Cell Division- meiosis
Introduction – The term meiosis coined by J.B Farmer and J.F Moore, and the cell in meiosis was found termed meiocytes.
In sexual reproduction, the production of offspring takes place by the fusion of gametes with a haploid set of chromosomes. This division is also called reduction division because the number of chromosomes gets reduced by half in daughter cells. In plants and animals, meiosis is found during gametogenesis.
Some important features of meiosis–
- By this type of division, the number of chromosomes remains constant in a species generation after generation. For example-onion cell-16, potato-48, horse-64, man-46 number of chromosomes.
- It takes place during sexual reproduction organisms form gametes such as sperms in males and ova in females. The male and female gametes fuse to form the zygote. which develops into a new individual.
- It is categorized by two divisions as meiosis 1 and meiosis 2.
- the pairing of homologous chromosomes takes and recombination between non-sister chromatids of homologous chromosomes takes place during meiosis and the formation of four haploid cells takes place after meiosis 2.
Meiosis 1– same as in mitosis, Meiosis-I also consists of four stages; Prophase-I, Metaphase-I, Anaphase-I, and Telophase-I.
Prophase-I – The Prophase-I stage of Meiosis-I is much longer as compared to the prophase of mitosis. It is subdivided into five sub-stages as-Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
Leptotene (GK ‘leptos’ – thin; ‘tene – thread)
- In this stage, the chromosomes become distinct and appear as long and thin threads having beads due to condensation (coiling of DNA) at specific points.
- Two chromatids held together by a centromere of each chromosome, visible in the light microscope.
Zygotene (GK. ‘Zygos’-pairing)
- In this stage, Chromosomes continue coiling and become thicker and shorter.
- During this stage, homologous chromosomes start pairing together from one end and this process of association is called synapsis.
- Nuclear membrane and nucleolus are distinct
- Through the synapsis formation of a complex structure takes place called the synaptonemal complex.
- Each pair of homologous chromosomes is termed bivalent.
Pachytene (GK. ‘pachus’ – thick)-
- Due to further coiling, the chromosomes become shorter and thicker.
- ‘bivalent’ pair shows four chromatids hence bivalents are also known as tetravalent
- Crossing-over occurs at the end of pachytene means to break and the exchange of genetic material occurs between two homologous chromosomes.
- At the chiasmata, the exchange of segments of chromatids (genetic material) between homologous chromosomes has taken place. The process of gene exchange is known as genetic recombination.
- Chromosomes continue coiling and become shorter and paired homologous chromosomes having apparent repulsion takes place in the region devoid of chiasmata.
- Chiasmata formation – Interchange and re-joining at the point of crossing over appears X-shaped and is known as chiasma, plural-chiasmata.
- Nucleolus and nuclear membrane become indistinct.
Diakinesis (dia = through, in different directions, kinesis = motion)
- The bivalents become shortest and thickest due to maximum coiling and during staining stains quite deeply.
- Terminalisation of chiasmata takes place.
- The nuclear membrane and nucleolus disappear.
- By the end of this phase, spindle fibers have organized for the establishment of poles, from here orientation of chromosomes for metaphase 1 takes place.
- The bivalent chromosomes arrange themselves at the equatorial plate.
- At this time the centromeres are pulled, each towards one pole, and, therefore the chiasmata on different arms are brought closer.
- The spindle fibers are attached at the centromere of the chromosomes.
- Spindle fibers extend between the poles and these are attached only to the kinetochores of the chromosomes.
- The spindle fibers shorten
- Now homologous chromosomes get separated as chiasmata lose retention influence.
- Centromeres of homologous chromosomes are then pulled along by the spindle fibers towards the opposite poles
- The nuclear membrane and nucleolus reappeared.
- There is an interphase that is short and during interkinesis replication of DNA does not take place.
- Cytokinesis occurs in this phase, The daughter cells now have half the amount of DNA as compared to that at Anaphase-I, that is 2n this is termed as a dyad of cells.
- Now from here, the daughter nuclei enter into the second meiotic division.
Second Meiotic Division i.e. meiosis II (equational)–
Has the same four stages as Earlier-Prophase II, Metaphase II, Anaphase II, Telophase II
- Arms of chromatids are widely separated, no reduction in coiling found.
- Compaction of chromosomes takes place.
- Nucleolus and nuclear membrane begin to disappear and the appearance of spindle fibers in the cytoplasm takes place.
- It is very short in the period.
- The chromosomes get arranged themselves along the equatorial region and arms of chromosomes extend outwards.
Formation of spindle apparatus is completed as each chromatid has its own kinetochore
- The centromere in each chromosome divides so each chromatid gets its own centromere and each chromatid is now a complete chromosome.
- Due to the contraction of spindle fibers daughter chromosomes and begin to move towards the opposite poles.
- Meiosis ends with the stage telophase II, In this phase, the nucleolus and nuclear membrane reappear.
- Chromosomes after reaching the poles, organize themselves into haploid daughter nuclei.
- four haploid daughter cells i.e. tetrad, formed after cytokinesis.
Cytokinesis – occurs in two successive stages, first occurs after meiosis I and then after meiosis II, in some instances, it may occur only after meiosis II. So in the context of cytokinesis four haploid cells are formed after meiotic cell division.
Significance of meiosis
- It helps to maintain constant numbers of chromosomes in different generations of a species undergoing sexual reproduction.
- It also helps to increase genetic variation in a population of a species or organisms that breed sexually.
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