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Mitosis , Meiosis and the Cell Cycle By: Alexus Greene What is Meiosis? Meiosis is a cell division process for reproduction. During this process sex cells or “gametes” are created. Meiosis is important because without it you will not be able to produce the egg or sperm to reproduce or achieve fertility. Phases of Meiosis Interphase I The cell increases in mass and prepares for cell division. The cell synthesizes protein and continues to grow in size. Prophase I & II During prophase the nuclear envelope and the nuclei disappear, and the spindle starts to form. As DNA condensates the chromosomes become visible. The chromosomes with two chromatids become visible and begin to condense. Metaphase I & II Tetrads line up, and the spindle at this point has been completely formed. Genetic recombination begins, but only begins once there are tetrads. Anaphase I & II Tetrads pull apart and chromosomes with two chromatids move toward the poles. Chromosomes split so that one chromosome with a chromatid can go toward each pole. Telophase I & II Chromosomes with two chromatids de- condense and a nuclear envelope reforms around them. Each nucleus is now haploid. Purpose? At the end of meiosis I, each chromosome still had two chromatids. That is double the amount of DNA that a cell should have. So, the entire reason to go through meiosis II is to reduce the amount of DNA back to normal basically, to split the chromosomes so that each daughter cell has only one chromatid per chromosome. What is Mitosis? Mitosis is a process of cell division that results in two daughter cells and its purpose is to repair and replace die and/or damaged cells. Mitosis is important because our body needs new cells in certain parts of our bodies every second and Mitosis makes sure that it happens. Phases of Mitosis Interphase The DNA replicates and remains loosely coiled “Messy DNA”. The nuclear membrane is stable to protect the DNA molecules from undergoing mutation. Prophase DNA molecules shorten then condense to form chromosomes the nucleolus and nuclear membrane become invisible. The spindle apparatus moves to the opposite poles. Metaphase Spindle fibers attach to centromeres of the chromosomes and align. Anaphase The spindle fibers shorten and the centromere splits, separated sister chromatids are pulled along behind the centromeres. Telophase The chromosomes reach the poles. The nuclear envelope reforms before the chromosomes uncoil. The spindle fibers dissolve. Cytokinesis This is the last stage of mitosis. It is the process of splitting the daughter cells apart. A furrow forms and the cell is split in two. Each daughter cell contains the same number and same quality of chromosomes. Purpose? Before mitotic division DNA is replicated to produce; two identical chromatids attached by a centromere. This is necessary so that each new daughter cell formed receives one copy of each chromosome. What is the Cell Cycle? The cell cycle, or cell-division cycle, is the events that take place in a cell leading to its division and duplication. These events include mitosis, cytokinesis, and meiosis. Cell Cycle Regulation Cycling Cycling undergo a constant cycle of synthesis and splitting during cell division. When cycling are synthesized, they activate proteins and bind to CDKS forming a cyclin-CDK complex; it is a message to the cell to pass to the next cell cycle phase. The cyclins degrade, deactivating the CDK. G1 cyclins G1 cyclins attach to CDK proteins. When the cell reaches the correct size and the cellular environment is correct for DNA replication, the cyclins begin to degrade. G1 cyclin degradation deactivates the Cdk and leads to entry into S phase. Mitotic Cyclins Mitotic cyclins happens during G2. Once they reach a high enough concentration, they can bind to CDK. When mitotic cyclins bind to CDKS in G2, the resulting complex is known as mitosis-promoting factor (MPF). Once the mitotic cyclin degrades, MPF is inactivated and the cell exits mitosis by dividing and re- entering G1. Cancer from Cell Division?? It is a fact that due to an undivided cell, cancer can occur. In the article “Critical pathway in cell cycle may lead to cancer development” the source is Salk Institute for Biological Studies; published July 11, 2013. From this article, I have learned that the cell cycle causes cancer because when the cells divide they become shorter than the original cell; when that happens the cell either becomes inactive or dies. Activating some enzymes can stop this from occurring; this is an enzyme called telomerase. Telomerase prevents the shortening of cells when the enzyme is duplicated, this helps to continue growth which can lead to uncontrolled cell growth, and in result can lead to cancer. 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