The Cell Life Cycle
From fertilization to physical maturity, the cells undergo many divisions. When a single cell divides, it forms two daughter cells that are identical to the original cell. A cell may live from a few days to many years, depending on the cell type. Most cells have a gene, which is triggered to self-destruct at a specific time.
Cells divide in two ways: mitosis and meiosis. Mi-tosis is common and is the process of division seen in somatic cells, involving the separation of the dupli-cated chromosome into two identical nuclei. The cy-toplasm and the nucleus then separate into two new cells.
Meiosis can be seen in the testis and ovary during the formation of sperm and ova, in which the daugh-ter cells end up with half the number of chromo-somes found in somatic cells. When the ovum and sperm fuse during fertilization, the fused cell then has the right number of chromosomes.
When cells are not dividing, they continue to func-tion fully. This phase is known as the interphase. Cells that do not multiply after birth, such as neu-rons, are said to be in the interphase.
Cell division is regulated by peptides known as growth factors, which are present in the extracellularfluid. Growth factors bind to receptors in the cell membrane and trigger cell division. Growth hormone, nerve growth factor, epidermal growth factor, and ery-thropoietin are a few of the growth factors identified.
Specific genes, known as repressor genes, op-pose,cell division. When the rate of growth exceeds that of inhibition, the tissue enlarges. If uncontrolled cell growth occurs, a tumor or neoplasm results.
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