Because this particular step includes so many events, it is further subdivided into six substages, the first of which is leptonema. The dividing cell may spend more than 90 percent of meiosis in Prophase I. Prophase I takes up the greatest amount of time, especially in oogenesis. Each stage is identified by the major characteristic events in its span which allow the dividing cell to progress toward the completion of meiosis. This first step is further subdivided into four main stages: prophase I, metaphase I, anaphase I, and telophase I. The two-stage process of meiosis begins with meiosis I, also known as reduction division since it reduces the diploid number of chromosomes in each daughter cell by half. For clarity, the process is artificially divided into stages and steps in reality, it is continuous and the steps generally overlap at transitions. While replicating somatic cells follow interphase with mitosis, germ cells instead undergo meiosis. In order to undergo replication during interphase, the DNA (deoxyribonucleic acid, the carrier of genetic information and developmental instructions) is unraveled in the form of chromatin. In preparation for meiosis, a germ cell goes through interphase, during which the entire cell (including the genetic material contained in the nucleus) undergoes replication. Meiosis occurs in the primordial germ cells, cells specified for sexual reproduction and separate from the body’s normal somatic cells. While they occur at different times and different locations depending on the sex, both processes begin meiosis in essentially the same way. The male counterpart is spermatogenesis, the production of sperm. In females, the process of meiosis is called oogenesis, since it produces oocytes and ultimately yields mature ova(eggs). Some twenty years later, in 1911, Thomas Hunt Morgan examined meiosis in Drosophila, which enabled him to present evidence of the crossing over of the chromosomes.īoth males and females use meiosis to produce their gametes, although there are some key differences between the sexes at certain stages. However, it wasn’t until August Weismann’s work in 1890 that the reduction role that meiosis played was recognized and understood as essential. Edouard Van Beneden expanded upon Hertwig’s descriptions, adding his observations about the movements of the individual chromosomes within the germ cells. The process of meiosis was first described in the mid-1870s by Oscar Hertwig, who observed it while working with sea urchin eggs. Problems during meiosis can stop embryonic development and sometimes cause spontaneous miscarriages, genetic errors, and birth defects such as Down syndrome. While parts of meiosis are similar to mitotic processes, the two systems of cellular division produce distinctly different outcomes. However, the primary function of meiosis is the reduction of the ploidy (number of chromosomes) of the gametes from diploid (2n, or two sets of 23 chromosomes) to haploid (1n or one set of 23 chromosomes). As sexually-reproducing, diploid, multicellular eukaryotes, humans rely on meiosis to serve a number of important functions, including the promotion of genetic diversity and the creation of proper conditions for reproductive success. Meiosis, the process by which sexually reproducing organisms generate gametes (sex cells), is an essential precondition for the normal formation of the embryo.
#Steps of prophase full#
You can find the full image and all relevant information here. Editor's note: Katherine Koczwara created the above image for this article.
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