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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