Meiosis is a type of cell division in which the cell divides into two daughter cells possess half the number of chromosomes. Meiosis completes in two divisions called the first meiotic division and the second meiotic division. The first meiotic division is preceded by the interphase in which the duplication of DNA takes place just like the mitosis.
Meiosis I or first meiotic division
The prophase of meiosis I is long and is divided into many stages that are
At this stage the chromosomes become visible. Each chromosome contains two sister chromatids that cannot be distinguished at this time. Initially, the chromosome appears as a thread one and if which is attached to the nuclear membrane. In leptotene, the chromosomes become shorter and thicker.
b. Zygotene. At this stage, the chromosomes pairs come and lay parallel to each other and are carefully opposed. This pairing is called synapsis or conjunction.
c. Pachytene. At this stage, both chromatids of each chromosome become specific and the bivalent now has four chromatids and is called a tetrad. Tetrad consists of two central and two peripheral chromatids. The central chromatids become coiled over one another and this is called crossing over. At the site where the chromatids combine become adherent and the factors of adhesion are called chiasmata.
d. Diplotene. At this stage, the bivalent chromosomes now try to move apart. Because they try to move the chromatids break at the site of the crossing and the lost parts become mounted on the opposite chromatids. This stage results in an exchange of hereditary materials between the chromatids. A study shows that the crossing over of the genetic materials each of the four chromatids of the tetrad now has a distinctive hereditary content.
After the diplotene stage, the metaphase comes. The 46 chromosomes become attached to the spindle at the equator in which the two chromosomes of the pair present near to each other.
The anaphase of Meiosis I is different from the anaphase of mitosis. The main difference is that in the anaphase of meiosis I there is absolutely no splitting of the centromeres. One whole chromosome of each pair goes to each pole of the spindle. As a result of this, the daughter cells have 23 chromosomes, each composed of two chromatids.
The telophase of meiosis I is comparable to that in mitosis.
The first meiotic division is then accompanied by a brief interphase. This brief interphase usually differs from the usual interphase for the reason that there is no duplication of DNA. The reason for no duplication is that the duplication is already done in the first meiotic division.
Meiosis II or second meiotic division
The meiosis II is comparable to mitosis because in meiosis II there is absolutely no reduction in chromosome number. However, in meiosis II the DNA content of the daughter cells is reduced to 50%. Due to the crossing over that occurs during the first meiotic division the daughter cells are not identical in genetic material.
Due to the following reasons, meiosis II is different from the typical mitosis.
At this time the 46 chromosomes that contain 23 pairs, one chromosome of each pair being produced from the mother and one from the father. During the first meiotic division, the chromosomes derived from the mother and father are distributed randomly between the daughter cells.
This process and the process of crossing over leads to the thorough shuffling of the genetic material to the cells produced. And due to this reason, the daughter cells have a definite genetic content.
CHROMOSOMAL SEX AND SEX CHROMATIN
We’ve seen that every cell of the individual male has 44+X+Y chromosomes; and that each cell of a lady has 44+X+X chromosomes. We have also seen that through the formation of gametes by meiosis the chromosome number is reduced to half. Because of this, all ova contain 22+X chromosomes. Spermatozoa are of two types. Some have the chromosomal constitution 22+X and others have the constitution 22+Y.
If an ovum is fertilized by a sperm bearing an X-chromosome the resulting child has (22+X)+(22+X) = 44+X+X chromosomes and is a girl. On the other hand, if an ovum is fertilized with a sperm bearing a Y-chromosome the kid has (22+X) +(22+Y) = 44+X +Y chromosomes and is a guy. Of both X-chromosomes in a lady, only 1 is functionally energetic.
The other (inactive) X-chromosome forms a mass of heterochromatin that lies slightly below the nuclear membrane. This mass of heterochromatin can be identified in suitable arrangements and can be handy in identifying whether a particular tissue belongs to a male or a female.
As a result of this association with sex, this mass of heterochromatin is named the sex chromatin. It is also called a Barr-body following the name of the scientist who found out it.
In a few cells, the sex chromatin occupies a different position from that described above. In neurons, it forms a curved mass lying very close to the nucleolus and it is, therefore, called a nucleolar satellite. In neutrophil leucocytes it could show up as an isolated round mass mounted on the rest of the nucleus by a narrow band, thus resembling the looks of the drumstick.
Rarely, a lot of people may have significantly more than two X-chromosomes. In such cases, only one X chromosome is energetic (and therefore euchromatic) while others are represented by the public of heterochromatin.
Thus in a person with three X-chromosomes, two public of sex chromatin are seen. In some cases, the sex of a person may not be clear (at delivery) because of abnormalities in the genital organs.
In such cases, the real sex of the average person may be determined by looking for sex chromatin. Methods are also designed for identifying the Y-chromosome in cells. However, a very important thing to do is to make a karyotype.
The abnormal separation of chromosomes in the first meiotic division or sister chromatids in the second meiotic division is called nondisjunction. Nondisjunction can result in the production of gametes that have few or more chromosomes than normal.
Non-disjunction can cause the following conditions
1. Down syndrome
2. Patau syndrome
3. Edwards syndrome
4. Klinefelter syndrome
5. Turner syndrome
6. Triple X syndrome
7. XYY syndrome.