| Gaia |
| A hypothetical superorganism composed of the Earth's four spheres: the biosphere, hydrosphere, lithosphere, and atmosphere. |
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| gametes |
| Haploid reproductive cells (ovum and sperm). |
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| gametophyte |
| The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process of mitosis. |
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| ganglia |
| Clusters of neurons that receive and process signals; found in þatworms and earthworms. |
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| gap junctions |
| Junctions between the plasma membranes of animal cells that allow communication between the cytoplasm of adjacent cells. |
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| gastric pits |
| The folds and grooves into which the stomach lining is arranged. |
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| gastrin |
| A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids. |
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| 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. |
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| gene |
| The functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.
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| gene amplification |
| An increase in the number of copies of any particular piece of DNA. A tumor cell amplifies, or copies, DNA segments naturally as a result of cell signals and sometimes environmental events.
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| gene expression |
| A highly specific process in which a gene is switched on at a certain time and begins production of its protein.
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| gene mapping |
| Determining the relative positions of genes on a chromosome and the distance between them.
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| gene pool |
| The sum total of genes, with all their variations, possessed by a particular species at a particular time.
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| gene therapy |
| An evolving technique used to treat inheritied diseases. The medical procedure involves either replacing, manipulating, or supplementing nonfunctional genes with healthy genes.
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| gene transfer |
| Insertion of unrelated DNA into the cells of an organism. There are many different reasons for gene transfer: for example, attempting to treat disease by supplying patients with therapeutic genes. There are also many possible ways to transfer genes. Most involve the use of a vector, such as a specially modified virus that can take the gene along when it enters the cell.
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| genera |
| Taxonomic subcategories within families (sing.: genus), composed of one or more species. |
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| 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. |
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| 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. |
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| genetic code (ATGC) |
| The instructions in a gene that tell the cell how to make a specific protein. A, T, G, and C are the "letters" of the DNA code; they stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out 3-letter "words" that specify which amino acid is needed at every step in making a protein.
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| genetic counseling |
| A short-term educational counseling process for individuals and families who have a genetic disease or who are at risk for such a disease. Genetic counseling provides patients with information about their condition and helps them make informed decisions.
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| 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. |
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| 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 alone. |
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| genetic map |
| (Also known as a linkage map) a chromosome map of a species that shows the position of its known genes and/or markers relative to each other, rather than as specific physical points on each chromosome.
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| genetic maps |
| Diagrams showing the order of and distance between genes; constructed using crossover information. |
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| genetic marker |
| A segment of DNA with an identifiable physical location on a chromosome and whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of a gene that has not yet been identified, but whose approximate location is known.
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| genetic screening |
| Testing a population group to identify a subset of individuals at high risk for having or transmitting a specific genetic disorder.
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| genetics |
| The study of the structure and function of genes and the transmission of genes from parents to offspring. |
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| genital herpes |
| A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes of the mouth or genitals. |
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| genome |
| All the DNA contained in an organism or a cell, which includes both the chromosomes within the nucleus and the DNA in mitochondria.
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| genotype |
| The genetic identity of an individual that does not show as outward characteristics.
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| geographic isolation |
| Separation of populations of a species by geographic means (distance, mountains, rivers, oceans, etc.) that lead to reproductive isolation of those populations. |
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| geographic range |
| The total area occupied by a population. |
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| geological time |
| The span of time that has passed since the formation of the Earth and its physical structures; also, a timescale that focuses on events on the order of thousands of years or more. |
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| geotropism |
| Plants' response to gravity: roots grow downward, showing positive geotropism, while shoots grow upward in a negative response. |
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| germ cells |
| Collective term for cells in the reproductive organs of multicellular organisms that divide by meiosis to produce gametes. |
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| germ line |
| A sequence of cells, each descended from earlier cells in the lineage, which will develop into new sperm and egg cells for the subsequent generation.
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| gibberellins |
| A group of hormones that stimulate cell division and elongation in plants. Gibberellic acid (GA), the first of this class to be discovered, causes bolting (extreme elongation) of stems. GA is also applied to certain plants to promote larger fruits. |
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| gill slits |
| Opening or clefts between the gill arches in Þsh. Water taken in by the mouth passes through the gill slits and bathes the gills. Also, rudimentary grooves in the neck region of embryos of air-breathing vertebrates such as humans; a characteristic of chordates. |
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| ginkgos |
| Group of seed plants today restricted to a single genus (Ginkgo biloba); ginkgos were more diverse during the Mesozoic Era. |
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| glial cells |
| Nonconducting cells that serve as support cells in the nervous system and help to protect neurons. |
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| glomerulus |
| A tangle of capillaries that makes up part of the nephron; the site of Þltration. |
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| glucagon |
| A hormone released by the pancreas that stimulates the breakdown of glycogen and the release of glucose, thereby increasing blood levels of glucose. Glucagon and insulin work together to maintain blood sugar levels. |
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| glucocorticoids |
| A group of steroid hormones produced by the adrenal cortex that are important in regulating the metabolism of carbohydrates, fats, and proteins. |
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| glucose |
| A six-carbon single sugar; the most common energy source. |
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| glycolipids |
| Polysaccharides formed of sugars linked to lipids, a part of the cell membrane. |
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| glycolysis |
| The universal cellular metabolic process in the cell's cytoplasm where 6-carbon glucose is split into two 3-carbon pyruvate molecules, and some ATP and NADH are produced. Click here to view the On-Line Biology Book chapter on glycolysis. |
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| glycoproteins |
| Polysaccharides formed of sugars linked to proteins. On the outer surface of a membrane, they act as receptors for molecular signals originating outside the cell. |
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| gnetales |
| Group of seed plants restricted to three genera today (Gnetum, Ephedra, and Welwitschia); the possible outgroup for flowering plants. |
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| golden brown algae |
| Common name applied to the protistan division Chrysophyta. |
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| Golgi complex |
| Organelles in animal cells composed of a series of þattened sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum. |
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| gonadotropin-releasing hormone (GnRH) |
| A hormone produced by the hypothalamus that controls the secretion of luteinizing hormone. |
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| gonadotropins |
| Hormones produced by the anterior pituitary that affect the testis and ovary; include follicle-stimulating hormone and luteinizing hormone. |
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| gonads |
| The male and female sex organs. |
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| gonorrhea |
| A sexually transmitted disease that is caused by a bacterium that inþames and damages epithelial cells of the reproductive system. |
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| grana |
| A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloroplasts. |
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| grasslands biome |
| Occurs in temperate and tropical regions with reduced rainfall or prolonged dry seasons; characterized by deep, rich soil, an absence of trees, and large herds of grazing animals. |
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| green algae |
| Common name for algae placed in the division Chlorophyta. |
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| greenhouse effect |
| The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radiate it back to the surface; so-called because the gases are transparent to sunlight but not to heat and thus act like the glass in a greenhouse. |
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| ground system |
| Plant tissue system, composed mainly of parenchyma cells with some collenchyma and sclerenchyma cells, that occupies the space between the epidermis and the vascular system; is involved in photosynthesis, water and food storage, and support; one of the four main tissue systems in plants. |
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| growth hormone (GH) |
| A peptide hormone produced by the anterior pituitary that is essential for growth. |
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| guanine |
| One of the nitrogenous bases in nucleic acids, guanine is one of the two purine bases. |
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| guard cells |
| Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange and water loss. |
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| gymnosperms |
| Flowerless, seed-bearing land plants; the Þrst seed plants; living groups include the pines, ginkgos, and cycads. Naked seeds. |
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