In other words, gametes have only 23 chromosomes (referred to as the haploid number (1n). In order for the fertilized egg to end up with the correct diploid number, sperm cells and eggs must be produced such that each has only one chromosome from each pair. The 46 chromosomes are referred to as the diploid (2n) number, because there are two of each. Recall that humans have 22 pairs of homologous chromosomes and one pair of sex chromosomes one member of each pair came from the mother, and the other from the father. Meiosis is the specialized process by which gametes (sperm and eggs) are produced for sexual reproduction in the ovaries and testes. This is discussed in greater detail in the online module on cancer. Loss of control mechanisms can result in abnormal cell division and a progression to to tumor formation. The cell cycle is normally carefully controlled by a number of biochemical mechanisms. M, which stands for mitosis, i.e., when the cell actually divides into two identical cells.G2, when there is continued cell growth.S, when synthesis of new DNA (replication) takes place.G 1, when the cell grows in size in preparation for division.The phases in dividing cells are as follows: These are cells in the G 0 phase this is sometimes called the "resting phase," but these cells are actively functioning, and they are resting only in the sense that they aren't replicating. Many of our cells are mature functioning cells that are not actively dividing. The Cell CycleĬells normally follow a carefully controlled cell cycle, depicted below. Benign and malignant tumors also growth through mitosis. This provides a way of replacing cells, such as skin cells the epithelial cells that line the respiratory, digestive, and urogenital tracts and blood cells. These "stem cells" divide by mitosis and produce daughter cells which then differentiate into a particular cell type. In humans some cells retain the capacity to divide throughout life. It is the process by which new cells are formed in the growing embryo and after birth, and mitosis also replaces cells that have died or been shed. Mitosis is the process by which eukaryotic cells replicate by dividing into two genetically identical cells. A similar process is used to replicate mitochondria within eukaryotic cells, but the overall process of cell replication in eukaryotes is more complicated (see below). The cell membrane then begins to invaginate and eventually separates into two genetically identical bacteria. The single chromosome replicates and each copy attaches to a different location on the cell membrane. Prokaryotes reproduce by the relatively simple process of binary fission.
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