The Meiosis term is given by Farmer and Moore in 1905. Meiosis-cell division is also known as Reduction division.
Meiosis is a division where a single cell divides twice to produce four daughter cells containing half the original amount of chromosomes. These daughter cells are our sex cells – sperm in males, eggs in females.
It includes two divisions –
Reductional division, also known as Heterotypic division is the first Meiotic division.
Equational division, also known as Homotypic division is the second Meiotic division.
First Meiotic division
It separates homologous chromosomes producing two haploid cells (n), thus it is also known as Reductional division.
The meiotic division is further divided into-
It is the longest phase of meiosis. It is further divided into 5 substages- Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
It is the first stage of Prophase I.
Also known as Leptonema.
In this stage, individual chromosomes each consisting of 2 sister chromatids change from the diffused states they exist during the cell period of growth and gene expression.
The two sister chromatids are still to be slightly bound that they are indistinguishable from one another.
The chromosomes become visible as long thread-like structures.
It is of very short duration.
Also known as Zygonema.
Here the chromosomes line up with each other forming homologous chromosomes pair. This is known as pairing or synapsis. There are various points of contact known as the synaptonemal complex.
The paired chromosomes are known as Bivalent or tetrad.
Also known as Pachynema.
Here crossing over occurs. Non-sister chromatids of homologous chromosomes may exchange segments at the synaptonemal complex. The exchange of information between non-sister chromatids results in the recombination of chromosomes.
Also known as Diplonema.
Here, homologous chromosomes begin to separate from one another. Chromosomes themselves uncoil a bit. The homologous chromosomes of each bivalent remain tightly bound at the chiasmata. The chiasmata remain on chromosomes until they serve as in Anaphase I.
It is the final substage of Prophase- I.
Here the 4 parts of the bivalents are actually visible effectively as very short and thick. Overlapping chiasmata is also clearly visible.
The nuclear membrane and nucleolus disappear and the formation of the spindle starts.
The bivalents chromosomes move towards the equatorial plane. Each pair of chromosomes has one centromere on each side by which it attaches to the spindle fiber.
At this stage, the chromosomes which were arranged at the equatorial plane get separated from each other. The pair of chromosomes after separation move to the opposite pole.
Two daughter nuclei each with a pair of chromatids are present at the poles thus the two daughter nuclei contain haploid no of chromosomes are formed.
Each nucleus then may rest for a while (Interphase) after which it passes on the second division which is mitotic in nature.
Second Meiotic division
Meiosis II is similar to mitosis which is why it is also known as meiotic mitosis.
It is also divided into 4 stages-
Chromosomes appear as double structures.
Nucleolus and nuclear membrane disappear.
It is similar to the prophase of mitosis except that here haploid no of chromosomes are under process.
Chromosomes arrange themselves in the equatorial plane and the centromere gets attached to the spindle.
The behaviour of chromosomes is the same as in the case of Anaphase I.
Four daughter nuclei are formed each with haploid no. of chromosomes.
Significance of Meiosis
- Meiosis is responsible for the formation of gametes or sex cells.
- It maintains the constant no. of chromosomes from generation to generation by reducing the chromosomes no. by halves in the gametes.
- It brings genetic variations due to the process of crossing over. Variations play an important role in evolution.
- Chromosomal mutations are introduced by meiosis.
- Recombination at meiosis plays an important role in the repair of genetic defects.
- Meiosis helps in the segregation, assortment, and recombination of genes.
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