Meiosis

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
•Leptoteneis 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 fromone other.
•During this phase the lateral elements of the synaptonemalcomplex assemble.
•This stage is of very short duration and progressive condensation and coiling of chromosome takes place.

6. Zygotene
•Zygoteneis 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
synaptonemalcomplex.
•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. SynaptonemalComplex

8. SynaptonemalComplex
•Montrose J. Moses (1956) firstly described synaptonemalcomplex (SC) in primary spermatocytes of crayfish and D. Fawcett in spermatocytes of pigeon, cat and man.•The synaptonemalcomplex 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
•Synaptonemalcomplex stabilizes the pairing of homologous chromosomes i.e. synapsis.
•It facilitatescrossing over.
•Synaptonemalcomplex 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 inchromosome number or structure

10. Pachytene
•The pachytenestage is also known as pachynemaand is derived from Greek which means "thick threads".Thisis the stage where chromosomal crossing over occurs.
•Nonsisterchromatids of homologous chromosomes exchange segments over homologous regions.
•Sex chromosomes are not identical and they exchange information over a small region of homology.
•Chiasmatais 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. Pachytenecrossing over

12. Diplotene
•The diplotenestage is also known as diplonema, which is derived from Greek word meaning "two threads".
•During this stage there is degradation of synaptonemalcomplex 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,wherecrossing over occurred.
•The chiasmataregions remains on the chromosomes until they are separated in the anaphase.

13. Diakinesis
•During the stage of diakinesisthe chromosomes condense further.
•The word diakinesisis 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 chiasmatavisible clearly.
•This stage resembles the prometaphaseof mitosis, where the nucleoli disappear and the nuclear membrane disintergratesinto vesicle and also there is formation of the meiotic spindle.

15. Metaphase-I
•Metaphase I
•The homologous pairs of chromatids move together along the metapahseplate.
•The kinetochore microtubules fromthe 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 diversityas the chromosomes end up in either of the daughter cells.
•The nonkinetochoremicrotubules 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 telophaseI 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 metaphseII 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 telophaseI.
•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. MeiosisMitosis
End resultNormally there are four cells, each cell
hashalf thechromosomes as the parent cell.
Two cells, each cells has the
same numbeof
chromosomesas
that of the parent.
FunctionSexual 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, MetaphseI
anaphase I, TelophaseI Prophase II, Metaphase II,
Anaphase II, TeloophaseII.
Prophase, Metaphase,
Anaphase,
Telophase.
Genetical
compositionNot similar to parentsUsually similar to parents.
Crossing over
processOccurs in Prophase ISometimes
Pairing of
Homologous
chromosomes
Yes No
CytokinesisOccurs in TelophaseI and Telophase
II Occurs in Telophase
Splitting of
chromosomes
Does not occurin Anaphase I; occurs
inAnaphase IIOccurs in Anaphase