Today we will discuss another very important cell function, which is known as Meiosis Cell Division. Meiosis cell division do occur in plant cell and as well as in animal cell. Living organisms produce gametes by this process. It is very complicated too. So let us check out what happens in the process, which is described below.
Definition: Meiosis is the germ mother cell division in which the chromosome number of the mother cell is reduced to half in each daughter cell (reductional division).
Site of occurrence: In plant meiosis occurs in the microspore mother cell and megaspore mother cell and in animal it occurs in the germ mother cell of the testis and ovary.
- Leptotene (lepto-thin/tene-band):
- Chromosomes start to dehydrate
- It appears as distinct longitudinal threads
4. Each chromosome has a beaded appearance
5. The appearance is for the chromomeres
6. In some insects the chromosomal ends are arranged in a particular direction. This is called a bouquet stage.
N.B. Draw two centrioles distinctly.
Homologous chromosomes approach each other and are arranged along their longitudinal axis this phenomenon is called synapsis. This pairing is accomplished through a unique ultrastructure compound, called synaptonemal complex.
Two homologous chromosomes together called bivalent or diad, since there are 2n number of chromosomes in the cell.
Each member of bivalent chromosome gets longitudinally split up and forms chromatids thus a bivalent bears four chromatids called tetrad.
The non-sister chromatids of a tetrad now experience a force of attraction between them and as a result they tend to approach each other at one or more points.
The non-sister chromatids overlap each other along more than one point and gets so re-constituted that each shares the part of the other. This process is called crossing over and the points of crossing over are known as chiasmata.
As a result of crossing over the mode of arrangement of genes on the chromosomes are re-shuffled leading to the variation.
Suddenly repulsion is observed between the chromosome, which first starts at the centromeric zone and moves apart.
Chiasmata starts moving towards the terminal ends of these chromosomes, this phenomenon is called terminalization.
Chromosome shrinks and dehydrate.
Nuclear membrane and nucleolus are on the verge of disappearance.
Spindle or mitotic apparatus also at the end of completion. The chiasmata reach the terminal end of the chromosome.
Chromatids further shrinks and thickens.
Mitotic apparatus is fully formed.
Chromosomes with their chromatids are placed to the equatorial position of a cell through the centromere, and they are easily seen under the microscope, and they are easily countable.
Due to contraction of spindle fibers half of the chromosomes start to move towards one pole and another half towards opposite pole. This is the polar migration of chromosome.
They appear ‘V’, ‘L’, ‘J’ and ‘I’ shaped.
This is called as disjunction.
Polar migration is completed.
Thus two daughter nuclei are formed.
Types of Meiosis:
- A) Terminal or Gametic meiosis: It is characterized by diploid individual forms haploid gametes by meiosis during gametogenesis in the gonads. Then haploid gametes fuse to form diploid zygote, which by mitosis forms diploid adult. All animals except Protozoans
Diploid generation (2n) à gametic meiosis à gametes (n) à fertilization à zygote (2n) à growth à Diploid generation (2n).
B) Initial or Zygotic meiosis: It is characterized by haploid individual forms haploid gametes by mitosis. Then the haploid gametes fuse and form diploid zygote, which undergo meiosis followed by mitotic division to form haploid adult. Algae and Protozoans.
Haploid generation (n) à mitosis à gametes (n) à fertilization à zygote (2n) à zygotic meiosis à spores (n) à growth à Haploid generation (n)
C) Intermediary or Sporogenetic meiosis: Diploid sporocytes or spore mother cells of sporophytic plant, undergo meiosis to form the haploid spores in the sporangia. Haploid spores germinates to form haploid gametophyte, which produces the haploid gametes by mitosis. These haploid gametes fuse to form diploid zygote, which develops into diploid sporophyte by mitotic divisions. Bryophytes and Pteridophytes (alternation of generation in moss and fern)
Sporogenic Meiosis Growth
Diploid generation (2n) Haploid generation (n)
Growth Mitosis (n)
Zygote (2n) Fertilization Gametes
Significance of Meiosis:
- Meiosis produced haploid gametes.
- Meiosis helps to maintain the species specific chromosome number unchanged through out the generations in the sexually reproducing organisms.
- During crossing over genes recombine to give rise to variations leading to organic evolution.
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