Metaphase Metaphase begins when the nuclear envelope has broken down. As it disappears, more space is created so that the chromosomes can move into position during their division. The sister chromatids align themselves on the microtubules in the middle, or equator, of the spindle and are attached by their centromeres. Anaphase During anaphase, the centromeres split and the sister chromatids are pulled apart and move towards the centrioles at opposite sides, or poles, of the cell as the spindle fibres shorten. Each sister chromatid is now called a chromosome again. Telophase Once the two sets of chromosomes reach their opposite poles, the spindle fibres break down and a nuclear envelope forms around each set of chromosomes. At the same time, the chromosomes uncoil and become invisible through a light microscope. Following telophase, in animal cells, the plasma membrane pinches in and the two new nuclei become separated. Eventually, during cytokinesis, the two sides of the plasma membrane meet and two completely new cells are formed. Each has a complete set of chromosomes, cytoplasm, organelles and a centriole. In plant cells, the cytoplasm divides in a slightly different way. Firstly, a cell plate forms along the centre of the cell, separating the cytoplasm into two regions.Vesicles accumulate at the edges of the cell plate and release cellulose and pectins, which are needed to form a new cell wall. Gradually a cell wall builds up along the cell plate separating the two nuclei and dividing the cytoplasm to form two new cells.
Cyclins Cyclins are compounds that are involved in the control of the cell cycle. Cyclins interact with other proteins called CDKs (cyclin-dependent kinases) to form enzymes that direct cells through the cell cycle and control specific events such as microtubule formation and chromatid alignment. Cyclins were discovered by Timothy Hunt in 1982 when he was studying the cell cycle of sea urchins. In 2001, Hunt, together with Lee Hartwell and Paul Nurse who also contributed to the discovery, were awarded the Nobel Prize in Physiology or Medicine for their work. Understanding factors that control the cell cycle is important in the study of cancer and the way cell division can be disrupted. Cyclins are divided into four types based on their behaviour in vertebrate and yeast cells (Figure 1.32) but some cyclins have different functions in different types of cell.
Mitotic index When studying cells under the microscope, the ratio of the number of cells undergoing mitosis to the total number of cells in view is called the mitotic index. The mitotic index in an important prognostic tool used in predicting the response of cancer cells to chemotherapy – a low mitotic index indicates a longer survival time, and suggests that the treatment is working. It can be less accurate when used with elderly patients, whose cells divide more slowly. In such patients, a low mitotic index may not indicate that a treatment is working. In the laboratory, it is possible to work out the mitotic index of growing and dividing cells from an electron micrograph.
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