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Life scientists use the term life cycle to mean the sequence of development through which an individual organism passes from its first stage as an individual to the point at which it reproduces itself. Thus the life cycle of humans begins with the formation of the single-celled zygote by the fusion of egg and sperm, progresses through embryo, birth, childhood to sexual maturity, and completes the circle when the egg or sperm of the individual contribute to the formation of a zygote. A life cycle is often characteristic of a given group of organisms and is closely adapted to environment and lifestyle. It may include specialized stages for dispersal, reproduction, dormancy, migration, etc. Some organisms, such as the sockeye salmon, may die immediately after reproduction while others may live on to reproduce again or to care for their young. Simple, single-celled organisms, such as bacteria, divide by binary fission and thus have a life cycle which is closely related to the cycle of cell division. RB
See also developmental biology; metamorphosis. |
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