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 termthat is, they make copies of themselves in the form of offspringbut they do so in a variety of ways, many involving complex organs and elaborate hormonal mechanisms. the process by which all organisms replicate themselves and perpetuate their species. The characteristics that an organism inherits are controlled by its genes, which are arrayed along one or more chromosomes. Genes consist of deoxyribonucleic acid (DNA), the molecular structure of which determines the genetic code. There are two basic types of reproduction: asexual and sexual. Asexual reproduction gives rise to an offspring that is genetically identical to its single parent. In one-celled organisms (e.g., bacteria and protozoa), reproduction is usually by fission, an asexual process in which the parent organism splits into two identical daughters. Other forms of asexual reproduction include spore formation, in which a reproductive cell gives rise to a new organism; budding (e.g., hydra and yeast), as when a small protuberance on the parent grows into a new individual; regeneration (e.g., flatworms), whereby an organism can be divided into two or more pieces, each of which grows into a new individual; and vegetative reproduction, the formation of new individuals by nonreproductive parts of an organism (e.g., rhizomes, bulbs, and tubers in plants). Sexual reproduction involves the creation of a new individual through the union of special sex cells called gametes; usually the gametes come from different parents. Gametes result from meiosis, a type of cell division that produces cells with half the number of chromosomes of the original cell (haploidy). During fertilization, two haploid gametes unite to form a zygote, the first cell of a new organism. The zygote has the full number of chromosomes typical of the species (diploidy). Sexual reproduction ensures that each offspring is genetically unique (except in cases of multiple offspring derived from divisions of one zygote). Most animals reproduce sexually; among vertebrates, it is the only form of reproduction. Higher plants also reproduce sexually, but among plants there is a regular alternation of sexual (gametophyte) and asexual (sporophyte) generations in the life cycle. The reproductive systems of land vertebrates include elaborate structures to assure union of the gametes and to nourish and protect the embryo as it develops. Fertilization is usually internal. The development of the embryo may occur primarily outside the female or within her body. In birds, for example, the new organism develops externally, sheltered in a hard-shelled egg. In mammals the embryo grows inside the mother's body and, after birth, is fed on milk from the mammaries. In females, eggs are produced in the paired ovaries. Other sexual organs in the female are the oviducts, in humans called the fallopian tubes, one extending from each ovary to the uterus (where the embryo develops), and the vagina, or birth canal. In males, gametes called sperm are produced in the two testes. A system of tubules conveys the sperm from the testes to the penis; along the way, various glands (including the prostate) secrete seminal fluid into the tubules. In a mature female an egg is released from one of the ovaries about every four weeks. After copulation the sperm move up into the uterus and fallopian tubes, where, if an egg is present, fertilization may occur. The zygote lodges itself in the lining of the uterus, where it is sheltered and nourished by the maternal blood supply until birth about nine months later. If fertilization does not occur, the egg degenerates, and the uterine lining, which has thickened in preparation for pregnancy, is shed in menstruation. The human reproductive system may be adversely affected by abnormal hormone secretions. Disorders of the reproductive system may also be caused by genetic abnormalities, congenital defects, sexually transmitted infections, and tumours. Additional reading The nature of reproduction is addressed in John Tyler Bonner, Size and Cycle (1965), a very general discussion of the evolutionary significance of life cycles in animals and plants; and James Watson, Molecular Biology of the Gene, 4th ed., 2 vol. (1987), an up-to-date summary of molecular replication by one of the pioneers in the field. John Tyler Bonner The Editors of the Encyclopdia Britannica