Useful for B. Sc. Botany Syllabus of Dr. Babasaheb Ambedkar Marathwada University

1. MEIOSIS
B. Sc. III
Dr. Anjali Naik

2. Meiosis overview

5. Leptotene
• Leptotene is the first stage of prophase I and is
also known as leptonema, which is derived from a
Greek word which means "thin threads".
• In this stage, individual chromosomes consists of
two sister chromatids.
• The chromosomes condense into visible strands
within the nucleus.
• The two sister chromatids are tightly bound, that
they are not distinguishable from one other.
• During this phase the lateral elements of the
synaptonemal complex assemble.
• This stage is of very short duration and
progressive condensation and coiling of
chromosome takes place.

6. Zygotene
• Zygotene is also known as zygonema, it is
derived from Greek word which means 'paired
threads'.
• Homologous chromosome pairs.
• This stage is known as bouquet stage, due to
the way the telomeres cluster at on end of the
nucleus.
• Synapsis of homologous chromosomes takes
place in this stage, it is facilitated by the
assembly of central element of the
synaptonemal complex.
• Pairing of chromosomes happens in a zipper like
fashion and starts at the centromere
(procentric) or at the chromosome ends
(proterminal) or at any other portion
(intermediate).
• Two chromosomes in a pair are equal in length
and in position of the centromere, making the
pairing highly specific and exact.
• These paired chromosomes are called bivalent
or tetrad chrmosomes.

7. Synaptonemal Complex

8. Synaptonemal Complex
• Montrose J. Moses (1956) firstly
described synaptonemal complex
(SC) in primary spermatocytes of
crayfish and D. Fawcett in
spermatocytes of pigeon, cat and
man.
• The synaptonemal complex is a
tripartite, protein structure. It forms
between two homologous
chromosomes.
• It consists of two parallel lateral
regions and a central element. All are
electron dense elements that are
separated by less electron dense
areas.
• Central element: Has ladder like
configuration. Contains mainly RNA
and proteins and has little DNA. It is
spanned between the two lateral
elements.
• Lateral element: Composed of
transverse filaments, called as
synaptomers. These are rich in DNA,
RNA and proteins.

9. Functions of SC
• Synaptonemal complex stabilizes
the pairing of homologous
chromosomes i.e. synapsis.
• It facilitates crossing over.
• Synaptonemal complex appears
during the meiotic stages in which
pairing and recombination occurs.
So it may be useful to probe the
presence of pairing abnormalities
in individuals carrying
chromosomal abnormalities, either
in chromosome number or
structure

10. Pachytene
• The pachytene stage is also known as pachynema
and is derived from Greek which means "thick
threads".This is the stage where chromosomal
crossing over occurs.
• Nonsister chromatids of homologous
chromosomes exchange segments over
homologous regions.
• Sex chromosomes are not identical and they
exchange information over a small region of
homology.
• Chiasmata is formed where exchange happens.
• There is exchange of information between the non-
sister chromatids and this results in a
recombination of information.
• Every chromosome has a complete set of
information and there are no gaps formed as the
result of the process.

11. Pachytene crossing over

12. Diplotene
• The diplotene stage is also known as
diplonema, which is derived from Greek
word meaning "two threads".
• During this stage there is degradation of
synaptonemal complex and the
homologous chromosomes separate a
little from one another.
• The chromosomes in this stage uncoil a
little, this allows transcription of DNA.
• The bivalent homologous chromosomes
remain tightly bound at the region of the
chiasmata,where crossing over occurred.
• The chiasmata regions remains on the
chromosomes until they are separated in
the anaphase.

13. Diakinesis
• During the stage of diakinesis the
chromosomes condense further.
• The word diakinesis is derived form Greek
word which means "moving through".
• This stage is the first part of meiosis where
the four arms of the tetrads are visible.
• The sites where crossing over has occurred
entangle together, overlapping effectively
and making the chiasmata visible clearly.
• This stage resembles the prometaphase of
mitosis, where the nucleoli disappear and the
nuclear membrane disintergrates into vesicle
and also there is formation of the meiotic
spindle.

15. Metaphase-I
• Metaphase I
• The homologous pairs of chromatids
move together along the metapahse
plate.
• The kinetochore microtubules
from the centrioles attach to their
kinetochores respectively.
• The homologous chromosomes align
along the equatorial plane, this
alignment happens due to the
continuous counterbalancing forces
exerted on the bivalents by the
microtubules emanating from the
kinetochores of the homologous
chromosomes.

16. Anaphase-I
• microtubules shorten, and pulls the
homologous chromosomes apart.
• the whole chromosomes are pulled
towards the opposite poles which results
in the formation of two haploid sets.
• Each chromosomes contains a pair of
sister chromatids.
• Disjunction occurs during this time, this is
one of the process that leads to genetic
diversity as the chromosomes end up
in either of the daughter cells.
• The nonkinetochore microtubules
lengthen and pushes the centrioles
farther apart. The cell is elongated and it
prepares for division at the center.

17. Telophase-I
• The first phase of meiotic division ends when the
chromosomes arrive at the poles. The daughter cells now
have half the number of chromosomes, the chromosomes
consists of a pair of chromatids.
• The microtubules of the spindle network disappear and
nuclear membrane surrounds each haploid set.
• The chromosomes uncoil and return back to the chromatin
stage.
• The process of cytokinesis occurs where, the cell membrane
in the animals cells is pinched off, and in plant cells there is
formation of the cell wall in between the daughter cells.
• This completes the creation of two daughter cells. The sister
chromatids remain attached during the telophase I stage.

18. Prophase-II
• During this stage there is
disappearance of the nucleoli
and the nuclear envelope, also
there is shortening and
thickening of the chromatids.
• The centrioles move to the polar
region and the spindle fibers are
arranged for the second meiotic
division.

19. Metaphase-II
• During this stage the
centromeres that contain two
kinetochores attach to the
spindle fibers at each pole from
the centrioles.
• The equatorial plate formed
here is rotated by 90 degrees,
compared to meiosis I and is
perpendicular to the previous
metaphase plate.

20. Anaphase-II
• The metaphse II is followed by
the anaphase II, in the anaphase
II stage the centromeres are
cleaved, this allows the
microtubules attached to the
kinetochores to pull the sister
chromatids apart.
• The sister chromosomes move
towards the opposing poles

21. Telophase-II & Cytokinesis
• The meiosis II process ends at this
stage, this stage is similar to the
telophase I.
• In this phase there is uncoiling and
lengthening of the chromosomes
and disappearance of the spindle.
There is also reformation of
nuclear envelope.
• Cleavage or cell wall forms which
eventually produces a total of four
daughter cells, each cell having its
own haploid set of chromosomes.

22. Meiosis-Overview

23. Meiosis Mitosis
End result
Normally there are four cells, each cell
has half the chromosomes as the parent cell.
Two cells, each cells has the
same numbe of
chromosomes as
that of the parent.
Function
Sexual reproduction of gametes
(sex cells).
Cellular reproduction,
growth, repair, wear and tear
of cells, sexual production.
Where it occurs?
Animals, fungi, plants, and some
protists.
Occurs in all eukaryotic
organisms.
Stages
Steps in the process Prophase I, Metaphse I
anaphase I, Telophase I Prophase II, Metaphase II,
Anaphase II, Teloophase II.
Prophase, Metaphase,
Anaphase,
Telophase.
Genetical
composition
Not similar to parents Usually similar to parents.
Crossing over
process
Occurs in Prophase I Sometimes
Pairing of
Homologous
chromosomes
Yes No
Cytokinesis
Occurs in Telophase I and Telophase
II
Occurs in Telophase
Splitting of
chromosomes
Does not occur in Anaphase I; occurs
in Anaphase II
Occurs in Anaphase