Meiosis I vs Meiosis II
Meiosis is a complex cellular and biochemical process of which the chromosome number is halved during the formation of gametes in an organism. In this process, a diploid cell is converted into four daughter cells, each having haploid number of chromosomes. Meiosis is a special kind of cell division that occurs in the production of sperm cells and eggs. In males, the formation of gametes is referred to as spermatogenesis whereas, in females, it is known as oogenesis. It consists of two nuclear divisions, but only one involves DNA replication, hence resulting daughter cells with only one chromosome of each pair (haploid cells). In humans cells, reproductive cells with 46 chromosomes yields four daughter cells (either sperms or eggs), each with 23 chromosomes. The process steps of meioses can be sub-divided into two major phases; meiosis I and meiosis II. As soon the cells finished undergoing meiosis I, they start undergoing meiosis II. There is no any interphase between these two phases.
Meiosis I is the first phase of meiosis that consists of sub phases including Prophase I, Metaphase I, Anaphase I, and Telophase I. During meiosis I, two homologous chromosomes come near each other and are aligned next to one another. This process is referred to as ‘synapsis’. Since, each homologous chromosome has two chromatids, there are actually four chromatids. Therefore, this arrangement is known as ‘tetrad’. During synapsis, the two homologous chromosomes can swap equal amounts of DNA, so called ‘crossing-over’. Crossing-over makes four completely different unique chromatids, thus increases the genetic variation among gametes of the same organism.
Meiosis II is the second phase of meisos, in which longitudinal division of the duplicated chromatis and further cell division take place. During meiosis I, daughter cells produced by meiosis I continue their further division so that each daughter cell coming from meiosis I produces two gametes. The sub phases of meiosis II are, Prophase II, Metaphase II, Anaphase II and Telophase II. These phases are very much similar to sub phases of meiosis I, with one big exception; prophase II does not have DNA replication unlike the prophase I of meiosis I.
What is the difference between Meiosis I and Meiosis II?
• Meiosis I is the first phase of gamete production while Meiosis II is the second.
• The phases of meiosis I are Prophase I, Metaphase I, Anaphase I, and Telophase I, whereas that of meiosis II are Prophase II, Metaphase II, Anaphase II, and Telophase II.
• Unlike in the meiosis II, the separation of homologous chromosomes called synapsis only occurs in the meiosis I.
• Cross-over only happens during the meiosis I.
• Meiosis-I starts with a diploid parent cell and ends with two haploid cells while meiosis II starts with the resulting two haploid cells and ends with four haploid cells.
• DNA replication occurs during meiosis I while it does not occur during meiosis II.
• Meiosis I separates homologous chromosomes while meiosis II separates sister chromatids.