Meiosis is a special type of cell division, in which the genetic material from one cell is divided into four cells. Meiosis produces gametes or spores containing one set of haploids. The gametes combine at the time of fertilization to supply a diploid set of chromosomes. This leads to the fusion of gametes with many unique combinations of maternally and paternally derived chromosomes among the haploid complement.
Meiosis is the main reason behind variation. Crossover, which occurs in meiosis, leads to genetic exchange between members of every homologous pair of chromosomes. This leads to the production of offspring that always differ from either parent.
We divide Meiosis into Meiosis 1 or reductional division and meiosis 2 or equational division. Meiosis 1 has five substages:
Figure 1 - Meiosis I Stages And Processes Explained
The chromatin condenses and coils into visible chromosomes. Each member of a homologous pair of chromosomes undergoes synapsis. Synapsis is that process where the pairing of homologous chromosomes occurs. This is often where crossover happens.
Prophase 1 is further divided into:
The chromosomes are now in a shortened form and are considerably thick as well. The tetrad is arranged on the central metaphasic plate and spindle fibers now attach these chromatids and are ready to pull the pair towards opposite poles.
Each pole pulls half the tetrad towards it. A series of dyads (one-half of every tetrad) equivalent to the haploid number is present at each pole. If crossover had not happened, each dyad at each pole would have contained paternal or maternal chromatids. But the exchange produces mixed chromatids of paternal and maternal origin.
Nuclear membranes form around the dyads. This phase is brief. The cell moves on meiosis 2.
Meiosis 2 also has prophase, metaphase, anaphase, and telophase. In prophase 2, each dyad containing a pair of sister chromatids is attached by a standard centromere. In Metaphase 2, the centromeres are placed on the metaphase plate. In Anaphase 2, the sister dyads are pulled towards the other poles.
The number of binary groups is adequate for the number of haploids. Telophase 2 is where one member of every pair is present at each pole. So now, each chromosome is a monad. Following telophase 2, cytokinesis occurs.
So, four haploid gametes are formed at the end of one meiotic event. The haploid state is achieved after the completion of the Meiosis 2