Meiosis- ‘Meiosis’ means To reduce.

- It is a special kind of cell division that takes

place in reproductive / germ cells only.

- Cells in the reproductive tissues multiply in

number by mitotic divisions.

- The final division that produces mature- The final division that produces mature

gametes however, are not mitotic.

- In fact these division are of the reducingtype.

- The number of chromosomes in the cell is

halved in gametes.

- When these haploid gametes fuse to forma zygote the diploid chromosome number of organism is restored.

- In man, meiosis occurs in testesproducing sperm and in female it occurs in ovaries producing ova.ovaries producing ova.

- In plant kingdom meiosis occurs in anthers and ovaries of angiosperms(flowering plants) producing pollen grainsand ovules respectively.

In Meiosis the cell divides in two

sequences as below1). Heterotypic or

1st Meiotic Division- Here the diploid

chromosome number ofparent cell is reduced tohalf (haploid) in daughtercells.cells.

2). Homotypic or2nd Meiotic Division

- This is normal mitoticdivision with no changein the number ofchromosome.

1). Heterotypic Division (Meiosis-I)

• It is also known as First Meiotic Division

• Here the chromosome number is halved.

• It consists of following phases

1. Prophase I1. Prophase I

2. Metaphase I

3. Anaphase I

4. Telophase I

Prophase 1• It is further consist of 5 substages

- Leptotene (L)

- Zygotene (Z)

- Pachytene (P)- Pachytene (P)

- Diplotene (D)

- Diakinesis (D)

(L Z P D D)

Leptotenemeans thin threads

chromosomes condenses into visiblestrands within the nucleus. 2 sisterchromatids are still intact with each other.Synaptonemal complex assembles.

It is of very short duration. Nuclear membraneand nucleolus are present during this stage.

Progressive condensation and coiling ofchromosome fibers takes place.chromosome fibers takes place.Chromosome assume a long thread likeshape, they contract and become thick.

Each chromosome is made up of only onechromatid and half of the total chromosomeare paternal and half maternal. For everypaternal chromosome there is acorresponding maternal chromosome similarin size, shape and nature of inheritedcharacters and are called HOMOLOGOUSCHROMOSOME.

ZygoteneMeans paired threads

Pairing of homologouschromosomes takes place and itis called SYNAPSE / SYNAPSIS.

The paired homologouschromosomes are joined by aprotein containing frame workcalled SYNAPTONEMALcalled SYNAPTONEMALCOMPLEX.

Individuals of a pair are equal inlength and in position ofcentromere. Thus pairing ishighly specific and exact. Thepaired chromosomes are calledBivalent or Tetrad chromosome.

PachyteneEach paired chromosome (bivalent) becomes

shorter and thicker than in earlier substages

and splits into two sister chromatids except at

the region of the centromere.

As a result of the longitudinal division of each

homologous chromosome into two

chromatids there are four groups of

chromatids in the nucleus lying parallel to

each other called TETRADS.each other called TETRADS.

An exchange of chromosomal or genetic

material between non-sister chromatids of

each tetrad takes place with a common

intersection called CHIASMATA.

Such exchange of genetic materials between

non-sister chromatids is called CROSSING-

OVER and it provides the raw material, the

hereditary variations for the organic evolution.

DiploteneThe attraction between thehomologous chromosomes in abivalent is stopped and replaced byrepulsion.

The repulsion starts at thecentromere region and proceedstowards the arms.

As a result the paired chromosomesbegin to separate from each otherbegin to separate from each otherexcept at sites of crossovers.

Separation of homologouschromosome in a bivalent is calleddisjunction.

The chiasmata are displaced alongthe length of the chromosome andare called TERMINALISATION.

DiakinesisIn the diakinesis stage thebivalent chromosome becomemore condensed and evenlydistributed in the nucleus.

The nucleolus detaches fromthe nucleolar organizer andthe nucleolar organizer andultimately disappears.

The nuclear envelope breaksdown.

The chiasmata disappear. Thespindle fibres originate fromthe poles.

All these phases be summed up as under:-

a). Pairing of Chromosomes: Homologous chromosomes pairsseem to attract each other. They are similar chromosomes-one each from either parent. Each pair of homologouschromosomes is also known as tetrad because it has fourchromatids.

b). Crossing Over: While the homologous chromosomes are inthe tetrad formation the chromatids seem to cross each otherat one or at more points. The chromatids actually break andat one or at more points. The chromatids actually break andrejoin at these points of intersection known as “Chiasmata”.Alongs with parts of chromatids exchanged, genetic materialgets recombined into new combinations of genes.

c). Nuclear envelope, nucleoli etc… disappear and thechromosomes move apart to the opposite poles and 2 newcells are formed. Thus at the end of first meiotic division, 2haploid daughter cells are form from a diploid parent cell.

Significance of Crossing Over- During Prophase-I crossing over takes place.

- The maternal and paternal homologouschromosomes of an organism pair up andexchange certain portions of their chromatids.

- This crossing over is the basis of geneticvariations in organisms.variations in organisms.

- This is why all offspring of 2 organisms are notalike.

- Crossing over makes the genetic informationin each haploid cell unique.

- Each chromosome pair can crossover at many pointsmore than once.

2). Homotypic Division (Meiosis-II)- This division is identical to Mitosis.- Here the chromosome number (haploid) is

maintained and at the end, four haploid daughtercells are produced.

- The phases of second meiotic division are divisibleinto Prophase-II, Metaphase-II, Anaphase-II andTelophase-II.

- Cytokinesis in meiosis varies greatly.- Cytokinesis in meiosis varies greatly.- Sometimes two daughter cells produced at end of

Meiosis-I undergo only Karyo-kinesis and gothrough Meiosis-II before dividing into four haploiddaughter cells.

- At other times Cytokinesis occur at end of Meiosis-Iand thus two haploid daughter cells produced undergo Meiosis-II.

Characteristics of Meiosis

• Occur in germ cells only.

• Results in formation of gametes.

• Results in formation of four haploid daughter cells.daughter cells.

• Chromosome number of parent cell is halved.

• Daughter cells differ from parent cell.

Significance of Meiosis

• Due to Meiosis, chromosome number is reduced to half in gametes and chromosome number of species is maintained.

• Due to crossing over in Prophase-I, genetic • Due to crossing over in Prophase-I, genetic variations occur.

• Due to Meiosis, the zygote on fertilizationhas one maternal and one paternalchromosome in each homologous pair.