Problems during meiosis can stop embryonic development and sometimes cause spontaneous miscarriages, genetic errors, and birth defects such as Down syndrome. The process of meiosis was first described in the mids by Oscar Hertwig, who observed it while working with sea urchin eggs. Some twenty years later, in , Thomas Hunt Morgan examined meiosis in Drosophila , which enabled him to present evidence of the crossing over of the chromosomes. Both males and females use meiosis to produce their gametes, although there are some key differences between the sexes at certain stages.
In females, the process of meiosis is called oogenesis, since it produces oocytes and ultimately yields mature ova eggs. The male counterpart is spermatogenesis, the production of sperm. While they occur at different times and different locations depending on the sex, both processes begin meiosis in essentially the same way. 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.
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. While replicating somatic cells follow interphase with mitosis , germ cells instead undergo meiosis. For clarity, the process is artificially divided into stages and steps; in reality, it is continuous and the steps generally overlap at transitions. 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.
This first step is further subdivided into four main stages: prophase I, metaphase I, anaphase I, and telophase I. Each stage is identified by the major characteristic events in its span which allow the dividing cell to progress toward the completion of meiosis. Prophase I takes up the greatest amount of time, especially in oogenesis. The dividing cell may spend more than 90 percent of meiosis in Prophase I. Because this particular step includes so many events, it is further subdivided into six substages, the first of which is leptonema.
During leptonema, the diffuse chromatin starts condensing into chromosomes. Each of these chromosomes is double stranded, consisting of two identical sister chromatids which are held together by a centromere; this arrangement will later give each chromosome a variation on an X-like shape, depending on the positioning of the centromere.
In the next substage, zygonema, there is further condensation of the chromosomes. As they come into closer contact, a protein compound called the synaptonemal complex forms between each pair of double-stranded chromosomes.
As Prophase I continues into its next substage, pachynema, the homologous chromosomes move even closer to each other as the synaptonemal complex becomes more intricate and developed.
This process is called synapsis, and the synapsed chromosomes are called a tetrad. The tetrad is composed of four chromatids which make up the two homologous chromosomes. During pachynema and the next substage, diplonema, certain regions of synapsed chromosomes often become closely associated and swap corresponding segments of the DNA in a process known as chiasma. At this point, while still associated at the chiasmata, the sister chromatids start to part from each other although they are still firmly bound at the centromere; this creates the X-shape commonly associated with condensed chromosomes.
The nuclear membrane starts to dissolve by the end of diplonema and the chromosomes complete their condensation in preparation for the last substage of prophase I, diakinesis.
During this part, the chiasmata terminalize move toward the ends of their respective chromatids and drift further apart, with each chromatid now bearing some newly-acquired genetic material as the result of crossing over. When a cell begins to divide by mitosis, it makes a second copy of the DNA in its nucleus. Chromosomes contain this DNA, and humans have 46 chromosomes.
Once DNA production is done, there are still 46 chromosomes, but each is twice the normal size. Next, chromosomes line up across the cell's center and each chromosome splits in half, with one half moving to each end of the cell. Finally, a new membrane forms across the middle of the cell, making two new cells, each containing 46 new chromosomes. In the absence of spontaneous changes to DNA, called mutations, mitosis produces two new cells that are identical to the parent cell. In all your cells, one member of each of the 23 pairs of chromosomes came from your father and one from your mother.
When a developing egg or sperm cell begins meiosis, it doubles the size of each of its chromosome by making new DNA. Then, unlike mitosis where chromosomes divide, in meiosis one member of each pair of chromosomes moves to each end of the cell, which then divides into two new cells.
Called the first meiotic division, the new cells have only 23 chromosomes. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate. Then, in anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.
During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell. Meiosis II is a mitotic division of each of the haploid cells produced in meiosis I.
During prophase II, the chromosomes condense, and a new set of spindle fibers forms. The chromosomes begin moving toward the equator of the cell.
During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells. Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell.
Finally, during telophase II, the chromosomes are enclosed in nuclear membranes.
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