Sexual reproduction is the process through which two parents produce offspring which are genetically different from themselves and have new combinations of their characteristics. This contrasts with asexual reproduction , where one parent produces offspring genetically identical to itself. During sexual reproduction, each parent contributes one haploid gamete (a sex cell with half the normal number of chromosomes). The two sex cells fuse during fertilization and form a diploid zygote (which has the normal number of chromosomes). Recombination, which is the production of variations in gene combinations, occurs at fertilization, so bringing together new combinations of alleles . Crossing-over, the exchange of pieces of chromosomes by two homologous chromosomes, also brings about genetic variation during sexual reproduction. Sexual reproduction is advantageous because it generates variations in characters that can adapt a species over time and improve its chances of survival.
Sexual reproduction occurs in practically all forms of life. Even bacteria , which are always haploid, exchange genetic material. Eukaryotes, organisms possessing a nuclear membrane , generally produce haploid gametes (or sex cells). A gamete, such as an egg or a sperm, possesses half the normal number of chromosomes, and is produced by meiosis , which is reduction cell division , which reduces the number of chromosomes from diploid in the parent cell to haploid in the gametes. When the gametes fuse at fertilization, they restore the normal number of chromosomes. Conjugation, alternation of generations, and animal reproduction illustrate various modes of sexual reproduction.
Conjugation is a process of genetic recombination that occurs between two organisms (such as bacteria) in addition to asexual reproduction. Conjugation only occurs between cells of different mating types. In bacteria, cells designated F+ and F-lie close together, and a narrow bridge of cytoplasm forms between them. F+ cells contain a plasmid or reproductive factor that is made of DNA, which replicates within the bacterial cell. A copy is transferred from a donor F+ cell to a recipient F-. Spirogyra, a freshwater filamentous alga, also exhibits conjugation, where two nearby filaments develop extensions that contact each other. The walls between the connecting channels disintegrate, and one cell moves through the conjugation tube into the other. The cells fuse to form a diploid zygote, the only diploid stage in the life of Spirogyra. The black bread mold , Rhizopus, reproduces asexually by spores and sexually by conjugation. During conjugation, the tips of short hyphae act as gametes, and fuse. The resulting zygote develops a protective wall and becomes dormant. Finally, meiosis occurs, and a haploid bread mold germinates and grows spore-producing sporangia.
Alternation of generations
In plants, sexual and asexual reproduction unite in a single cycle called alternation of generations. During alternation of generations, a gametophyte, (a haploid gamete-producing plant ), alternates with a sporophyte (a diploid spore-producing plant). In Ectocarpus, a brown aquatic alga, the two generations are equally prominent, whereas in mosses, the gametophyte generation dominates. In ferns and seed plants, the sporophyte dominate, because the sporophyte generation is better adapted to survive on land.
Mosses are small plants that lack vascular tissue and do not produce seeds , and depend on a moist environment to survive. The green leafy ground cover of mosses that we are familiar with is the haploid gametophyte. The gametophyte develops sex organs, a male antheridium and a female archegonium on the same or different plants. The antheridium produces flagellated sperm cells that swim to the egg cells in the archegonium. After fertilization, the zygote grows into a diploid sporophyte. The sporophyte consists of a foot, stalk, and capsule. It remains attached to the gametophyte. Cells in the capsule undergo meiosis and develop into haploid spores. When released, spores grow into gametophytes with rootlike, leaflike and stemlike parts.
Ferns, in the form of the familiar green leafy plants, represent the diploid sporophyte generation. Ferns have a vascular system and true roots, stems, and leaves, but they do not produce seeds. Sporangia, or spore cases, develop on the leaves of ferns, and produce haploid spores by means of meiosis. The spores germinate into haploid green gametophytes. The fern gametophyte is a tiny heart-shaped structure that bears antheridia and archegonia. Flagellated sperm swim to the eggs in a layer of ground water . Although the sporophyte is adapted to land life, this need for water limits the game-tophyte. After fertilization, the diploid zygote develops into the sporophyte.
In flowering plants, the diploid sporophytes are plants with roots, leaves, stems, flowers and seeds. Anthers within the flower contain four sporangia. Cells in the sporangia undergo meiosis and produce haploid microspores. The wall of each microspore thickens, and the haploid nucleus of the microspore divides by mitosis into a generative nucleus and a tube nucleus. These microspores are now called pollen, and each pollen grain is an immature male gametophyte. Pollination occurs when pollen escapes from the anthers and lands on the stigma of a flower, either of the same plant or a different plant. There, a pollen tube begins to grow down the style toward the ovary of the pistil, and the two nuclei move into the pollen tube. The generative nucleus divides to form two haploid sperm cells. The germinated pollen grain is now a mature male gametophyte. Finally, the pollen tube penetrates the ovary and the sperm enter. The ovary contains sporangia called ovules. Meiosis occurs within each ovule forming four haploid megaspores. Three disintegrate, and the remaining megaspore undergoes repeated mitosis to form the female gametophyte. The female gametophyte is a haploid seven-celled structure. One of the seven cells is an egg cell. Another of the seven cells contains two nuclei called polar nuclei. When the two sperm cells enter, double fertilization occurs. One sperm fertilizes the egg, forming a zygote that develops into a diploid embryo sporophyte. The two polar nuclei fuse and their product unites with the second sperm forming a triploid endosperm. The endosperm serves as stored food for the embryo sporophyte. After fertilization, the ovule matures into a seed, consisting of embryo, stored food, and seed coat. In angiosperms, the ovary usually enlarges to become a fruit. Upon germination , the seed develops into a mature diploid sporophyte plant. Internal fertilization and seeds help adapt flowering plants to life on land.
During sexual reproduction in animals, a haploid sperm and unites with a haploid egg cell to form a diploid zygote. The zygote divides mitotically and differentiates into an embryo. The embryo grows and matures. After birth or hatching, the animal develops into a mature adult capable of reproduction. Some invertebrates reproduce by self-fertilization, in which an animal's sperm fertilizes its own eggs. Self-fertilization is common in tapeworms and other internal parasites , which lack the opportunity to find a mate. Most animals, however, use cross fertilization, in which different individuals donate the egg and the sperm. Even hermaphrodites animals (such as the earthworms) that produce both types of gametes use cross-fertilization.
Animals exhibit two patterns for bringing sperm and eggs together. One is external fertilization, whereby animals shed eggs and sperm into the surrounding water. The flagellated sperm need an aquatic environment to swim to the eggs, the eggs require water to prevent drying out. Most aquatic invertebrates, most fish , and some amphibians use external fertilization. These animals release large numbers of sperm and eggs, thereby overcoming large losses of gametes in the water. In addition, courting behavior in some species brings about the simultaneous release of the gametes, which helps insure that sperm and egg meet.
The other pattern of sexual reproduction is internal fertilization, whereby the male introduces sperm inside the females reproductive tract where the eggs are fertilized. Internal fertilization is an adaption for life on land, for it reduces the loss of gametes that occurs during external fertilization. Sperms are provided with a fluid (semen) that provides an aquatic medium for the sperm to swim when inside the male's body. Mating behavior and reproductive readiness are coordinated and controlled by hormones so that sperm and egg are brought together at the appropriate time.
After internal fertilization, most reptiles and all birds lay eggs that are surrounded by a tough membrane or a shell. Their eggs have four membranes, the amnion, the allantois, the yolk sac and the chorion. The amnion contains the fluid surrounding the embryo; the allantois stores the embryo's urinary wastes and contains blood vessels that bring the embryo oxygen and take away carbon dioxide . The yolk sac holds stored food, and the chorion surrounds the embryo and the other membranes. After the mother lays her eggs, the young hatch.
Mammals employ internal fertilization, but except for the Australian montremes such as the duckbill platypus and the echidna, mammals do not lay eggs. The fertilized eggs of mammals implant in the uterus which develops into the placenta, where the growth and differentiation of the embryo occur. Embryonic nutrition and respiration occur by diffusion from the maternal bloodstream through the placenta. When development is complete, the birth process takes place.
See also Chromosome.
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KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—Sporangium in a seed plant that gives rise to the female gametophyte and after fertilization becomes the seed.
—Circular piece of DNA in the cytoplasm of bacteria that replicates independently of the cell's chromosome.
—Process where genes from two individuals are contributed to an offspring.