Meiosis is a cell division process that produces gametes (sex cells) with half the normal number of chromosomes. It occurs in two stages, Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and crossover can occur, then the homologous chromosomes separate. This reduces the chromosome number from diploid to haploid. Meiosis II then separates the sister chromatids, without further reducing chromosome number, producing four haploid gametes. Fertilization restores the diploid number when a sperm and egg fuse.

1. Meiosis
Biren Daftary

2. Background
• Occurs in sex cells or gametes
• After undergoing meiosis, the chromosome
number of a cell is reduced by half
• Meiosis is necessary in the formation of male &
female gametes
• Hence gametes have half the number of
chromosomes as compared to other normal cells
• Entire meiosis is composed of two parts: Meiosis I
& meiosis II

4. Meiosis I
• In Meiosis I, a sex cell divides into two cells and
the chromosome number is halved
• It has 4 phases: Prophase I, Metaphase I,
Anaphase I & Telophase I
• Meiosis I is followed by meiosis II immediately
without the cell going into interphase
*Interphase is the duration between two successive mitotic/meiotic
phases consisting of 3 phases: G1, S & G2

6. Prophase I
• Longest and most complex phase (90%).
• Chromatin condenses to form chromosomes.
• Synapsis occurs: homologous chromosomes
come together to form a tetrad.
• Tetrad is two chromosomes or four chromatids
(sister and non-sister chromatids).
• Crossing over of genes occurs in the tetrad
*Homologous chromosomes are a pair of chromosomes, one obtained
from each parent. They are similar but not identical

8. Tetrad & Crossing over
Homologous
chromosomes after
crossing over

9. Metaphase I
•It is the shortest phase
•The tetrads align on the
metaphase plate
•Spindle fibers attach to the
kinetochores of the
centromeres

10. Anaphase I
• Homologous chromosomes
separate and move towards
the poles.
• Sister chromatids remain
attached at their centromeres.

11. Telophase I
• Each pole now has haploid
set of chromosomes.
• Cytokinesis occurs and two
haploid daughter cells are
formed.
*Cytokinesis is the physical process of cell division
in which the nucleus & cytoplasm divide giving
rise to 2 cells

12. Meiosis II
• In meiosis II, the chromosome number doesn’t
change when compared to meiosis 1 products
• Hence, Meiosis II is similar to mitosis
• Meiosis II consists of 4 stages: Prophase II,
Metaphase II, Anaphase II & Telophase II

14. Prophase II
• Same as prophase in mitosis
• Chromatin get condensed to
form chromosomes
• The spindle fibers are formed
• The centrosomes begin to move
towards either of the poles

15. Metaphase II
• Same as metaphase in Mitosis
• The chromosomes align
themselves on the metaphase
plate
• The spindle fibers attach to the
kinetochores on the centromeres

16. Anaphase II
• Same as anaphase in Mitosis
• Homologous chromosomes do
not separate
• Sister chromatids separate
• Each chromosome is thus
broken down into sister
chromatids which move towards
either of the poles

17. Telophase II
• Same as telophase in Mitosis
• Nuclei are formed.
• Cytokinesis occurs.
• 4 haploid daughter cells produced.
These cells are called as gametes

18. Meiosis
summary

19. Fertilization
• The fusion of a sperm(n) and egg(n) to form a zygote.
• A zygote is a fertilized egg (2n)
• 2n=diploid, n=haploid