Meiosis is the process in which a cell reduces its cell from being a diploid, having two sets of chromosomes, to being a haploid, having one set of the twenty-three chromosomes, when creating eggs and sperm. A haploid cell will combine with another haploid cell (one from an egg and one from a sperm) to form a diploid cell. This will result in the right number of chromosomes, 46. During this process, abnormalities of the number of chromosomes usually occur in humans.
The Process of Meiosis
This process happens in the cell nucleus, where there is a pair of chromosomes from each parent. Chromosomes replicates when the chromosomes from both parents are copied and paired to exchange portions of their own DNA. Chromosomes, that are similar, will pair up. The paired chromosomes will swap portions of their DNA, creating a mix of new genetic material in the offspring's cell. When the nucleus divides up into daughter cells, the chromosome pairs are divided. The daughter nuclei will divide again, resulting in further division of chromosomes. The daughter nuclei will end up having single chromosomes and a new mix of genetic material.
The Steps of Meiosis
Unlike mitosis, meiosis undergoes two replications rather than one. It is because of these two divisions that allow for the resulting daughter cells to end up with only half of the number of chromosomes, or haploid, from the total amount of chromosomes, or diploid. The steps goes as follows:
Prophase I: just like in mitosis, during the prophase step, DNA is condensed into very thick like rodes. In additon the nuclear membrane or envelope along with the nucleoli dissappear which allows for free DNA roaming. The spindle apparatuses begin to form ready for the metaphase step. One difference between mitosis prophase and meiosis prophase I is that as the DNA condenses, the chromosomes are visible as tetrads, in other words, homologous DNA pairs, which is not seen in mitosis.
Metaphase I: The tetrads of chromosomes are lined up on the equator of the cell with the spindle apparatus having already been completely formed. It is during prophase I and metaphase I where genetic recombination occurs. When the homologous pairs of DNA, or tetrads, are first lined up against each other, a event called crossing over occurs at the chaismata. Crossing over events happen between neighboring tetrads which swap genetic information with each other. More genetic variation comes from metaphase I specifically in the sense that tetrads can line up completely randomly on the equator of the cell, also known as independent assortment. It is these two events that derive most of genetic variation.
Anaphase I: The tetrads of chromosomes that were previously lined up on the equator are pulled apart in this phase. They are pulled apart in such a fashion that the spindles that are attached to the centromere of all the tetrads lined up on the equator and are pulled apart to each pole on either side of the cell, towards the centrioles. The resulting chromosomes are now two chromatids.
Telophase I: In this phase the chromosomes with two chromatids that have moved to eeither side of the cell decondense and a nuclear envelope begins to form around the genomic material.
Prophase II: The chromosomes that have two chromatids from the previous meiosis I cycle recondense, with their nuclear envelope and nucleoli disappear similar to prophase I. In addition the spindle forms in this phase.
Metaphase II: The chromosomes with the two chromatids line up at the equator similar to how they lined up in metaphase I. Although genetic variation is seen in this phase as well as it was seen in metaphase I, the degree is very much diminished. In prophase I, crossing over occurred, however in prophase II, since homologous chromosomes are non existent in this phase, the chromatids cannot react with any neighboring pieces of DNA. The only source of variation in this phase is independent assortment of genes on the equator of the cell. The possibility of chromatids either going left or right on the cell and being chosen for fertilization is the source of variation here.
Anaphase II: The chromosomes split by the same mechanics explained in Anaphase I except only one chromatid head moves towards each pole of the cell.
Telophase II: This phase has the same exact mechanics explained in Telophase I. The only difference is that instead chromatids being de-condensed with a nuclear envelop developing around it, it is solely a chromatid head. The cell is now ready to develop into sperm or eggs at this stage.
Mutations and Birth Defects
According to Angelika Amon, a molecular biologist from the Massachusetts Institute of Technology in Cambridge, the leading cause of human birth defects and miscarriages are the mistakes in dividing DNA between daughter cells during meiosis. Miscarriages occur when embryos have an incorrect number of chromosomes and do not go to full term.
The likelihood that chromosomes will not be apportioned properly increases with age in women. Studies have shown that one out of every eighteen babies born to women who are over the age of forty-five has three copies of chromosomes 13, 18, or 21 instead of the normal amount of two. This can lead to birth defects or mutations. An example is that Down Syndrome is caused by three copies of chromosome 21.
Amon studies yeast cells, which separate their chromosomes almost exactly the same way cells of humans do. The exception is that yeast cells' chromosomes separate much faster. A yeast cell copies its DNA and produces daughter cells in about half an hour. Humans cells, on the other hand, takes about a whole day.
Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman and, 2006. Print.