Audio Glossary

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  1. T cell: Any of a specialized class of lymphocytes that form part of the vertebrate immune system. See helper T cell, killer T cell.
  2. taproot system: A root system in which a dominant taproot grows directly downward, giving rise to branch roots that extend to the sides before also descending downward. Compare fibrous root system.
  3. tar: A sticky brown material that condenses as tobacco smoke cools.
  4. target cell: A cell that receives and responds to a signaling molecule.
  5. taste bud: A contact chemoreceptor located on the vertebrate tongue.
  6. taxon: A group defined within the Linnaean hierarchy; for example, a species or a kingdom.
  7. telomere: A bit of DNA that caps the end of, and apparently protects, a chromosome.
  8. telophase: The stage of mitosis during which chromosomes arrive at the opposite poles of the cell and new nuclear envelopes begin to form around each set of chromosomes.
  9. temperate forest: A terrestrial biome, the forest communities of which are dominated by trees and shrubs that grow in regions with cold winters and moist, warm summers.
  10. template strand: Of the two strands in a DNA molecule, the strand that contains the promoter and hence serves as the template from which an mRNA molecule is synthesized. Compare coding strand.
  11. tendon: A collagen-rich connective structure that attaches muscle to bone in vertebrate skeletons. Compare ligament.
  12. terminator: A DNA sequence that, when reached by RNA polymerase, causes transcription to end and the newly formed mRNA molecule to separate from its DNA template.
  13. tertiary structure: The overall folding of a protein into a three-dimensional form.
  14. testis (pl. testes): The reproductive structure in male animals that produces sperm.
  15. testosterone: One of a group of steroid hormones, called androgens, that maintain male sexual and behavioral characteristics.
  16. thalamus (pl. thalami): The portion of the vertebrate brain that relays sensory information to the cerebral cortex.
  17. therapeutic cloning: A technology used to produce stem cells in which the (haploid) nucleus of an unfertilized egg cell is replaced with the (diploid) nucleus of a nonreproductive donor cell, such as a skin cell. Next, chemicals are used to stimulate the egg to divide so that it begins to form an embryo. Finally, stem cells are removed from the developing embryo and stimulated to grow into a wide range of human cell types. Compare reproductive cloning.
  18. thermoreceptor: A sensory cell that initiates an action potential in response to heat.
  19. thrust: The force generated by an organism that propels it forward.
  20. thylakoid: One of a series of flattened, interconnected membrane sacs that lie one on top of another within a chloroplast in stacks called grana.
  21. thylakoid membrane: The membrane that encloses the thylakoid space inside a chloroplast. The thylakoid membrane houses both photosystems and their associated electron transport chains.
  22. thylakoid space: The space enclosed by the thylakoid membrane inside a chloroplast; the innermost compartment of the chloroplast.
  23. thyroid gland: An endocrine gland in the neck that releases calcitonin.
  24. thyroid-stimulating hormone (TSH): A hormone released by the pituitary gland in response to thyrotropin-releasing hormone. Thyroid-stimulating hormone acts on the thyroid gland to trigger the release of thyroid hormones.
  25. thyrotropin-releasing hormone (TRH): A hormone released by the hypothalamus that acts on the pituitary gland to trigger the release of thyroid-stimulating hormone.
  26. tight junction: A structure made up of strands of protein arranged in a belt beneath the plasma membrane of each of two animal cells that holds those cells together and prevents the passage of ions and small molecules between them.
  27. tissue: A collection of coordinated and specialized cells that together fulfill a particular function for the organism.
  28. toxin: A poisonous protein.
  29. trachea (pl. tracheae): In vertebrates, the wide, rigid tube, also called the windpipe, that connects the pharynx with the bronchi. In insects, one of the tubes that carries air between the environment and the tracheoles.
  30. tracheole: In insects, a tube that carries air between the tracheae and respiring cells.
  31. trait: A feature of an organism, such as its height, flower color, or the chemical structure of one of its proteins.
  32. transcription: Synthesis of an RNA molecule from a DNA template. Transcription is the first of the two major steps in the process by which genes specify proteins; it produces mRNA, tRNA, and rRNA molecules, all of which are essential in the production of proteins. Compare translation.
  33. transfer RNA (tRNA): A type of RNA that transfers the amino acid specified by mRNA to the ribosome during protein synthesis.
  34. transformation: A change in the genotype of a cell as a result of the incorporation of external DNA by that cell.
  35. translation: The conversion of a sequence of nitrogen bases in an mRNA molecule to a sequence of amino acids in a protein. Translation occurs at the ribosomes and is the second of the two major steps in the process by which genes specify proteins. Compare transcription.
  36. translocation: A mutation in which a segment of a chromosome breaks off and is then attached to a different, nonhomologous chromosome.
  37. transposon: A DNA sequence that can move from one position on a chromosome to another, or from one chromosome to another; known informally as a "jumping gene."
  38. trichome: An outgrowth from a dermal cell in a leaf, stem, or root. Such outgrowths serve a variety of functions, including the uptake of nutrients, defense against herbivores, and the secretion and storage of salt (in plants that can tolerate high-salt environments).
  39. trimester: One of the three stages of human pregnancy, each lasting about 3 months.
  40. trisomy: In diploid organisms, the condition of having three copies of a chromosome (instead of the usual two).
  41. tRNA: See transfer RNA.
  42. trophic level: A level or step in a food chain. Trophic levels begin with producers and end with predators that eat other organisms but are not fed on by other predators.
  43. trophoblast: The outer layer of the mammalian blastocyst, which will become the developing embryo's portion of the placenta.
  44. tropical forest: A terrestrial biome, the forest communities of which are dominated by a rich diversity of trees, vines, and shrubs that grow in warm, rainy regions.
  45. tubule: The portion of the nephron in the vertebrate kidney where valuable water and solutes are reabsorbed before the filtrate entering the nephron is released as urine.
  46. tubulin: The protein monomer that makes up microtubules.
  47. tumor suppressor: A gene that inhibits cell division under normal conditions.
  48. tundra: A terrestrial biome, the communities of which are dominated by low-growing shrubs and nonwoody plants that can tolerate extreme cold.