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Unformatted text preview: Mitosis and the Cell Cycle - 1 Growth and reproduction are two of the characteristics of life. Cell division is the process by which all the cells of a multicellular organism are formed during growth and development. Cell division is used for replacement of cells and tissues during one's lifetime. Asexual reproduction, a means of making more individuals for many groups of organisms, is also accomplished by cell division. For those organisms that have sexual reproduction in their life cycle, a special cell division, in which chromosome number is reduced, also occurs. Asexual Reproduction Growth and Development Tissue Replacement We shall focus on the processes of cell reproduction in eukaryotic organisms. The process of cell division in prokaryotic organisms, all of which are unicellular organism, is called binary fission, and will be briefly illustrated; the single molecule of DNA and absence of a nucleus in the prokaryotic cell account for a number of differences in the "mechanics" of the process by which prokaryotes divide and increase their numbers. The collection of one's genetic information, or DNA, is known as the genome. In eukaryotic organisms, the genome consists of a number of chromosomes. Each species has a fixed chromosome number, a number that does not change from generation to generation. For organisms that have a sexual reproduction in their life cycles, their genome includes two sets of genetic information, one set contributed by each parent at fertilization. Cells having two sets of genetic information are said to be diploid. ( Details later ) Our genetic molecule, DNA, is identical in each cell within a multicellular organism, so that when cells divide, new cells formed must have exactly the same DNA as the original cell. To ensure that chromosomes and DNA remain the same in new cells (the genome remains constant) when cells divide, it is crucial to have a mechanism that exactly duplicates (replicates) the DNA of the original cell and distributes, or segregates, the copied DNA equally to the new cells. To form new cells, we must also divide up the cytoplasm and critical organelles, such as mitochondria and chloroplasts, of the original cell into the new cells formed. Cells must also "know" when it's time to divide; there must be appropriate signals to initiate the process, and checkpoints to ensure that cell division is proceeding accurately. Mitosis and the Cell Cycle - 2 We will look at the mechanism by which cells duplicate DNA in a later unit. At this time we shall focus on the eukaryotic cell cycle, which includes mitosis, the process in eukaryotic organisms by which the duplicated chromosomes are equally distributed to new nuclei, cytokinesis, the distribution of the cytoplasm of the original cell into new cells, and also look at some of the controls of cell division. In a later unit we will see how normal cell division controls are affected by cancer. We will also address in our next section the process of meiosis, which reduces...
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