What is Meiosis? Stages of Meiosis I and II

Our chromosome number is 46. For the existence of any species chromosome number should be maintained. We inherited the genetic program from our parents. 23 chromosomes derived from father and 23 from mother. That is why we have many characters similar to our parents. We often use the term ‘hereditary’ or heredity to call this similarity. But to maintain the chromosome number to 46, a vital process has occurred, Reduction in chromosome number to half in egg and sperm. This division is termed as meiosis. 

Definition: Meiosis is reduction division that occurs only in germ cells where gametes are produced with half the chromosome number to that of the parent cell. This reduction is essential for maintenance of chromosome number in a species.

 The egg and sperm each with 23 chromosomes unite and form a zygote or first cell with 46 chromosomes. Then ‘mitosis’ will take over producing billions of identical cells in a controlled fashion. Meiosis is restricted to germ cells where gametes are produced. Crossing over or shuffling of genes during meiosis is the major reason for genetic variation within species.

In this division, chromosomes duplicate only once, but the cell divides twice. One parent cell produces 4 daughter cells, each having half the chromosome number and DNA amount. It occurs in germ cells to produce gametes. It also known as reductional division.Meiosis comprises meiosis I and meiosis II. Each one includes Prophase, Metaphase, Anaphase and Telophase. 

The essential stages that take place during meiosis are

• Two successive divisions without any DNA replication.
• Formation of chiasmata and crossing over.
• Segregation of homologous chromosomes.
• Separation of sister chromatids

Meiosis I

Meiosis I

Prophase I

It is along process during which homologous chromosomes pair closely and interchange hereditary material. It is divided into 5 sub stages.

Leptotene:

• Each chromosome is attached at both of its end to the nuclear envelope.
• Each chromosome replicates into two sister chromatids.

Zygotene:

• It begins as pairing between the two members of each homologous chromosome pair is initiated by synapsis.
• The homolohous chromosome comprises one chromosome derived from each parent.
• As a result of synapsis each gene is brought into close contact with its allele located on hologous chromosome.
• The two homologous chromosomes are brought together through synaptonemal complex.
• The chromosome so produced is called bivalent.
• Each bivalent contains four chromatids and is also called tetrad.

Pachytene:

• After completion of synapsis, large recombination nodules appear at intervals on the synaptonemal complex.
• The non sister chromatids twist around and exchange segments witheach other.

Diplotene:                                                                                              

• Separation of paired homologous chromosomes.
• Homologous chromosomes however remain attached at one or more points where crossing over has occurred.These points of attachment are called chiasmata.

Diakinesis:

• Termination of chiasmata takes place.
• RNA synthesis stops and the chromosome s condense, thicken and become attached to nuclear envelope. Each pair of sister chromatids is attached to nuclear envelope.
• Each pair of sister chromatids is attached at their centromeres.
• Non-sister chromatids of homologous chromosomes are in contact with each other at or      near their telomeres.

Metaphase-I

• The bivalents become arranged in the plane of the equator forming equatorial plate.
• The centromere of each chromosome is directed towards the opposite poles and the arms of chromosomes face the equatorial plate.

Anaphase I

• The two members of each bivalent repel each other and move towards the opposite poles.
• Each pole receives half the number of chromosomes.   
• Reduction occurs and movement of chromosomes is brought by the spindle fibres.

Telophase I

• The nuclear membranes are formed during this stage by endoplasmic reticulam around the group of daughter chromosome.
• One nucleus appears in each nucleus.
• Formation of two daughter cells with haploid number of chromosomes.
• Intrameiotic interphase: This is the stage between the Telophase of first meiotic division and prophase of second meiotic division.

Meiosis-II

• It is essentially similar to mitosis. It divides each haploid meiotic cell into two daughter cell.
• It is described in four stages.

Meiosis II

Prophase-II

• Nuclear membrane disappears and spindle formation takes place.

Metaphase -II

• Chromosomes become oriented on the equatorial plate.
• They have similar arrangements with spindle as in mitosis.

Anaphase-II

• The centromere divides and two chromatids of each chromosome separate and move towards the poles.
• After separation each chromatids become chromosome.

Telophase -II

• The four groups of chromosomes become organised into four haploid nuclei.
• Chromosomes return to the interphase condition.
• The endoplasmic reticulam forms the nuclear envelope around the chromosomes and nucleolus reappears.
• Each nucleus at this stage contains the haploid number of chromosomes and form four cells.

Major events in each stage of meiosis

Significance of Meiosis:

• It maintains a definite and constant number of chromosomes by producing haploid gametes.
• Due to crossing over in meiosis, organisms may exchange genes and cause genetic variation in species. This variation serves the raw material of evolutionary process.
• Chromosomal and genomic mutations caused by non disjunction etc. Are also the sources of useful variations.

Glossary: 

Homologous chromosomes: Each pair of a chromosome in a diploid cell has one chromosome derived from the male parent and the other from the female parent. They are morphologically and genetically similar and are called homologous chromosomes.

Crossing over (Genetic recombination): Reshuffling of the genes on chromosome that occurs during meiosis as a result of breakage and reunion of segments of homologous chromosomes

Synapsis: The process by which homologous become joined to one another during meiosis.

Synaptonemal complex (SC):  A ladder like structure composed of three parallel bars with many cross fibers. The SC holds each pair of homologous chromosomes in the proper position to allow the continuation of genetic recombination between strands of DNA.