In most cases, the DNA copy is not exact, so that the genetic makeup of the newly formed offspring differs from that of its parent. This programmed variability is accomplished primarily by sexual reproduction. The genetic process that defines sexual reproduction occurs only during a very brief specialized phase of cell division, and only within cells belonging to the germ cell line. A germ cell is defined as a cell belonging to a cellular lineage that, at some point, will deviate from normal cell division (i.e., mitosis, which results in exact duplicate copies of DNA) to engage in meiosis. Meio-sis is often known as reductional division, because it results in reducing the number of chromosomes by half in preparation for an exchange with the complementary chromosome of a mating partner that restores the full set. The most important aspect of meiosis, however, occurs long before this reduction of chromosome number, which occurs late in meio-sis. Early in meiosis, gene segments are actually recombined by the exchange of DNA sequences, so that the individual chromosomes are transformed into genetically unique combinations of genes. This process is known as crossing over; it is the defining event that distinguishes sexual from asexual reproduction. In this way, germ cells become genetically unique while they are still in the parent animal, long before they become fully formed sperm or oocytes, and even longer before they fuse with gametes from another parent. At the later point of gamete fusion, the recombined genes in the chromosomes from two parents will fertilize each other to produce new chromosome combinations. Thus, the genetic recombination that takes place during meiosis, combined with the reaggre-gation of the chromosomes that occurs during fertilization, results in the distinctive offspring that characterize most animals. It is important, however, to recognize that sexual reproduction does not require either fertilization or two parents. If meiosis takes place, the reproduction is sexual. In some lower metazoans, a single parent animal may produce gametes by meiosis, which can then develop into fully formed offspring by various processes known collectively as sexual parthenogenesis.
Hydras reproduce by budding. (Illustration by Barbara Duperron)
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