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3.3: Meiosis

  1. Meiosis:-Meiosis is the process by which gametes, sex cells, are formed. In this division the mother cell produces four daughter cells called gametes in which the chromosome number reduced to half, so this division is called reductional division. The gametes carry genetic recombination and therefore unidentical with that of mother cell, because of that this division is known as heterotypic division Thus if the parental cell has 14 chromosomes, each daughter has 7.

Sexual reproduction involves the two alternating processes of meiosis and fertilization.

  1. In meiosis, the chromosome number is reduced from the diploid to the haploid number.
  2. In fertilization, the nuclei of two gametes fuse, the zygote is formed and the chromosome number changes from haploid to diploid.

In most plants meiosis and fertilization divide the life of the organism into two distinct phases or "generations".

  1. The gametophyte generation begins with a spore produced by meiosis. The spore is haploid, and all the cells produced from it (by mitosis) are also haploid. In due course, this multicellular structure produces gametes — by mitosis — and sexual reproduction then produces the diploid sporophyte generation.
  2. The sporophyte generation thus starts with a zygote. Its cells contain the diploid number of chromosomes.Some reproductive cells undergo meiosis, forming spores and starting a new gametophyte generation.

Farmer and Moore (1905)- proposed the name meiosis (= reduction) to the whole process of the two successive nuclear divisions.It is the type of cell division, results in each daughter cell having half the number of chromosomes as the parent. This division maintains the chromosome number constant in all sexually reproducing plants.

Meiosis occur in diploid or polyploid cells, absent in haploid cells.

In lower plants, meiosis takes place in the zygote, this type of meiosis is known as zygotic meiosis(or initial meiosis) e.g. Chlamydomonas

Premeiotic interphase An interphase in meiosis is just like that of mitosis. It consists of G1,S and G2phases. G2 phase in meiosis is either very short or is completely absent. Meiotic division starts just after DNA synthesis is complete During meiosis, the nucleus divides twice but chromosomes divided only once, Gregoire (1904) called the first division as meiosis I and the second one as meiosis II

Phases of Meiosis: The end result of one round of meiosis will be four cells with half the number of chromosomes as the parent cell. There are two successive divisions in meiosis, which in plants occur without a pause.

A.  Meiosis I –Reduction division—the chromosome number is reduced to half the parent cell chromosome number. Therefore meiosis I is also known as reductional division. As a result two haploid cells are from one diploid cell . It comprises four phase:-

  1. Prophase I

First prophase is of a very long duration. It has five stages:-

  1. Leptotene(leptonema):-The chromatin condenses into long thin thread- like chromosomes.The bead-like thickening called chromomeres are found all along the length of chromosomes.The number , size and position of chromomeres is constant.The chromosomes are arranged parallel and well separated but at the end of leptotene the homologous paternal maternal chromosomes come close together at a point . This condition is known as bouquet stage
  2. Zygotene (zygonema):- The chromosomes each derived from one parental gamete nuclei (homologous chromosomes) come together and form pairs. This process of pairing is called synapsis and the resulting structure synaptic complex. The process pairing starts one or many points and extends in a zipper-like manner across the whole length of the homologous chromosomes. Synapsis is either:- Procentric (starting at the Centromere) or Proterminal (starting at the end) or Localized/random (starting at various points).
  3. Pachytene (pachynema)The paired chromosomes are now called a bivalents. They become shorter and thicker. Each of the homologous chromosomes in meiotic prophase I consists of two closely apposed sister chromatids, thus each bivalent contains four chromatids, and is also called tetrad (two chromosomes of two chromatids each, or four total chromatids). At this stage, large recombination nodules appear at intervals on the synaptonemal complex. These recombination nodules are believed to mediate for chromosomal recombination -crossing over. Two non-sister chromatids (chromatids of different chromosomes of a homologous pair) of each tetrad get coiled around each other and exchange Segments. For this, transverse breaks occur at the same level of non-sister(of the homologues) chromatids, exchanged their parts mutually at one , two, or many points between non-sister(of the homologues) Such points where the chromatids physically contact each other are called chiasmata at this stage the chromosomes appear as X shaped structure. During the formation of chiasmata the chromatids first break due to the action of an enzyme called endonuclease.The broken chromatid segments mutually exchange with each other and get united by the action of enzyme ligase. The formation of chiasmata leads to the exchange of genetic material and results in the recombination of characters , this process is known as crossing over. It is responsible for the origin of new species and thus leads to evolution

  1. Diplotene (diplonema)The synaptonemal complex dissolves allowing the two homologous chromosomes of the bivalent to pull away from each other .The separation is however not completed. The homologous chromosomes remain attached at one or more points where crossing over has occurred. These points of attachment are called Chiasmata by Janssens (1909). This state is marked by the formation of cross-like structures, single or multiple loops. The chromosomes decondense and engage in RNA synthesis. Due to crossing over genes from one chromosome(parental) get exchanged to genes of other chromosomes resulting in the formation of new gene combination -this process is extremely important for creating genetic diversity.
  2. Diakinesis Diakinesis is marked by terminalisation of chiasmata (displacment of chiasmata):-the chiasmata begin to move towards the chromosome ends. RNA synthesis stops and the chromosomes condense, thicken, and get detached from the nuclear envelope. Each pair (bivalent) of chromosomes has four chromatids and they have a centromere attached in the center holding the four strands together.whereas non-sister chromatids of homologous chromosomes are linked by chiasmata. Single chiasma on bivalent forms cross, two and three chiasma prouce rings and loops respectively. Nucleolus disassociates and nuclear envelope dissolves. Some spindle fibers are forming and some are attaching to the centromeres of the chromosomes. The fibers extend from each pole of the cell.
  1. Metaphase IThe bivalents become arranged in the plane of the equator forming the equatorial plate (Fig. 11.7). The centromere of each chromosome is directed towards the opposite poles and the arms of chromosomes face the equatorial plate. The two chromatids, from each chromosome, function as a single unit.

  1. Anaphase I—One entire chromosome, consisting of two chromatids, migrates from the equator to a pole. From each tetrad, two chromatids joined by their centromere, move as a unit (dyad) to one pole of a spindle ,while remaining two chromatids bound by their centromere migrate to opposite pole. Thus each pole receives half the number of chromosomes or the haploid set of the chromosomes. Thus, actual reduction in number of chromosomes occurs. The movement of chromosomes is brought about by the shortening of spindle fibres, similar to that in during mitosis. When sister chromatids go to same pole it is called reductional or disjunctional division ; on the other hand, when they separate and go to two poles it is an equational division as in mitosis.
  2. Telophase I—The nuclear membranes are formed during this stage by the endoplasmic reticulum around the groups of daughter chromosomes with the appearance of one nucleolus in each nucleus. It results in the formation of two daughter cells each with haploid number of chromosomesand only half amount of DNA.

Cytokinesis It occurs by cell wall formation in plants.But in many plants cytokinesis doesnot take place and cell directly passes into meiosisII.

  1. Meiosis II- equational division First meiotic division is followed second meiotic division with or without intervening interphase. Meiosis II is necessary due to crossing over, the chromatids are not identical and must be separated similar to mitosis and is also called equational division It consists of a transient interphase II,in which S phase is absent- no further DNA replication occurs, followed by :-
    1. Prophase II:- Chromosomes of both nuclei become shorter and thicker. The two-stranded nature becomes apparent once again and the nuclear membrane disappears.

  1. Metaphase II Spindle formation takes place. The chromosomes become oriented on the equatorial plate and have the same relationship to the spindle as in mitosis. The spindles in meiosis II are oriented at right angle to that in meiosis I.
  2. Anaphase II The centromere divides and the two sister chromatids of each chromosome separate and move towards the poles. After separation, each chromatid behaves as a chromosome. Thus, a chromosome has one chromatid before and two chromatids after replication.
  3. Telophase II At this stage, the four groups of chromosomes become organised into four haploid nuclei. The chromosomes return to the interphase condition. The endoplasmic reticulum forms the nuclear envelope around the chromosomes and the nucleolus reappears due to association of rRNA with ribosomal proteins synthesised on rDNA templates. Each nucleus at this stage contains the haploid number of chromosomes and forms four cells.
  4. Cytokinesis occurs and the two nuclei are separated as in mitosis.

Significance of Meiosis

  • The meiosis maintains a definite and constant number of chromosomes in the sexually reproducing organisms by producing haploid gametes.
  • During synapsis of the homologous chromosomes, pieces of chromosomes can exchange with one another. This allows for exchange of genetic information. The ultimate result is to increase genetic variability. By crossing over, the meiosis provides an opportunity for genetic variation through gentic material by crossing over and random distribution of maternal and paternal chromosomes.
  • Variation is necessary for natural selection. natural selection favors individuals with characteristics that are best adapted to their environments. Variation is therefore necessary for species to become adapted to their environment and it enables them to change when the environment changes.

Differences between mitosis and meiosis

Mitosis Meiosis
There is only one set of divisions during mitosis. Chromomeres not visible There are two sets of divisions in meiosis, called meiosis I and meiosis II. Chromomeres visible
Homologous chromosomes are not paired during mitosis> Homologous chromosomes are paired during meiosis.
Crossing over not occurs during mitosis. Crossing over occurs during meiosis. chiasmata are formed due to this process
Two daughter cells are formed during mitosis. Four daughter cells result from meiosis.
Number of chromosmes present in daughter cells equal to that in parent cells. Number of chromosomes present in daughter cells half of that in parent cell Only one each pair of homologous chromosomes present in daughtercells.
Mitosis occurs in most cells (somatic cells), at least those capable of division ( meristems in plants). Meiosis occurs in sex cells.
The function of mitosis is growth and repair. The function of meiosis is for sexual reproduction.