This is about Meiosis step in cell cycle

1. SUBMITTED BY:
Soma
SUBMITTING TO:
Dr. VANITA CHANDEL

2. INTRODUCTION
• It is a type of cell division that reduce the number of chromosomes in the parent
cell by half and produce four gamete cells.
• It is also known as reductional division.
• 1st discovered by german biologist Oscar Hertwig (1876) in egg of Sea Urchin.
• In 1883 at the level of chromosome by Edouard Van Benden in Ascaris (round
worm) egg.
• Farmer & Moore gave the term of Meiosis.
• Meiocytes are the cells which undergo meiotic division.

3. • Meiosis is like mitosis but more complex. There are
two rounds of cell division:
1. Meiosis I
2. Meiosis II
INTERPHASE MEIOSIS
MEIOSIS I
(REDUCTIONAL DIVISION)
MEIOSIS II
(EQUATIONAL DIVISION)

4. MEIOSIS I MEIOSIS II
A. Karyokinesis I
1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I
B. Cytokinesis I
A. Karyokinesis II
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II
B. Cytokinesis II
Prophase I:
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

5. 2n
n
n
n
n
n
n
M I
M II

6. • Meiosis I
1. Prophase I
• It is the most complicated & longest phase of Meosis I.
• Formation of spindle fibers.
• Crossing over and exchange of genetic material occur.
• Five sub stages: Leptotene, Zygotene, Patchytene, Diplotene & Diakinesis.
• Leptotene: Fibers are attached to nuclear membrane, chromatin network condense, each
chromosome have 2 sister chromatids but appear as single chromosome.
• Zygotene: Pairing of homologous chromosome: synapsis. the pairs at this stage called
bivalent.

7. • Patchytene:
1. Chromosomes become short & thick,
2. Bivalent appears to be composed of 4 chromatids (tetrad).
3. Crossing over takes place and formation of chiasmata.
• Diplotene:
1. Homologus chromosomes start repelling each other for separation (but
still sttached to each other).
2. Nuclear membrane starting to disappear.
• Diakinesis:
1. Chromosome continue to shorten & thicken.
2. The nucleolus & nuclear membrane completely disappear.

8. 2. Metaphase I:
• Nucear membrane disappear
• Chromosomes are free, in cytoplasm
• Spindle fibers are fully formed
• Each pair of chromosomes moves to equator
• They are attached to spinle fibers.
3. Anaphase I:
• Homologus chromosomes are separated.
• They migrate to opposite poles with
centromere leading.
• Spindle fibers shorten & thus chromosomes
pulled.
• Sister chromatids Do not separate.
4. Telophase I:
• Chromosomes are densly packed
together.
• Nuclear membrane formed around a
set of chromosome.
• Finally cell divided into 2.

10. • Prophase II
1. Chromosomes become shorter
and thicker.
2. New spindle fibers start forming.
• Metaphase II
1. Chromosomes moves towards equator of cell.
2. Spindle fibers attached to centromere.
3. They orient themselves to the opposite poles.
Meiosis II

11. • Telophase II & Cytokinesis
1. Spinde fibers disappear.
2. Nuclear membrane formed around a
set of chromosome.
3. Chromatids become chromosome of
daughter cells.
4. They uncoil & regain their thread
lke structure.
5. Cytoplasm divided across the midle.
• Anaphase II
1. Sister chromatids separates.
2. Both migrates towards to opposite
poles.
3. Spindle fibers still attached.

12. Result:
Four daughter cells
having haploid number of
chromosomes.

13. DNA count & Chromosomes number in different stages of Meiosis
Meiosis I Chromosomes
(n)
Chromatids
(DNA=C)
Prophase I 46 (2n) 92 (4c)
Metaphase I 46 (2n) 92 (4c)
Anaphase I 46 (2n) 92 (4c)
Telophase I 46 (2n) 92 (4c)
Daughter
cells
23 (n) 46 (2c)
1n= 23, 1c=23
Meiosis II Chromosomes
(n)
Chromatids
(DNA=C)
Prophase II 23 (n) 46 (2c)
Metaphase II 23 (n) 46 (2c)
Anaphase II 46 (2n) 46 (2c)
Telophase II 46 (2n) 46 (2c)
Daughter
cells
23 (n) 23 (c)

15. Significance of meiosis:
• Generation of genetic diversity
• Halving of chromosome numbers
• Crossing over
• In sexual reproduction
• Independent assortment

16. Reference
• National Human Genomic Research Institute
https://www.genome.gov/genetics-
glossary/Meiosis#:~:text=Meiosis%20is%20a%20type%20of,(one%20from%20each%
20parent).
• Editorial: Meiosis: From Molecular Basis to Medicine
By:Wei Li1,2*, Liangran Zhang3*, Akira Shinohara4* and Scott Keeney5*
https://www.frontiersin.org/articles/10.3389/fcell.2021.812292/full
• Meiosis: An Overview of Key Differences from Mitosis
By: Hiroyuki Ohkura
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4448623/