four daughter cells
• All daughter cells have haploid no of chromosomes
• Genetic continuity is maintained by meiosis
• It also ensures genetic variety

3. Sub division of Meiosis
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Liptotene
Zygotene
Pachitene
Diplotene
Diakinesis

4. Prophase I
Chromatins Start to thicken and coiled into chromosomes like Mitosis
Each chromosome is a double structure held together by Cohesin
Each homologous pair of chromosomes pair up, undergoing synapsis
Crossing over occurs between chromatids of synapsed homologs

5. Stages of Prophase I
Liptotene
The interphase chromatin material begins to condense
and become visible
Along each chromosome are chromomeres, begins to localize
condensationsthat resemble beads on a string
A process called homology search begins and pairing of homologs
begins during this phase

6. Stages of Prophase I
Zygotene
Chromosome continue to shorten and thicken
Homologous chromosomes undergo initial alignment
with one another
This so-called rough pairing is complete by the end of zygonema*
synaptonemal complex begins to form between the homologs
*Noun form of Zygotene

7. Stages of Prophase I
Zygotene (Continue…)
Synaptonemal complex begins to form between the homologs
This complex is believed to be the vehicle responsible
for the pairing of homologs
At the completion of zygotene the paired homologs
are referred as bivalents
The number of bivalents in each species is
equal to the haploid (n) number

8. Stages of Prophase I
Pachytene
• Chromosomes continue to coil and shorten
• Further development of the synaptonemal complex occurs between the two
members of each bivalent
• Compared to the rough-pairing characteristic of zygonema, homologs are now
separated by only 100 nm
• Each homolog is now evident as a double structure
• Each bivalent contains four member chromatids

9. Stages of Prophase I
• Pachytene
In mitosis, replicates are called sister chromatids,
whereas chromatids from maternal and paternal
members of a homologous pair are called nonsister chromatids
The four-membered structure, also referred to
as a tetrad, contains two pairs of sister chromatids

10. Stages of Prophase I
Diplotene
Each pair of sister chromatids begins to separate
Chiasma is a point where non-sister chromatids
exchange genetic information by a process called Crossing Over
Whereas physical exchange between chromosome areas
occurred during the previous pachytene stage
Why crossing over is important?
Crossing over is an important source of genetic variability
New combinations of genetic material are formed during this process

11. Stages of Prophase I
Diakinesis
Final stage of prophase I
The chromosomes pull farther apart, but nonsister chromatids remain
loosely associated at the chiasmata
As separation proceeds, the chiasmata move toward the ends of the tetrad
The process of Terminalization begins at late Diplotene and ends in Diakises
The nucleolus and nuclear envelope breaks down
The two centromeres of each tetrad attach to the recently formed spindle
fibers
At the completion of Prophase I, the centromeres of each tetrad structure
are present on the metaphase plate of the cell

13. Metaphase I
The chromosomes have maximally shortened and thickened
Each tetrad interacts with spindle fibers, facilitating its movement to
the metaphase plate
Half of the Tetrad (one of Dyad) will be pulled towards one pole and
other towards opposite pole

14. Anaphase I
Cohesin is degraded between sister chromatids, except at the
centromere region, which, as in mitosis, is protected by a shugoshin
complex
one-half of each tetrad (a dyad) is pulled toward each pole of
the dividing cell
Occasionally, errors in meiosis occur and separation is not achieved and
called Nondisjunction
If crossing over had not occurred in the first meiotic prophase, each
dyad at each pole would consist solely of either paternal or maternal
chromatids

15. Telophase I
Shorter as compared to mitotic Telophase
Nuclear membrane forming around the dyads
The nucleus next enters into a short interphase period
But Chromosomes will not replicate because they already consist of
two chromatids
In some organisms the cells go directly from anaphase I to meiosis II

17. Meiosis II
Also divided into 4 stages
Prophase II
Metaphase II
Anaphase II
Telophase II

18. Prophase II
Each dyad is composed of one pair of sister chromatids attached by
the common centromeric region
Metaphase II
the centromeres are positioned on the equatorial plate
When the shugoshin complex is degraded, the centromeres separate
Anaphase II
Anaphase II is initiated, and the sister chromatids of each dyad are
pulled to opposite poles. Because the number of dyads is equal to the
haploid number

19. Meiosis II
Telophase II
one member of each pair of homologous chromosomes present at
each pole
Each chromosome is now a monad
At the conclusion of meiosis II, not only the haploid state has been
achieved, but if crossing over has occurred, each monad contains a
combination of maternal and paternal genetic information
As a result, the offspring produced by any gamete will receive a
mixture of genetic information originally present in his or her
grandparents

20. Gametogenesis
Formation of Gametes (Sex Cells)
Two types
Male gametes Female Gametes
(Sperms) (Eggs/Ova)
Produced by Testes Produces by Ovaries
Egg
1n
Sperm
1n

21. Spermatogenesis
(Formation of Sperms)
This process takes place in Testes
The process starts with enlargement of a diploid germ cell called Spermatogonium
This cell grows to become a primary spermatocyte
Which undergoes 1st meiotic division and its product called Secondary
Spermatocyte
Secondary Spermatocyte contains haploid # of Dyads
Secondary Spermatocytes then undergoes Meiosis II and each cell produces two
Spermatids
Spermatids undergo a series of changes (Spermiogenesis) to become motile
Spermatozoa or Sperms
All Sperms contain haploid number of chromosomes and equal amount of
cytoplasm

22. Spermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
4 sperm cells are produced
from each primary
spermatocyte.
Primary Spermatocyte
Secondary Spermatocyte
Secondary Spermatocyte

23. Oogenesis
(Formation of Ova/egg)
This process occur in Ovaries
The daughter cells receive equal amount of Genetic material
But Unequal amount of Cytoplasm
During Anaphase I the Tetrads of the primary oocyte separates and the
dyads move towards opposite poles
During telophase I, the dyads at one pole are pinched off with very
little surrounding cytoplasm to form the first polar body
The 1st polar body may or may not divide
Other daughter cell of meiosis I contains most of the cytoplasm called
(Secondary Oocyte)

24. Oogenesis
During 2nd division the cytoplasm of secondary oocyte again divides
unequally to produce an ootid and a secondary polar body
The ootid then differentiates into the mature ovum

25. Oogenesis
*** The polar bodies die… only one ovum (egg) is
produced from each primary oocyte.