Meiosis is a type of cell division that produces haploid gametes from a diploid cell. It involves two rounds of nuclear division and cell division. In meiosis I, homologous chromosomes pair and separate, reducing the chromosome number by half. Meiosis II separates sister chromatids, resulting in four haploid daughter cells each with half the number of chromosomes as the original parent cell. The key stages of meiosis I are prophase I, metaphase I, anaphase I and telophase I, followed by cytokinesis. Prophase I is the longest phase and involves further substages of leptotene, zygotene, pachytene, diplotene and diakinesis.

1. MEIOSIS
SUBMITTED TO :
DR. PRITI MATHUR
SUBMITTED BY:
SHIPRA SRIVASTAVA
(A7104421013)
BSC HONS BIOTECHNOLOGY
SESSION 2021-24

2. CELL CYCLE
It includes INTERPHASE and M PHASE.
Interphase- preparation for cell division.
G1 phase
S phase
G2 phase
M phase or meiotic phase- cell division.
Karyokinesis
cytokinesis

3. Meiosis
- A special type of cell division that occurs in order to produce haploid cells
(gametes).
- Meiotic cell division was first time described by Van Beneden in 1883.
- During this type of cell division, the genetic material of a diploid germ cell
undergoes two nuclear divisions and resulting into four haploid daughter cells.
- Each daughter cell has half of the number of chromosomes as present in the
parent cell.
- Meiosis includes two successive nuclear divisions –
Meiosis1- Reductional Division.
Meiosis2- Equational Division.

4. Interphase
-The interphase just prior to the entry of cell into meiosis is known as pre
meiotic interphase.
-G1 phase
Protein synthesis.
Replication of cell organelles.
-S phase
Centrosome replicates.
DNA duplication chromatid formation.
-G2 phase
Protein synthesis .
Replication of mitochondria and chloroplast.

5. Meiosis-1
-Meiosis 1 separates homologous chromosomes
and produce two cells with haploid chromosome
number (n) for that reason it is known as
reductional division.
It consist of four stages –
Prophase1
Metaphase1
Anaphase1
Telophase1

6. Prophase 1
It is the longest phase takes about 85-90 percent of total time
for meiosis and also much more complex.
It is divided into five stages –
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis

10. Leptotene
-The first stage of prophase 1 is called Leptotene or leptonema
-All the chromosome begin to condense, so they become visible as fine thread.
-There is marked increase in the nuclear volume.
-A process of homology search which is essential to initial pairing of homologs
begin during Leptotene.

11. Zygotene
-This stage begins with the initiation of pairing between homologous
chromosome and it ends with complete pairing.
-The process of pairing (at end to end) between homologous chromosomes is
known as Synapsis (homologous dyads).
-The synaptonemal complex is formed during these zygotene stage.
-At the completion of zygotene ,the paired homologs take the form of bivalents.
-The number of bivalents in each species is equal to the haploid number.

12. Pachytene
- The process of synapsis is complete.
- The two homologous of each bivalent appears to be attached with each other at one or
more points. These attachments are known as chiasmata.
- Crossing over is a precise breakage swapping and remain between two non sister
chromatids.
- Cross over makes new gene combinations and which are an important source of genetic
variation in populations.

13. Diplotene
- DNA recombination is complete.
- The chromatids continue to shorten and thicken and the four sister chromatids in a
group is called a tetrad.
- The synaptonemal complex begins to breakdown.
-The paired chromatids begin to pull apart, causing the strands to separate longitudinally.
- The chiasmata tend to become terminalized as the meiotic prophase continues.

14. Diakinesis
- The chromosome becomes shorter and thicker due to condensation.
- Nuecleolous and nuclear envelope disappear toward the end of diakinesis .
- The spindle apparatus becomes organised.
- The centrioles migrate away from one another.

15. Metaphase- I
- All the bivalents migrate within a cell to metaphase plate.
- One homologue is pulled above the metaphase plate,the other below.
- The centromeres of homologues chromosome of each bivalent stresss out
on either side
- The centrioles are at opposite poles of the cell.
- Spindle fibres from one pole of the cell attach to one chromosome and
spindle fibre from the opposite pole attach to the homologues
chromosome.

16. Anaphase- I
 Chromosomes move to the opposite poles
 The microtubules and the kinetochore fibres interact, which cause the
movement.
 A difference between mitosis and meiosis is that sister chromatids remain
joined after metaphase in meiosis-I, whereas in mitosis they separate.
 During anaphase I original chromosomes separate , so reduction in
number of chromosomes from 2n to n number yet the sister chromatids
remain together.

17. Telophase- I
 The homologous chromosomes complete their migration
to the poles shortning of spindle.
 The nuclear envelope organized around the two groups of
chromosomes.
 The nucleolus also reappears.
Cytokinesis
 It involves the formation of a cleavage furrow, resulting in
the pocketing of the cell into two cells.
 At the end of telophase I and cytokinesis , two daughter
cells are produced , each with one half of the number of
chromosomes( haploid set of replicated chromosomes ) of
the original parent cell.

19. Interkinesis
 Interkinesis (interphase II ) is similar to interphase.
Meiosis II
 Meiosis II is the second part of the meiotic process.
 It consist prophase II ,metaphase II, anaphase II,
telophase II and cytokinesis.

21. Prophase II
 Each dyad is composed of a pair of sister chromatids
attached by a common centromere.
Metaphase II
 Centromeres are positioned at the equatorial plate.
Anaphase II
 Centromeres divide and the sister chromatid of each
dyad are pulled to opposite poles.

22.  Telophase-II
 One member of each pair of homologous chromosome present in each
pole.
 Each chromosome is referred as monad ( a combination of maternal and
paternal genetic information).
 Nuclei reform around chromosome at the poles.
 Following cytokinesis ,finally four haploid gametes result from a single
meiotic event.

23.  Functions of Meiosis
 Production of haploid gametes ; so that fertilization restores the normal
somatic chromosomes number.
 Production of genetic variation.
 Recombination between the linked genes due to crossing over during
pachytene stage.
 In sexually reproducing organism meiosis is essential for continuity of
generation because it results in the formation of male and female gametes
and union of each gametes leads to the development of zygotes and
thereby new individuals.

24. Thank you