Meiosis – Reproductive Cell Division

Meiosis – Reproductive Cell Division

• Meiosis is a cell division in which four haploid cells are formed from a single diploid cell with half the no. of chromosomes in daughter cells.
• It usually occurs in reproductive organs or gonads of the organisms.
• Meiosis is also known as reductional cell division because four daughter cells produced contain half the number of chromosomes than that of their parent cell.

• Meiosis has two nuclear division phases:
  1. Meiosis-I (Reductional or Heterotypic division) – It includes a long prophase in which the homologous chromosomes become closely associated to each other and interchange of hereditary material takes place between them. Further, reduction of chromosome number takes place and, thus, two haploid cells are resulted by this division.
  2. Meiosis-II (Equational or Homotypic division) – the haploid cell formed in first meiotic division, divides mitotically and results into four haploid cells.

 

Meiosis-I (heterolytic or Reductional division)

• • Meiosis-I Karyokinesis has four different phases or stages:
    1. 1. Prophase-I
       2. Metaphase-I
       3. Anaphase-I
       4. Telophase-I

Prophase-I

• • It occupies the longest duration in Meiosis-I.
  • It is divided into five sub-stages or sub-phases.
    1. 1. Leptotene
       2. Zygotene
       3. Pachytene
       4. Diplotene
       5. Diakinesis

a. Leptotene

• • • This phase starts immediately after interphase.
    • The size of cell and nucleus increases
    • The chromosomes appear long, uncoiled thread-like in structure bearing many bead-like structures called chromomeres.
    • The nuclear membrane and nucleolus remain as it is.
    • The centriole duplicates and each daughter centriole migrates towards the opposite poles of the cell.
    • On reaching at the poles, each centriole duplicates and, thus, each pole of cell possesses two centrioles of a single diplosome.

b. Zygotene

• • • Homologous chromosomes come closer and starts to pair up along their length.
    • The pairing of homologous chromosomes is called Synapsis and the paired homologous chromosomes are referred as
    • The homologous chromosomes are held together by ribo-nuclear protein between them.
    • Crossing over takes place.

c. Pachytene

• • • The chromosome become twisted spirally around each other and become shorter and thicker.
    • Each chromosome of the bivalents splits longitudinally to form two chromatids such that bivalents is composed of four strands and is known as a tetrad.
    • The process of crossing over starts which is accompanied by the chiasmata formation.
    • The crossing over involves reshuffling, redistribution and mutual exchange of hereditary material of two parents between two non-sister chromatids homologous chromosomes.
    • Crossing over is the most important genetic phenomenon of meiosis which causes variation in genetic characters in offspring.

d. Diplotene

• • • In this stage crossing over takes place. Bivalents (chromatids) repel each other.
    • Homologous chromosome (two non-sister chromatids) begins to separate but separation is not complete, they remains attached to a point with a knot like structure called chiasmata. At chiasmata crossing over takes place.
    • The number of chiasmata varies. Depending upon the number of chiasmata, chromosome appears different shape.
      • • 1 chiasmata: cross like
        • 2 chiasmata: ring like
        • Many chiasmata: series of loop
    • Nuclear membrane and nucleolus begins to disappear.

e. Diakinesis

• • • The chiasma moves from the centromere towards the end of the chromosomes (tetrad) due to contraction of chromosome. Lastly slips over separating the homologous chromosome.
    • This movement of the chiasmata towards the end of chromosome is called terminalisation.
    • The bivalent chromosomes become more condensed and evenly distributed in the nucleus.
    • By the end of diakinesis, nuclear membrane and nucleolus get completely disappeared.
    • the chromosomes are free in the cytoplasm.
    • Spindle fibres begin to form.

Metaphase-I

• • The spindle fibres organized between two poles and get attached to the centromere of chromosomes.
  • Chromosome moves to equator.
  • The centromere of each chromosome is directed towards the opposite poles.
  • The bivalent chromosomes are arranged in the equatorial plate in such a way that 2 metaphasic plates are formed.
  • The repulsive forces between the homologous chromosomes increase greatly and the chromosomes become ready to separate.

 Anaphase-I

• • Spindle fibres contracts and pulls the whole chromosomes to the polar region.
  • The actual reduction and disjunction occurs at this stage. The separated chromosome is known as dyads.
  • No splitting of chromosomes occurs, so the centromere of each homologous chromosome does not divide. Thus, the chromosome number of the daughter nuclei is reduced to half.
  • Now the separated chromosome moves toward opposite poles.
  • during the chiasma formation out of two chromatids of a chromosome, one has changed its counterpart, therefore, the two chromatids of a chromosome do not resemble with each other in the genetically terms.

Telophase-I

• • Two groups of chromosome formed at each pole and organised into nuclei.
  • endoplasmic reticulum forms the nuclear envelope around the chromosomes.
  • The nuclear membrane and nucleolus reappears.
  • The chromosomes get uncoiled into chromatin thread.
  • The spindle fibres disappear totally.

Cytokinesis I

• • Cytokinesis may or may not follow nuclear division
  • meiosis-I Cytokinesis occurs by cell plate formation method in plant cell and by furrowing in animal cells.
  • Two haploid cells are formed.

Interphase II or Interkinesis

• • The two cells or nuclei thus formed pass through a short stage called interphase-II. Sometimes, interphase-II is absent.
  • It is the resting phase between meiosis-I and meiosis-II.
  • It is either very short or may be absent.
  • No DNA synthesis occurs so that chromosomes at the second prophase are the same double-stranded structures that disappeared at the first telophase.

Meiosis-II ( Homolytic or equational division)

• Meiosis-II is exactly similar to mitosis, so it is also known as meiotic mitosis.
• The two haploid chromosome in the cell splits longitudinally and distributed equally to form 4 haploid cells.
• It completes in 4 stages.
  • • Prophase-II
    • Metaphase-II
    • Anaphase-II
    • Telophase-II

Prophase-II

• • The dyads chromosome (with two chromatids) becomes thicker and shorter.
  • Centrioles divides into two and migrates to opposite pole.
  • Nuclear membrane and nucleolus disappear.
  • Spindle fibre starts to form.

Metaphase-II

• • The dyads chromosomes comes to equatorial plane.
  • Spindle fibres organize between poles and attaches to centromere of chromosome.

Anaphase-II

• • Centromere of each chromosome divides and sister chromatids separates to form two daughter chromosome.
  • Spindle fibre contracts and pull the daughter chromosome apart towards opposite pole.

Telophase-II

• • Chromatids migrate to the opposite poles and now known as chromosomes.
  • Chromosome become organise at respective pole into nuclei.
  • Chromosome elongates to form thin networks of chromatin.
  • Nuclear membrane and nucleolus reappears.

Cytokinesis-II

• • The result of cytokinesis is four haploid daughter cells (gametes or spores).
  • Cytokinesis takes place by cell plate formation in plant cell.
    • • Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid cells. Eg. Monocot plants
      • Simultaneous methods: cytokinesis occurs only after meiosis-II to form 4 haploid cells. Eg. Dicot plants
  • In animal cells, cytokinesis occurs by furrow formation or depression.
  • These cells have different types of chromosomes due to the crossing over in the prophase I.

Significance of Meiosis

• Meiosis helps to maintain a constant number of chromosomes by reducing the chromosome number in the gametes
• Essential for sexual reproduction in higher animals and plants
• Meiosis helps in the formation haploid gametes and spores for sexual reproduction.
• Number of chromosome remain fixed in a species from generation to generation
• By crossing over, the meiosis helps in exchange of the genes, thus, causes the genetically variations among the species. The variations are the raw materials of the evolutionary process.
• The random distribution of maternal and paternal chromosomes takes place into daughter cells during meiosis and it is a sort of independent assortment which leads to variation.

Difference b/w Mitosis – Somatic Cell Division & Meiosis – Reproductive Cell Division

 So, this was all about the Meiosis – Reproductive Cell Division and its Significance.

In the Next Post (Click Here), we will discuss about the Chemical Composition of Chromosome and its Structure.