Meiosis is the special type of cell division in which the number of chromosomes in daughter cells is reduced to half, as compared to the parent cell. Each diploid cell after meiosis produces four haploid cells, because it involves two consecutive divisions after single replication of DNA. It takes place in diploid cells only, in animals at the time of gamete formation, while in plants when spores are produced. Two divisions, are meiosis I and meiosis II. The first meiotic division is the reduction division, whereas second meiotic division is just like the mitosis. Both divisions can further be divided into substages like prophase 1, metaphase 1, anaphase 1, telophase 1 and same names are used for meiosis II also.
Prophase I
This is very prolonged phase, and differs from the prophase of mitosis, because in this chromosomes behave as homologous pairs. Each diploid cell has two chromosomes of each type, one member from each parent, because chromosomes have been replicated during interphase. The interphase of meiosis lack G2 stage. These similar but not necessarily identical chromosomes are called as homologous chromosomes. Each diploid cell has two chromosomes of each type, one member from each parent, because chromosomes have been replicated during interphase. The interphase of meiosis lack G2 stage. Prophase 1 further consists of the followings stages :
Leptotene
The chromosomes become visible, shorten and thick. The size of the nucleus increase and homologous chromosomes start getting closer to each other. The chromosomes become visible, shorten and thick. The size of the nucleus increase and homologous chromosomes start getting closer to each other. The chromosomes become visible, shorten and thick. The size of the nucleus increase and homologous chromosomes start getting closer to each other. The chromosomes become visible, shorten and thick. The size of the nucleus increase and homologous chromosomes start getting closer to each other.
Zygotene
First essential phenomenon of meiosis i.e. pairing of homologous chromosomes called synapsis starts. This pairing is highly specific and exactly pointed, but with no definite starting point(s). Each paired but not fused, complex structure is called bivalent or tetrad. First essential phenomenon of meiosis i.e. pairing of homologous chromosomes called synapsis starts. This pairing is highly specific and exactly pointed, but with no definite starting point(s). Each paired but not fused, complex structure is called bivalent or tetrad.
Pachytene
The pairing of homologous chromosomes is completed. Each bivalent has four chromatids, which wrap around each other. Chromosomes become more and more thick. Non-sister chromatids of homologous chromosomes exchange their segments due to chiasmata formation, during the process called crossing over. Pachytene may lasts for days, weeks or even years, whereas leptotene and zygotene can last only for few hours. In this way reshuffling of genetic material occurs which produces recombinations. Pachytene may lasts for days, weeks or even years, whereas leptotene and zygotene can last only for few hours. In this way reshuffling of genetic material occurs which produces recombinations.
Diplotene
The paired chromosomes repel each other and begin to separate. Each bivalent has at least one such point, the chromatids otherwise are separated. Separation however, is not complete, because homologous chromosomes remain united by their point of the interchange. The paired chromosomes repel each other and begin to separate. Each bivalent has at least one such point, the chromatids otherwise are separated. Separation however, is not complete, because homologous chromosomes remain united by their point of interchange. Each bivalent has at least one such point, the chromatids otherwise are separated. Separation however, is not complete, because homologous chromosomes remain united by their point of interchange (chiasmata).
Diakinesis
During this phase the condensation of chromosomes reaches to its maximum. At the same time separation of the homologous chromosomes (started during diplotene) is completed, but still they are united at one point, more oftenl at ends. Nucleoli disappear. During this phase the condensation of chromosomes reaches to its maximum. At the same time separation of the homologous chromosomes (started during diplotene) is completed, but still they are united at one point, more oftenl at ends. At the same time separation of the homologous chromosomes (started during diplotene) is completed, but still they are united at one point, more oftenl at ends. Nucleoli disappear.
