Meiosis involves two cell divisions that result in four haploid cells from one original diploid cell. In meiosis I, homologous chromosomes pair up and undergo recombination during prophase I before separating. This reduces the chromosome number by half. Meiosis II separates sister chromatids, resulting in four haploid cells each with half the number of chromosomes as the original cell. Key stages include prophase I with synapsis, metaphase I separation of homologs, and metaphase II/anaphase II separation of sister chromatids. Meiosis produces gametes for sexual reproduction and genetic variation.

1. Meiosis

2. Meiosis – Cell division where the
chromosome number is reduced by half.
This is accomplished by having one cycle of
chromosome replication followed by two
divisions
By reducing the number of chromosomes
the cells go from a somatic number of 2N
to a gametic number of N

3. The two divisions are designated the
reduction division and the equational
division.
Reduction division
– separation of homologous
chromosomes. This is the division
when the chromosome number is
reduced.

4. Equational division
- separation of sister chromatids. This
results in four cells having half the
number or chromosomes as the original
cell.

5. • The key features of meiosis are as follows:
• Meiosis involves two sequential cycles of nuclear
and cell division called meiosis I and meiosis II
but only a single cycle of DNA replication.
• Meiosis I is initiated after the parental
chromosomes have replicated to produce
identical sister chromatids at the S phase.
• Meiosis involves pairing of homologous
chromosomes and recombination between them.
• Four haploid cells are formed at the end of
meiosis II

6. Reductional Division (Meiosis I)
- In this division the chromosome number is
reduced and recombination between
homologous chromosomes can occur.
- There are four stages:
- prophase I
- metaphase I
- anaphase I
- telophase I

7. To describe everything that occurs in
prophase I it is divided into five sub-
stages:
- leptotene
- zygotene
- pachytene
- diplotene
- diakinesis

8. Leptotene
- chromosomes become
visible
- telomeres are in contact
with the nuclear
membrane
- nucleolus present

9. Zygotene
- chromosomes continue
to condense
- homologous
chromosomes pair
- pairing is known as
synapsis
- Nucleolus and nuclear
membrane are still
present

10. The homologous chromosomes are held
together by the synaptonemal complex.
The synaptonemal complex is a tripartite
ribbon made of two lateral protein bands
surrounding a medial protein complex.
The synaptonemal complex makes it
possible for recombination to occur.

12. • The synaptonemal complex (SC) is a large
proteinaceous structure that holds together
homologous chromosomes during Meiosis, providing
the structural framework for meiotic recombination
and crossover formation.
• It is an evoluntionarily-conserved protein assembly
which forms between homolgous chromosomes.
• The synaptonemal complex begins to form during the
zygotene phase of Prophase I in the first division in
Meiosis and is complete in the pachytene phase.
• Acting like a 'zipper' it holds the homologous
chromosomes together, aligning them perfectly.
• After complete synapsis, crossing over occurs and in
the diplotene phase, where the chiasma is visible, the
synaptonemal complex 'unzips' and disappears.

13. • The SC was discovered in 1956 and studies by
electron microscopy have shown that in all
sexually reproducing organisms in which it is
found the complex adopts the same tripartite
structure.
• It is made up of two lateral elements of
approximately 50nm each that coat the
chromosome axes, and in almost all organisms
a central element of 20-40nm wide. The
elements continue along the entire
chromosome axis.

14. Pachytene
- paired chromosomes
continue to condense
and shorten
- exchange between
non-sister chromatids
can occur
- exchange appears to
be protein mediated
- nucleolus and nuclear
membrane still
present

17. • During this stage bivalent chromosomes now clearly
appears as tetrads.
• This stage is characterized by the appearance of
recombination nodules, the sites at which crossing over
occurs between non-sister chromatids of the homologous
chromosomes.
• Crossing over is the exchange of genetic material between
two homologous chromosomes.
• Crossing over is also an enzyme-mediated process and the
enzyme involved is called recombinase.
• Crossing over leads to recombination of genetic material on
the two chromosomes.
• Recombination between homologous chromosomes is
completed by the end of pachytene, leaving the
chromosomes linked at the sites of crossing over.

18. Diplotene
- synaptonemal
complex starts to
break down
- homologous
chromosome pairs
start to separate
- areas of exchange
stay together longer,
called chiasma
- Nucleolus and nuclear
membrane start to
breakdown

20. Diakinesis
- chromosomes
continue to condense
and chiasmata
terminalize
- if chiasmata in both
arms get a ring
bivalent, if one arm a
rod bivalent
- nucleolus and nuclear
membrane start to
disappear

21. Formation of ring bivalent with terminalization of chiasmata
Formation of rod bivalent with terminalization of chiasmata

22. Metaphase I
- bivalents align on
metaphase plate
- presence of multiple
chromosomes pairing or
chromosomes not
pairing are indicators of
chromosome additions,
deletions or
modifications

23. Rod bivalents
Ring bivalents
Univalents

24. Anaphase I
• The homologous chromosomes separate,
while sister chromatids remain associated at
their centromeres

25. Telophase I
• The nuclear membrane and nucleolus
reappear, cytokinesis follows and this is called
as diad of cells.
• Although in many cases the chromosomes do
undergo some dispersion, they do not reach
the extremely extended state of the
interphase nucleus.
• The stage between the two meiotic divisions is
called interkinesis and is generally short lived.
• Interkinesis is followed by prophase II, a much
simpler prophase than prophase I.

28. Meiosis II

29. Prophase II:
• Meiosis II is initiated
immediately after
cytokinesis, usually before
the chromosomes have fully
elongated.
In contrast to meiosis I,
meiosis II resembles a
normal mitosis.
• The nuclear membrane
disappears by the end of
prophase II .
• The chromosomes again
become compact.

30. Metaphase II
• At this stage the
chromosomes align
at the equator and
• the microtubules
from opposite poles
of the spindle get
attached to the
kinetochores of
sister chromatids.

31. Anaphase II
• It begins with the
simultaneous
splitting of the
centromere of each
chromosome (which
was holding the
sister chromatids
together), allowing
them to move
toward opposite
poles of the cell.

32. Telophase II
• Meiosis ends with
telophase II, in which the
two groups of
chromosomes once again
get enclosed by a nuclear
envelope.
• cytokinesis follows
resulting in the formation
of tetrad of cells i.e., four
haploid daughter cells.

35. Significance of Meiosis
• It brings about a reduction in the
chromosome number from a diploid (2n)
condition to a haploid (n) condition. Such a
reduction becomes necessary for
maintaining the chromosome number.
• provides chance for the appearance of new
gene combinations as a result of crossing
over. This situation brings about variations.
• necessary for the formation of gametes in
animals and spores in plants.

36. Summary

37. Comparison between Mitosis and meiosis