On-Line Biology Book: GLOSSARY

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Gaia  A hypothetical superorganism composed of the Earth's four spheres: the biosphere, hydrosphere, lithosphere, and atmosphere.

gametes  Haploid reproductive cells (ovum and sperm). PICTURE

gametophyte  The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process of mitosis.

ganglia  Clusters of neurons that receive and process signals; found in þatworms and earthworms. PICTURE

gap junctions  Junctions between the plasma membranes of animal cells that allow communication between the cytoplasm of adjacent cells.

gastric pits  The folds and grooves into which the stomach lining is arranged.

gastrin  A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids.

gastroesophageal sphincter  A ring of muscle at the junction of the esophagus and the stomach that remains closed except during swallowing to prevent the stomach contents from entering the esophagus.

gene pool  The sum of all the genetic information carried by members of a population. Note: there is no diving in the deep end of the gene pool!

genera  Taxonomic subcategories within families (sing.: genus), composed of one or more species.

genes  SpeciÞc segments of DNA that control cell structure and function; the functional units of inheritance. Sequence of DNA bases usually code for a polypeptide sequence of amino acids.

gene therapy  The insertion of normal or genetically altered genes into cells through the use of recombinant DNA technology; usually done to replace defective genes as part of the treatment of genetic disorders.

genetic code  The linear series of nucleotides, read as triplets, that speciÞes the sequence of amino acids in proteins. Each triplet speciÞes an amino acid, and the same codons are used for the same amino acids in almost all life-forms, an indication of the universal nature of the code. PICTURE

genetic divergence  The separation of a population's gene pool from the gene pools of other populations due to mutation, genetic drift, and selection. Continued divergence can lead to speciation.

genetic drift  Random changes in the frequency of alleles from generation to generation; especially in small populations, can lead to the elimination of a particular allele by chance