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Sexual Reproduction - Animal Reproduction

fertilization eggs sperm embryo

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.


Resources

Books

Campbell, N., J. Reece, and L. Mitchell. Biology. 5th ed. Menlo Park: Benjamin Cummings, Inc. 2000.

Chinery, Michael. Partners and Parents. Crabtree Publishing, 2000.

Essenfeld, Bernice, Carol R. Gontang, and Randy Moore. Biology. Menlo Park: Addison Wesley, 1996.

Films for the Humanities and Sciences. The Chemistry of Fertilization. Princeton, 1994.

Taylor, Martha. Campbell's Biology Student Study Guide. Redwood City, CA: Benjamin/Cummings, 1990.

Periodicals

Adams, K.L., et al. "Repeated, Recent and Diverse Transfers of a Mitochondrial Gene to the Nucleus in Flowering Plants." Nature 408 (2000): 354-357.

Hardin, P. E. "From Biological Clock to Biological Rhythms." Genome Biology 1 (2000): 1023.1-1023.5.

Kerr, Richard A. "Timing Evolution's Early Bursts." Science (January 6, 1995).

Richardson, Sarah. "Guinness Book Gametes." Discover (March 1995).

Sikkel, Paul C., "Honey, I Ate the Kids." Natural History (December 1994).


Bernice Essenfeld

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ovule

—Sporangium in a seed plant that gives rise to the female gametophyte and after fertilization becomes the seed.

Plasmid

—Circular piece of DNA in the cytoplasm of bacteria that replicates independently of the cell's chromosome.

Recombination

—Process where genes from two individuals are contributed to an offspring.

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about 3 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. The two main processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination.
The evolution of sexual reproduction is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago. Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly in response to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the "ratcheting" of evolutionary speed as one clade competes with another for a limited resource.


In the first stage of sexual reproduction, "meiosis," the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization," haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored

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about 2 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organism..

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about 1 year ago

shoulder

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about 2 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organism..

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about 2 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organism..

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about 2 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organism..

Vote down Vote up

about 2 years ago

Sexual reproduction is the creation of a new organism by combining the genetic material of two organism..