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Reproduction process by which organisms replicate themselves. In a general
sense reproduction is one of the most important concepts in biology: it means
making a copy, a likeness, and thereby providing for the continued existence of
species. Although reproduction is often considered solely in terms of the
production of offspring in animals and plants, the more general meaning has far
greater significance to living organisms.
To appreciate this fact, the origin of
life and the evolution of organisms must be considered. One of the first
characteristics of life that emerged in primeval times must have been the
ability of some primitive chemical system to make copies of itself. At its
lowest level, therefore, reproduction is chemical replication. As evolution
progressed, cells of successively higher levels of complexity must have arisen,
and it was absolutely essential that they had the ability to make likenesses of
themselves. In unicellular organisms, the ability of one cell to reproduce
itself means the reproduction of a new individual; in multicellular organisms,
however, it means growth and regeneration. Multicellular organisms also
reproduce in the strict sense of the term--that is, they make copies of
themselves in the form of offspring--but they do so in a variety of ways, many
involving complex organs and elaborate hormonal mechanisms. Reproduction of
organisms In single-celled organisms (e.g., bacteria, protozoans, many algae,
and some fungi), organismic and cell reproduction are synonymous, for the cell
is the whole organism. Details of the process differ greatly from one form to
the next and, if the higher ciliate protozoans are included, can be
extraordinarily complex. It is possible for reproduction to be asexual, by
simple division, or sexual. In sexual unicellular organisms the gametes can be
produced by division (often multiple fission, as in numerous algae) or, as in
yeasts, by the organism turning itself into a gamete and fusing its nucleus with
that of a neighbour of the opposite sex, a process that is called conjugation.
In ciliate protozoans (e.g., Paramecium), the conjugation process involves the
exchange of haploid nuclei; each partner acquires a new nuclear apparatus, half
of which is genetically derived from its mate. The parent cells separate and
subsequently reproduce by binary fission. Sexuality is present even in primitive
bacteria, in which parts of the chromosome of one cell can be transferred to
another during mating. Multicellular organisms also reproduce asexually and
sexually; asexual, or vegetative, reproduction can take a great variety of
forms.
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