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STEM CELLS 101 7 The genome is the organismÊs full complement of genes, or DNA. With the exception of the red blood cells and a few types of cells in the bone marrow, every cell has the entire genome. It is amazing that a cell in the skin, as well as a cell in the muscle, has the complete genome of about 30,000 genes. Meiosis is the process by which germ cells in the ovaries and testes divide to produce „gametes,‰ the generic term for sperm and egg. In meio- sis the cells divide, and then divide again so that the sperm and egg have only half the original number, or 23 chromosomes. Meiosis occurs in phases. In phase one, chromosomes exchange genetic material so that they are not exact copies but may be mixed. In phase two, the cells contain two sets of chromosomes, which divide again to form half the number of chro- mosomes. These are packaged into sperm or eggs with only 23 chromo- somes. At the point of fertilization, natureÊs arithmetic will come out right, with 23 chromosomes from the mother and 23 from the father making 46 chromosomes·the number required to form a human being. STEM CELL CLASSIFICATION Stem cells can also be classified according to their plasticity, or how ver- satile they may be in their development. The classifications may describe a cellÊs commitment to become a particular kind of cell: the totipotent stem cell, the pluripotent stem cell, or the unipotent cell. Totipotent stem cells are the most versatile type. When sperm carrying the 23 chromosomes, chock-full of genetic material, fertilizes the female egg with its 23 chromosomes, the union occurs in the fallopian tube. The fertilized egg is called a zygote. This fertilized egg is said to be totipotent· from the Latin totus , meaning „entire.‰ It has the potential to generate all the cells and tissues that will make up the embryo and support its devel- opment in the uterus. In mammals, only the zygote and the cells resulting from the first few divisions have this all-encompassing ability to generate cells, the umbilical cord, and other structures. At this stage the cell is the ultimate mother stem cell, possessing potent stemness·the cellÊs poten- tial to generate multiple mature cell types. Pluripotent cells are like the totipotent cells in that they can give rise to all tissue types. Unlike totipotent cells, pluripotent cells cannot develop into an entire organism. On about the fourth day of development, the embryo forms into two layers that will become the tissues of the develop- ing body. The outer layer will become the placenta, which in inner cells can form nearly any human tissue. However, because the cells cannot sur- vive without the outer layer, they are now pluripotent, not totipotent.