This document provides an overview of cell cycle, cell division, and meiosis. It describes the stages of the cell cycle including interphase and mitosis. It then explains the two main types of cell division - mitosis, which produces identical daughter cells, and meiosis, which reduces the chromosome number by half to produce gametes like sperm and egg cells. The stages of meiosis, including meiosis I and meiosis II, are outlined in detail. Key events in meiosis I like homologous chromosome pairing, crossing over, and separation of homologous chromosomes are discussed. The phases of each meiotic division - prophase, metaphase, anaphase, and telophase - are also summarized.

1. Presented By :-
Tejendra Baghel

2. CONTENTS
 Cell Cycle
 Cell Division
 Meiosis
 Stages of Meiosis
 Meiosis-I
 Stages of Meiosis-I
 Homologous Chromosomes
 Crossing Over
 Meiosis-I
 Stages of Meiosis-I
 References

3. Cell Cycle
“Cell cycle refers to the series of
events that take place in a cell,
resulting in the duplication of
DNA and division of cytoplasm
and organelles to produce two
daughter cells.”

4. Phases of the Cell Cycle
1. Interphase:-
The resting stage between subsequent cell
divisions, but new research has shown it is a very
active phase.
 G0 Phase (Resting Phase): The cell neither
divides nor prepares itself for the division.
 G1 Phase (Gap 1): The cell is metabolically active
and grows continuously during this phase.
 S phase (Synthesis): The DNA replication or
synthesis occurs during this stage.
 G2 phase (Gap 2): Protein synthesis happens in
this phase.
 Quiescent Stage (G0): The cells that do not
undergo further division exits the G1 phase and
enters an inactive stage. This stage is known as
the quiescent stage (G0) of the cell cycle.
Schematic of the cell cycle
Outer ring:- I = Interphase, M = Mitosis
Inner ring:- M = Mitosis , G1 = Gap 1,
G2 = Gap 2 ,S = Synthesis
Not in ring:-G0 = Gap 0/Resting

5. 2. M Phase (Mitosis phase)
The actual cell division occurs in mitosis
phase. There are two key steps in this
phase, namely karyokinesis and
cytokinesis.
 Karyokinesis :- The process of the division
of a cell nucleus during mitosis or meiosis.
Karyokinesis occurs in four stages. They
are Prophase, Metaphase, Anaphase and
Telophase.
 Cytokinesis:- Cytokinesis is the physical
process of cell division, which divides the
cytoplasm of a parental cell into two
daughter cells.

6. Cell Division
The division of a cell into two
daughter cells with the same genetic
material. Cell division usually occurs
as part of a larger cell cycle.
These newly formed daughter cells
could themselves divide and grow,
giving rise to a new cell population
that is formed by the division and
growth of a single parental cell and its
descendant.

7. Types of Cell Division
● Mitosis: The process cells use to make
exact replicas of themselves. Mitosis is
observed in almost all the body’s cells,
including eyes, skin, hair, and muscle
cells.
● Meiosis: In this type of cell division,
sperm or egg cells are produced instead
of identical daughter cells as in mitosis.
● Binary Fission: Single-celled organisms
like bacteria replicate themselves for
reproduction.

8. Meiosis
“Meiosis is the type of cell division that
results in four daughter cells, each with
half the number of chromosomes of the
parent cell. Meiosis is the process in
which a single cell divides twice to form
four haploid daughter cells. These cells
are the gametes – sperms in males and
egg in females . “
Diploid (2n) → Haploid (n)

9. Stages Of Meiosis
The process of meiosis is divided into 2 stages.
1. Meiosis-I :- Meiosis-I separates the pair of
homologous chromosomes and reduces the
diploid cell to haploid. It is this step in meiosis
that generates genetic diversity.
1. Meiosis-II:- During Meiosis-II, sister chromatids
found in two daughter cells separate and form
four new haploid gametes.

10. Meiosis-I
“Meiosis I segregates homologous
chromosomes, which are joined as tetrads
(2n), producing two haploid cells which each
contain chromatid pairs. Because the ploidy
is reduced from diploid to haploid, meiosis I
is referred to as a reductional division.”

11. Homologous chromosomes are the
chromosomes which are similar in length,
gene position and centromere location. In
diplotene stage of Prophase1 of Meiosis 1
paired homologous chromosomes get
shortened and thickened.
Homologous pairs carry genes controlling
the same inherited traits.Each locus
(position of a gene) is in the same position
on homologues.
Humans have 23 pairs of homologous
chromosomes. 22 pairs of autosomes and 01
pair of sex chromosomes
Homologous Chromosomes

12. Crossing Over
Crossing over is a process that takes
place during prophase-I of meiosis-I,
in which non-sister chromatids of
homologous chromosomes exchange
segments between themselves. This
exchange of segments is referred to as
crossing over.
Crossing over (variation) may occur
between nonsister chromatids at the
chiasmata.
Crossing over: segments of nonsister
chromatids break and reattach to the
other chromatid. Chiasmata (chiasma)
are the sites of crossing over.

13. Phases of Meiosis-I
★ Prophase-I
Prophase I is the first stage in the first meiotic
division (meiosis I). It is essential mainly for the
exchange of DNA between homologous
chromosomes via a process called homologous
recombination and the crossover at chiasmata
between non-sister chromatids.
Prophase I is longer than the mitotic prophase
and is further subdivided into 5 substages.
❖ Leptotene
❖ Zygotene
❖ Pachytene
❖ Diplotene
❖ Diakinesis

14. ❖ Leptotene – The chromosomes begin to
condense and are attached to the
nuclear membrane via their telomeres.
❖ Zygotene – Synapsis begins with a
synaptonemal complex forming
between homologous chromosomes.
❖ Pachytene – Crossing over of genetic
material occurs between non-sister
chromatids.
❖ Diplotene – Synapsis ends with
disappearance of synaptonemal
complex; homologous pairs remain
attached at chiasmata.
❖ Diakinesis – Chromosomes become
fully condensed and nuclear membrane
disintegrates prior to metaphase I
Stages of Prophase-I

15. ★ Metaphase-I
The bivalents align at the equatorial plate
and microtubules from the opposite poles
attach to the pairs of homologous
chromosomes.
★ Anaphase-I
The two chromosomes of each bivalent
separate and move to the opposite ends of
the cells. The sister chromatids are
attached to eachother.

16. ★ Telophase- I
The chromosomes arrive at opposite poles of
the cell. In some organisms, the nuclear
membrane re-forms and the chromosomes
decondense, although in others, this step is
skipped—since cells will soon go through
another round of division, meiosis II.
★ Cytokinesis
Usually occurs at the same time as telophase I,
forming two haploid daughter cells.

17. Meiosis-II
Meiosis II is a shorter and simpler process than
meiosis I. The cells that enter meiosis II are the ones
made in meiosis I. These cells are haploid—have just
one chromosome from each homologue pair—but
their chromosomes still consist of two sister
chromatids.
In meiosis II, the sister chromatids separate, making
haploid cells with non-duplicated chromosomes.

18. Phases of Meiosis-II
★ Prophase-II
Chromosomes condense and the
nuclear envelope breaks down, if
needed. The centrosomes move
apart, the spindle forms between
them, and the spindle microtubules
begin to capture chromosomes from
opposite spindle poles.
★ Metaphase-II
Chromosomes line up individually
along the metaphase plate.

19. ★ Anaphase-II
The sister chromatids separate and are pulled
towards opposite poles of the cell.
★ Telophase-II
Nuclear membranes form around each set of
chromosomes, and the chromosomes
decondense.
★ Cytokinesis
Cytokinesis splits the chromosome sets into
new cells, forming the final products of meiosis:
four haploid cells in which each chromosome
has just one chromatid. In humans, the products
of meiosis are sperm or egg cells.

20. References
 Books :- NCERT Books
 Websites:-
 https://byjus.com/biology/cell-cycle-and-division/
 https://en.wikipedia.org/wiki/Cell_cycle#/media/File:Cell_Cycle
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21. T H A N K S