meiosis division

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Meiosis
Dr. Sonia Bajaj
(Head of Department)

2. Meiosis
 Term Meiosis was proposed by Farmer and Moore.
 Meiosis is also known as reduction division.
 Haploid gametes formed during sexual reproduction (gametogenesis) are due to meiosis.
 Meiosis is responsible for maintaining conservation of the number of chromosomes in different
individuals of the same species.
 Meiosis also accounts for the genetic variation or differences between two individuals of the
same species.
 There are two sequential cell divisions-
1 Meiosis I (Heterotypic division)
2. Meiosis II (Homotypic division)
 DNA replication occurs only once in the S phase before meiosis I.
 In meiosis I, pairing homologous chromosomes is followed by recombination between non-sister
chromatids and formation of two haploid daughter cells.
 Meiosis II is like a mitotic division and at the end of meiosis 4 haploid daughter cells are formed.

3. Meiosis I
A. First Meiotic Division (Meiosis I)
The first meiotic division results in reduction of chromosome number in each new cell to just half of the mother
cell, therefore, it is referred to as reductional division.
The first meiotic division consists of four different phases, viz:
(1) Prophase,
(2) Metaphase,
(3) Anaphase and
(4) Telophase.
1. First Prophase: (Prophase I)
This phase starts after interphase and is of maximum duration. This consists of five sub stages, viz., leptotene,
zygotene, pachytene, diplotene and diakinesis.
a.Leptotene-
 It is also called threaded stage.
 Volume of nucleus increase.
 Chromosomes are longest & thinnest.
 Chromosomes start condensing.
 All the chromosome in nucleus remain directed toward centrioles, so group of chromosomes in nucleus
appears like a bouquet in animal cell.so this stage is also called known as Bouquet stage.

4. b. Zygotene/Synaptotene-
 Zygotene is characterized by Pairing of homologous chromosomes Synapsis.
 In each pair, one chromosome is maternal and other is paternal.
 Two homologous chromosomes are held together by a filamentous nucleoprotein complex known as
synaptonemal complex.
 synaptonemal complex is composed of three thick lines of DNA and protein.
 synaptonemal complex help in pairing and chiasmata formation.
 Pairs of homologous chromosomes are called bivalent or tetrad.
c.Pachytene-
 It is also known as thick threaded stage.
 Further condensation of chromosomes take place.
 Each chromosome is now visibly formed of two sister chromatids joined at a centromere and is called dyad.
 Each bivalent (one homologous pair) has two dyads and so is called a tetrad. This stage is characterized by
appearance of recombination nucleus i.e. a chiasmata. This is the point where and non-sister chromatids
cross over.
 Crossing over is the exchange of genetic material between to homologous chromosomes.
 Crossing over is an enzyme mediated process and the enzyme involved is recombinase. Recombination is
completed by the end of pachytene.

5. d. Diplotene-
 Separation of homologous chromosome being due to the dissolution of syneptinemal complex except at
chiasmata.
 chiasmata start to move toward the end of the chromosome i.e. terminalisation starts.
 It is the longest and the most active sab phases of meiosis I.
 Lampbrush chromosomes found in the oocyte of amphibians are formed at the diplotene stage..
e. Diakinesis-
 It is the final stage of meiosis prophase I.
 By the end of diakinesis nuclear membrane and nucleolus disappear.
 Centrioles move towards the opposite poles.
 Marked by terminalisation of chiasmata (chiasmata open in Zip like manner).
 Terminalisation is composed during diakinesis is it is by zipper manner which it starts from the point of
crossing over and move towards the end of the chromosome.
 Complete dissolution of nuclear membrane and nucleoli take place.
 Spindles are completely formed.
 Bivalents are scattered in nucleo -cytoplasmic matrix.
 diakinesis represent transition to metaphase.

6. Metaphase I
 Chromosome are condensed, shorter and thicker.
 Spindle fibres are joined with the centromere of homologous chromosome.
 Chromosome pairs are arranged on the equatorial plate. This process is called aggregation.
• Bivalent chromosomes align at the equator and homologous chromosomes get attached to the spindles from
opposite poles.
Anaphase I
• Homologous chromosomes move to opposite poles.
• The shape of separating chromosome may appear like
- V, L, J or I, depending upon the position of centromere.
Telophase I
• Nucleolus and nuclear envelope reappears, chromosomes collect at the poles.
• This is followed by cytokinesis and dyad of haploid cells are formed.
• Asters and spindle fibres dissociate and disappear.
Cytokinesis I-In animal cells,it takes place by furrow formation and in plant cell by cell plate formation.
Significance of Meiosis I- crossing over results in recombination and variation .variation are essential for
evolution.

7. Interkinesis
• It is the period between meiosis I and meiosis II.
• It involves only protein and RNA synthesis and there is no S phase and no DNA synthesis .
• It does involve duplication of centriole.

8. Meiosis II
• It is also known as educational division because the number of chromosome remains the same as after
meiosis I.
• It is also completed in two parts –Karyokinesis II and Cytokinesis II
A. Karyokinesis II
• It involves separation of two chromatids of each chromosome and their movement to separates cells.
• It is divided in to 4 phase-
Prophase II
• The nuclear membrane and nucleolus disappears.
• Each chromosome is formed of two sister chromatids joined at a centromere.
• Condensation of chromatin fibres.
• Formation of asters bipolar spindle fibres.

9. Metaphase II
• . kinetochore of each centromere is joined by two chromosomal spindle fibres, one from each pole.
• Chromosome get arranged in one equatorial plate
Anaphase II
• Centromere splits and sister chromatids separate. They move to opposite poles and formation of two
daughter chromosome.
• Daughter chromosome are pulled towards the pole by chromosomal spindle fibres.
Telophase II
• Nuclear membrane reappears.
• Spindle fibres and asters disappear.
.

10. Cytokinesis II -In animal cells,it takes place by furrow formation and in plant cell by cell plate formation.
and we get 4 haploid daughter cells, i.e. tetrad.
Significance of Meiosis-II
By meiosis ,a diploid parents cell divides twice forming 4 haploid gametes.
It results in genetic variation which are essential for evolution.