Meiosis is derived from a Greek name meioun which means to diminish. The process of meiosis takes place in reproductive or germ cells that produce gamete, sperms and ova. In this division, chromosomes divide once and the nucleus divides twice. As a result of which the number of chromosomes is reduced to half. The number of chromosomes becomes half in each sex cell. It is a reductional division. During fertilization (fusion of male and female gametes) diploid number of chromosomes pairs is restored.
Meiosis has two nuclear divisions
- First meiotic division
- Second meiotic division
In all meiotic divisions, all the cell stages are repeated twice. During meiosis, the diploid cells are reduced to haploid cells.
Diploid (2n) ———- Haploid(n)
First Meiotic Division (Meiosis I)
Homologous chromosomes come together(associate) and later on segregate into daughter cells. The number of chromosomes is reduced from diploid to the haploid state that is why it is known as a reduction division.
Events in first meiotic division
Chromosomes pair and separate
The homologous chromosomes attract each other and come to lie in pairs. The pairing of homologous chromosomes is known as synapsis and the pair is known as bivalent.
Chaismata formation or crossing over
Chromosomes continue to shorten and thicken. Chromosomes split lengthwise into two chromatids. Each homologous pair has four chromatids is known as a tetrad. Non-sister chromatids of a tetrad break open and rejoin each other. This process is known as crossing over or chiasmata formation. The exchange of genes takes place between parental and maternal chromatids of homogenous chromosomes during meiosis. Due to the process of crossing over chromosomes separate out. Thus nucleolus and nuclear membrane disappear. The members of homologous chromosomes separate completely from each other and start moving towards opposite poles. The nuclear membrane reappears which leads to the formation of two daughter nuclei.
Second Meiotic Division (Meiosis ll)
It is very similar to mitosis. During this, the two chromatids of each chromosome separate and starts moving towards opposite poles. The nuclear membrane disappears and four cells are formed. A cell formed is haploid(n) in nature.
Significance of meiosis
- The number of chromosomes is reduced to half in each daughter cell.
- It leads to the formation of haploid germ cells but after fertilization, it restores the diploid number of chromosomes in the zygote.
- It leads to variation in genetic characters.
- It maintains a constant number of chromosomes.
- Crossing over helps to produce a new combination of traits.
- Mutations occur due to irregularities in the cell division by meiosis.