Audio Glossary

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  1. G0 phase: The stage of the cell cycle during which the cell pauses between mitosis and S phase. No preparations for S phase are made during this period.
  2. G1 phase: The stage of the cell cycle following mitosis and before S phase. The cell makes preparations for DNA synthesis during G1 phase phase.
  3. G2 phase: The stage of the cell cycle following S phase and before mitosis.
  4. gallbladder: A small sac where bile secreted by the liver is stored before its release into the small intestine.
  5. gamete: A haploid sex cell that fuses with another sex cell during fertilization. Eggs and sperm are gametes.
  6. ganglion: A structure consisting of densely packed neuronal cell bodies that allows the integration of signals.
  7. gap junction: A direct channel protein connection between the plasma membranes of two animal cells that allows the passage of ions and small molecules between them.
  8. gastrula: The stage of vertebrate development that results from the rearrangement of the three cell layers during gastrulation.
  9. gastrulation: The movement of the three cell layers (endoderm, mesoderm, and ectoderm) during animal development to the positions appropriate for the tissues to which these layers give rise.
  10. gel electrophoresis: A process in which DNA fragments are placed in a gelatin-like substance (a gel) and subjected to an electrical charge, which causes the fragments to move through the gel. Small DNA fragments move farther than large DNA fragments, thus causing the fragments to separate by size.
  11. gene: A DNA sequence that contains information for the synthesis of a protein or an RNA molecule. Genes are located on chromosomes.
  12. gene cascade: A process in which the protein products of different genes interact with one another and with signals from the environment, thereby turning on other sets of genes in some cells, but not in other cells. Organisms use gene cascades to control how genes are expressed during development.
  13. gene expression: The synthesis of a gene's protein or RNA product. Gene expression is the means by which a gene influences the cell or organism in which it is found.
  14. gene flow: The exchange of alleles between populations.
  15. gene therapy: A treatment approach that seeks to correct genetic disorders by repairing the genes that cause them.
  16. gene-for-gene recognition: In plants, a specific immune response in which many genes, which collectively have hundreds of alleles (known as R alleles), provide resistance against specific attackers.
  17. genetic bottleneck: A drop in the size of a population that results in low genetic variation or causes harmful alleles to reach a frequency of 100 percent in the population.
  18. genetic code: The code according to which each set of three nitrogen bases in mRNA specifies either an amino acid or a signal to start or stop the construction of a protein. The genetic code allows the cell to use the information in a gene to build the protein called for by that gene.
  19. genetic cross: A controlled mating experiment, usually performed to examine the inheritance of a particular characteristic.
  20. genetic drift: process in which alleles are sampled at random over time, as when chance events cause certain alleles to increase or decrease in a population. The genetic makeup of a population undergoing genetic drift changes at random over time, rather than being shaped in a nonrandom way by natural selection.
  21. genetic engineering: A three-step process in which a DNA sequence (often a gene) is isolated, modified, and inserted back into an individual of the same or a different species. Genetic engineering is commonly used to change the performance of the genetically modified organism, as when a crop plant is engineered to resist attack from an insect pest.
  22. genetic linkage: The situation in which different genes that are located close to one another on the same chromosome do not follow Mendel's law of independent assortment.
  23. genetic screening: The examination of an individual's genes to assess current or future health risks and status.
  24. genetic variation: The genetic differences among the individuals of a population.
  25. genetically modified organism (GMO): An individual into which a modified gene or other DNA sequence has been inserted, typically with the intent of improving some aspect of the recipient organism's performance.
  26. genetics: The scientific study of genes.
  27. genome: All the DNA of an organism, including its genes; in eukaryotes, the term "genome" refers to a haploid set of chromosomes, such as that found in a sperm or egg.
  28. genomics: The study of the structure and expression of entire genomes and how they change during evolution.
  29. genotype: The genetic makeup of an organism. Compare phenotype.
  30. genotype frequency: The proportion (percentage) of a particular genotype in a population.
  31. genus: The level in the Linnaean hierarchy that is above species and therefore comprises species, but that is below families and so composes families.
  32. geographic isolation: The physical separation of populations from one another by a barrier such as a mountain chain or a river. Geographic isolation often causes the formation of new species, as when populations of a single species become physically separated from one another and then accumulate so many genetic differences that they become reproductively isolated from one another. Compare reproductive isolation.
  33. gill: The gas exchange surface of an aquatic animal, typically consisting of finely folded sheets of thin epithelial tissue. Compare lung.
  34. gizzard: A portion of the digestive system that breaks down food by grinding it against rocks or sand collected from the environment.
  35. global change: Worldwide change in the environment. There are many causes of global change, including climate change caused by the movement of continents and changes in land and water use by humans.
  36. global warming: A worldwide increase in temperature. Earth appears to be entering a period of global warming caused by human activities; specifically, by the release of large quantities of greenhouse gases such as carbon dioxide into the atmosphere.
  37. glomerulus: The tiny capillary bed through which blood is filtered from the circulatory system into the tubules of the vertebrate kidney.
  38. glucagon: A hormone produced by the pancreas that acts as an antagonist to insulin in regulating blood sugar levels. Compare insulin.
  39. glucose: A monosaccharide that is the primary metabolic fuel in most cells.
  40. glycogen: A storage carbohydrate found primarily in the liver.
  41. glycolysis: A series of catabolic reactions that split glucose to produce pyruvate, which is then used in either oxidative phosphorylation or fermentation.
  42. GMO: See genetically modified organism.
  43. golgi apparatus: An organelle composed of flattened membrane sacs that routes proteins and lipids to various parts of the eukaryotic cell.
  44. gonad: An animal sex gland that produces gametes and sex hormones. See also testis, ovary.
  45. gonadotropin: Either of a pair of hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), that are produced by the pituitary gland and regulate the development and function of the reproductive organs.
  46. granum (pl. grana): A structure made up of a stack of membrane sacs, called thylakoids, that is part of the interconnected internal membrane system within a chloroplast.
  47. grassland: A terrestrial biome, the communities of which are dominated by grasses and many different types of wildflowers. Grasslands often occur in relatively dry regions with cold winters and hot summers.
  48. Green Revolution: A large increase in agricultural productivity that occurred in the last 50 years due to the development of new varieties of wheat, rice, and corn that allowed the production of large amounts of grain on a given amount of farmland.
  49. greenhouse gas: Any of several gases in Earth's atmosphere that let in sunlight, but trap heat.
  50. greenroof: On a building, a 2- to 4-inch-thick "living" rooftop that has a layer of soil or other material in which plants grow, under which there are one or more layers that absorb water and prevent roots and water from damaging the underlying roof structure.
  51. gross domestic product (GDP): A traditional mea sure of a nation's economic output that records the value of goods produced, but does not consider the social and environmental costs that result from producing those goods. Compare index of sustainable economic welfare.
  52. ground tissue: The tissue type in plants that forms most of the photosynthetic tissue in leaves and surrounds the vascular system.
  53. groundwater: Water from an underground source, such as an aquifer or belowground river.
  54. growth hormone (GH): A hormone produced by the pituitary gland that plays a key role in regulating the growth of bones and muscles.
  55. guard cell: A specialized cell type found on leaves and stems that can open and close, thereby controlling the rates at which CO2 is brought into the plant and O2 and water are lost from the plant.
  56. gut inhabitant mutualism: A mutualism involving organisms that live in the digestive tract of a host, receiving food from the host and digesting foods that the host otherwise could not use.
  57. gymnosperms: A group of plants that includes pine trees and other conifers, ginkgos, and cycads. Gymnosperms were the first plants to evolve seeds. Compare angiosperms.