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ENCYCLOP∆DIA BRITANNICA


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. Many multicellular lower plants give off asexual
spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or
multinucleate. In some cases the reproductive body is multicellular, as in the soredia of lichens
and the gemmae of liverworts. Frequently, whole fragments of the vegetative part of the
organism can bud off and begin a new individual, a phenomenon that is found in most plant
groups. In many cases a spreading rhizoid (rootlike filament) or, in higher plants, a rhizome
(underground stem) gives off new sprouts. Sometimes other parts of the plant have the
capacity to form new individuals; for instance, buds of potentially new plants may form in the
leaves; even some shoots that bend over and touch the ground can give rise to new plants at
the point of contact.

Among animals, many invertebrates are equally well endowed with means of asexual
reproduction. Numerous species of sponges produce gemmules, masses of cells enclosed in
resistant cases, that can become new sponges. There are many examples of budding among
coelenterates, the best known of which occurs in freshwater Hydra. In some species of
flatworms, the individual worm can duplicate by pinching in two, each half then regenerating the
missing half; this is a large task for the posterior portion, which lacks most of the major
organs--brain, eyes, and pharynx. The highest animals that exhibit vegetative reproduction are
the colonial tunicates (e.g., sea squirts), which, much like plants, send out runners in the form
of stolons, small parts of which form buds that develop into new individuals. Vertebrates have
lost the ability to reproduce vegetatively; their only form of organismic reproduction is sexual.

In the sexual reproduction of all organisms except bacteria, there is one common feature:
haploid, uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate
zygote. At some later stage in the life history of the organism, the chromosome number is
again reduced by meiosis to form the next generation of gametes. The gametes may be
in size (isogamy), or one may be slightly larger than the other (anisogamy); the majority of
forms have a large egg and a minute sperm (oogamy). The sperm are usually motile and the
egg passive, except in higher plants, in which the sperm nuclei are carried in pollen grains that
attach to the stigma (a female structure) of the flower and send out germ tubes that grow down
to the egg nucleus in the ovary. Some organisms, such as most flowering plants, earthworms,
and tunicates, are bisexual (hermaphroditic, or monoecious)--i.e., both the male and female
gametes are produced by the same individual. All other organisms, including some plants (e.g.,
holly and the ginkgo tree) and all vertebrates, are unisexual (dioecious): the male and female
gametes are produced by separate individuals.

Some sexual organisms partially revert to the asexual mode by a periodic degeneration of the
sexual process. For instance, in aphids and in many higher plants the egg nucleus can develop
into a new individual without fertilization, a kind of asexual reproduction that is called
parthenogenesis.



Natural selection and reproduction

The significance of biological reproduction can be explained entirely by natural selection (see
evolution: The concept of natural selection). In formulating his theory of natural selection,
Charles Darwin realized that, in order for evolution to occur, not only must living organisms be
able to reproduce themselves but the copies must not all be identical; that is, they must show
some variation. In this way the more successful variants would make a greater contribution to
subsequent generations in the number of offspring. For such selection to act continuously in
successive generations, Darwin also recognized that the variations had to be inherited, although
he failed to fathom the mechanism of heredity. Moreover, the amount of variation is particularly
important. According to what has been called the principle of compromise, which itself has been
shaped by natural selection, there must not be too little or too much variation: too little
produces no change; too much scrambles the benefit of any particular combination of inherited
traits.

Of the numerous mechanisms for controlling variation, all of which involve a combination of
checks and balances that work together, the most successful is that found in the large majority
of all plants and animals--i.e., sexual reproduction. During the evolution of reproduction and
variation, which are the two basic properties of organisms that not only are required for natural
selection but are also subject to it, sexual reproduction has become ideally adapted to produce
the right amount of variation and to allow new combinations of traits to be rapidly incorporated
into an individual.

The evolution of reproduction

An examination of the way in which organisms have changed since their initial unicellular
condition in primeval times shows an increase in multicellularity and therefore an increase in
the size of both plants and animals. After cell reproduction evolved into multicellular growth, the
multicellular organism evolved a means of reproducing itself that is best described as life-cycle
reproduction. Size increase has been accompanied by many mechanical requirements that have
necessitated a selection for increased efficiency; the result has been a great increase in the
complexity of organisms. In terms of reproduction this means a great increase in the
permutations of cell reproduction during the process of evolutionary development.

Size increase also means a longer life cycle, and with it a great diversity of patterns at different
stages of the cycle. This is because each part of the life cycle is adaptive in that, through
natural selection, certain characteristics have evolved for each stage that enable the organism
to survive. The most extreme examples are those forms with two or more separate phases of
their life cycle separated by a metamorphosis, as in caterpillars and butterflies; these phases
may be shortened or extended by natural selection, as has occurred in different species of
coelenterates.

To reproduce efficiently in order to contribute effectively to subsequent generations is another
factor that has evolved through natural selection. For instance, an organism can produce vast
quantities of eggs of which, possibly by neglect, only a small percent will survive. On the other
hand, an organism can produce very few or perhaps one egg, which, as it develops, will be
cared for, thereby greatly increasing its chances for survival. These are two strategies of
reproduction; each has its advantages and disadvantages. Many other considerations of the
natural history and structure of the organism determine, through natural selection, the strategy
that is best for a particular species; one of these is that any species must not produce too few
offspring (for it will become extinct) or too many (for it may also become extinct by
overpopulation and disease). The numbers of some organisms fluctuate cyclically but always
remain between upper and lower limits. The question of how, through natural selection,
numbers of individuals are controlled is a matter of great interest; clearly, it involves factors
that influence the rate of reproduction.



reproduction

Levels of reproduction

Molecular replication

The characteristics that an organism inherits are largely stored in cells as genetic information in
very long molecules of deoxyribonucleic acid (DNA). In 1953 it was established that DNA
molecules consist of two complementary strands, each of which can make copies of the other.
The strands are like two sides of a ladder that has been twisted along its length in the shape of
a double helix (spring). The rungs, which join the two sides of the ladder, are made up of two
terminal bases. There are four bases in DNA: thymine, cytosine, adenine, and guanine. In the
middle of each rung a base from one strand of DNA is linked by a hydrogen bond to a base of
the other strand. But they can pair only in certain ways: adenine always pairs with thymine, and
guanine with cytosine. This is why one strand of DNA is considered complementary to the other.

The double helices duplicate themselves by separating at one place between the two strands
and becoming progressively unattached. As one strand separates from the other, each acquires
new complementary bases until eventually each strand becomes a new double helix with a new
complementary strand to replace the original one. Because adenine always falls in place
opposite thymine and guanine opposite cytosine, the process is called a template
replication--one strand serves as the mold for the other. It should be added that the steps
involving the duplication of DNA do not occur spontaneously; they require catalysts in the form
of enzymes that promote the replication process.

Molecular reproduction

The sequence of bases in a DNA molecule serves as a code by which genetic information is
stored. Using this code, the DNA synthesizes one strand of ribonucleic acid (RNA), a substance
that is so similar structurally to DNA that it is also formed by template replication of DNA. RNA
serves as a messenger for carrying the genetic code to those places in the cell where proteins
are manufactured. The way in which the messenger RNA is translated into specific proteins is a
remarkable and complex process. (For more detailed information concerning DNA, RNA, and
the genetic code, see the articles nucleic acid and heredity: Chromosomes and genes). The
ability to synthesize enzymes and other proteins enables the organism to make any substance
that existed in a previous generation. Proteins are reproduced directly; however, such other
substances as carbohydrates, fats, and other organic molecules found in cells are produced by
a series of enzyme-controlled chemical reactions, each enzyme being derived originally from
DNA through messenger RNA. It is because all of the organic constituents made by organisms
are derived ultimately from DNA that molecules in organisms are reproduced exactly by each
successive generation.


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