Meiosis

Meiosis is a special kind of cell division in which chromosomes replicate only once but with two cell divisions that occur in rapid succession. So, a mother cell produces four daughter cells having half of the chromosome number than in the mother cell. It is also called a reductional division.

Occurrence of meiosis

In animals, it occurs at the time of gamete formation but in plants at the time of spore formation. Spore ultimately forms gametes. Any cells in which meiosis occur are called meiocytes. Eg;

1. Sporocytes (2n) ? Spores(n)
2. Oocytes (2n) ? Ovum(n)
3. Spermatocytes(2n)? Sperms(n)

Types of meiosis

• Gametic meiosis: Formation of gametes. Eg; Animal
• Sporic meiosis: Formation of spores. Eg; Marchantia
• Zygotic meiosis: After formation of zygote. Eg; Spirogyra

Mechanism of meiosis

It consists of two cell division which is Meiosis I and Meiosis II

Meiosis I

It is reductional or heterotypic division because daughter cells contain half of the chromosome than the mother cell. It consists of two phases interphase and Karyokinesis.

Karyokinesis

Prophase I

It is the longest phase and is divided into following sub-phases.

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene

• Chromatin fibres condense into chromosomes by coiling and dehydration.
• Chromosomes are thin beaded like appearance.
• Chromosomes are single stranded though they have become replicated during interphase.
• The ends of chromosomes remain directed towards one side of the nucleus and is called bouquet stage.

Zygotene

• Homologous chromosomes(paternal and maternal) undergo pairing or synapsis. A paired of synapse chromosome is called bivalent.
• Synapsis occurs due to the formation of a synaptonemal complex between them.

Pachytene

• Chromosomes become much thicker and the chromatids become visible.
• Now at this stage, these bivalent appear in a four-stranded structure called the tetrad.
• At this tetrad stage crossing over (exchange of chromatid segment between non-sister chromatids of the homologous chromosome) occurs at one or more specific points called Chiasmata.
• Crossing-over involves breakage and fusion of chromatid segment and this is caused by enzymes known as the endonuclease(cutting and joining).
• Crossing over brings genetic recombination or variation in the daughter cell.

Diplotene

• After crossing over homologous chromosome undergo desynapsis and separate from each other except at chiasmata.
• The chiasmata then shift towards the end of the chromosome and the process is called thermalization.
• Nuclear membrane and nucleolus start to disappear.

Diakinesis

• Chromosomes become much condensed so that all the chiasmata get finally terminalized.
• Nuclear membrane and nucleolus completely disappear.
• Chromosomes get scattered in the cytoplasm.
• Spindle fibres begin to reappear.

Metaphase I

• All the chromosome get arranged at the equator in two metaphysic plates which lie equidistance from the equator.
• The arrangement and the orientation of each bivalent are independent of the other so that the chromosomes assort independently to poles.
• The centromere face towards the pole and arms towards the equator.
• The centromere is joined by only one spindle fibre coming from one of the two poles.

Anaphase I

• The homologous chromosomes separate or undergo disjunction and move towards opposite poles along with undivided centromere and two chromatids. These chromosomes are called dyad chromosomes.
• The chromosomes are pulled towards the pole by the contraction of tractile fibres.
• The chromosomes appear in different shape V, U, L, J.
• At the end of Anaphase I only haploid or n number of chromosome reach the pole.

Significance: It brings the reduction in the number of chromosomes in the daughter cells.

Telophase I

• It may or may not occur when it occurs it consist of the changes which are opposite to those of prophase-I.
• Chromosomes decondense into chromatid fibre.
• The nucleolus is formed and nuclear membrane appears around each polar group of dyad chromosomes.
• Spindle fibre disappears.
• In the end, two haploid nuclei are formed.

Cytokinesis

It occurs by two usual methods;

• Cell plate method
• Cell cleavage method

The cytokinesis takes place either successively (occurs both after 1^st and 2^nd miotic division) or simultaneously (occurs only after 2^nd meiotic division).

Meiosis-II

It is an equational or homotypic division which is similar to mitosis or mitotic cell division.

Interphase

It is only a short resting period in which there is no replication of DNA.

Karyokinesis

Prophase-II

• Chromatin fibres condensed into chromosomes.
• Chromosomes are thin and consist of two chromatids which are genetically unlike.
• Nucleolus and nuclear membrane disappear.
• Spindle apparatus is formed at the right angle to the axis of previous spindle apparatus.

Metaphase-II

• Chromosomes get arranged at the equator in a single metaphoric plate.
• Only the centromeres lie on the equator and the arms face towards poles.
• Chromosomes are short and thick.
• Each centromere is joined by two spindle fibres.

Anaphase-II

• Like in mitosis, the centromere divides, chromatids separate and then move towards pole by the contraction of the spindle or tractile fibres.
• Chromosomes appear in V, U, L, J shapes.
• The anaphase II ends when all the chromosomes reach the pole.

Telophase II

• Nuclear membrane reappears around each polar group of haploid chromosomes.
• The nucleolus is also formed.
• Chromosomes decondensed into chromatin fibres.
• At last four haploid daughters, nuclei are formed

Cytokinesis: It always occurs by the same method.

Significance of Meiosis II

• It forms gametes in animals and spores in plants.
• Meiosis along with fertilisation helped to maintain constant chromosome number in organisms.
• It maintains alternation of generation.
• It brings variation by the process of crossing over.
• It helps in an assortment of paternal and maternal chromosomes into the daughter cells.
• It includes chromosomal mutation.