What is Meiosis?
In the process of meiosis cell division, the chromosome actually divides only once but the cell divides twice. In the first case there are two separate divisions, common among more is temporary and total number is reduced from diploid (2n) to haploid(n), while second division just resembles a mitotic division. So, the end product is meiosis always include four cells wheat haploid numbered of chromosomes, commonly referred to as gametes. By this reducing process the chromosome number is maintained in a particular spaces generation after generation.
So, meiosis is a complex cell division process in which the diploid number of chromosomes of the mother cell is reduced to haploid number of chromosomes in the four daughter cells.
Where the Meiosis occur?
Meiosis takes place in the eukaryotic diploid germ cells of the sex organs of plants and animals. These cells are called meiocytes. In case of plants the meiocytes are called sporocytes. The meiocytes of the gonads in animals are called gonocytes, whereas in male it is called spermatocytes and in females is called oocytes.
Types of meiosis: –
Meiosis classified into three types on the basis of the type of cell, where it take place.
i) Initial meiosis or zygotic: -the process of meiosis takes place immediately after the formation of zygote by syngamy. It occurs in lower plants like diatoms and certain fungi and some sporozoan animals.
ii) Intermediate meiosis or Gametic: – the meiotic process occuring at the time of gamete formation. Examples are lowered plans and higher animals.
iii) Intermediary meiosis or Sporic: – at the time of spore formation these meiotic process occur in lower plants as well as flowering plants.
Process of Meiosis: -
The process of meiotic cell division consists of two complete divisions of the diploid cell producing four haploid daughter cells. Those two divisions are fast meiotic division and second meiotic division.
First meiotic division: –
The first meiotic division as the following divisional stages: –
Prophase-I: – the meiotic prophase-I division is very important from cytological point of view and it is the longest of the meiotic stages and it includes six number of sub stages. This sub stages are as follows: –
i) Preleptotene or preleptonema.
ii) Leptotene or leptotena.
iii) Zygotene or zygonema.
iv) Pachytene or pachynema.
v) Diplotene or diplonema.
Description of sub stages: –
Preleptotene or preleptonema: - the short stage before leptotene when the uncoiling of Cromartiin starts with the following events:
i) Decoiling of chromatin starts.
ii) Chromosomes begun to appear as slender thin structures.
Leptotene or leptotena: -the chromosomes appear as single thread like structures bearing chromomeres. Leptotene is the first visible step. The meiocytes together with their nuclei appear longer and prominent than the neighbouring cells. The events are:
i) nucleus become dehydrated, chromatin network becomes lose to form thin, uncoiled slender chromosomes.
ii) Chromosomes have a definite configuration and they are granulare chromomeres are arranged at irregular intervals along their length.
iii) The chromosomes clump at one side of the nucleus in some plant cells.
iv) In animals cells, the bouqute stage occur due to specific orientation of chromosomes in such a way that their converged ends from multiple loops of chromosomes on one side of the nucleus.
v) The animals cells, the divided centrioles gradually move towards the, apart of 180°.
vi) On reaching the two opposite poles, each centriole duplicates into two but remain within the single diplosome.
Zygotene or zygonema: - it is a stage in which pairing of homologus chromosomes and formation of synaptonemal complex takes place.
Pachytene or pachynema: -it is the stage in which crossing over and recombination of chromosome parts or genes between homologous chromosomes takes place. Crossing over is a biological process of exchange of the corresponding parts of homologous chromosomes. The nucleus still remains attached with the nuclear organisation region of a particular chromosome in this important stage of the meiosis.
Diplotene or diplonema: -this is the stage of meiosis in which the non-sister homologous chromosomes touch each other followed by their separation. The chromosomes become more shorter, thicker and the nucleus is less distinct. The chromatids show some amount of infolding and RNA synthesis.
Diakinesis: -this is the last upstage of meiotic prophase-I, in which reduction in number of chiasmata take place. Nucleolus gets detached from the nucleus organiser of a particular chromosome and it ultimately disappears. At the end of this stage the nuclear membrane disappears.