| T cells |
| The type of lymphocyte responsible for cell-mediated immunity; also protects against infection by parasites, fungi, and protozoans and can kill cancerous cells; circulate in the blood and become associated with lymph nodes and the spleen. |
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| taiga biome |
| The region of coniferous forest extending across much of northern Europe, Asia, and North America; characterized by long, cold winters and short, cool summers and by acidic, thin soils. |
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| tap root |
| A primary root that grows vertically downward and gives off small lateral roots; occurs in dicots. Root system in plants characterized by one root longer than the other roots. Example: carrot. |
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| target cell |
| A cell that a particular hormone effects by its direct action (either passing through the membrane or binding to a surface receptor). 1 2 |
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| tarsals |
| The bones that make up the ankle joint. |
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| taxis |
| The behavior when an animal turns and moves toward or away from an external stimulus (pl.: taxes). |
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| taxonomy |
| A systematic method of classifying plants and animals. Classification of organisms based on degrees of similarity purportedly representing evolutionary (phylogenetic) relatedness. |
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| technology transfer |
| The process of providing the private sector access to technological advances developed by scientists in the Federal government.
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| tectonic plates |
| Segments of the lithosphere that comprise the surface of the Earth much the way a turtle shell is composed of its plates. |
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| telophase |
| The Þnal stage of mitosis in which the chromosomes migrate to opposite poles, a new nuclear envelope forms, and the chromosomes uncoil. The last phase of nuclear division in eukaryotes when the segregated chromosomes uncoil and begin to reform nuclei. This is immediately followed (in most cases) by cytokinesis. |
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| temperate forest biome |
| Extends across regions of the northern hemisphere with abundant rainfall and long growing seasons. Deciduous, broad-leaved trees are the dominant plants. |
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| template strand |
| The strand of DNA that is transcribed to make RNA. |
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| temporal lobe |
| The lobe of the cerebral cortex that is responsible for processing auditory signals. |
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| tendons |
| Bundles of connective tissue that link muscle to bone. Fibrous connective tissue that connects muscle to bone. |
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| terminal buds |
| Buds located at the end of a plant shoot. |
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| termination |
| The end of translation; occurs when the ribosome reaches the stop codon on the messenger RNA molecule and the polypeptide, the messenger RNA, and the transfer RNA molecule are released from the ribosome. |
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| termination codon |
| One of three three-base sequences that initiate termination of the protein synthesis process. See stop codon. |
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| tertiary structure |
| The folding of a protein's secondary structure into a functional three-dimensional conÞguration. Shape assumed by protein due to interactions between amino acids far apart on the chain. |
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| test cross |
| Genetic crossing of an organism with known genotype (one that exhibits the recessive phenotype) with an individual expressing the dominant phenotype but of unknown heritage. |
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| testes |
| The male gonad; produce spermatozoa and male sex hormones. Male gonads in mammals. Singular, testis. Paired organs that contain seminiferous tubules in which sperm are produced. 1 2 |
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| testosterone |
| Male sex hormone that stimulates sperm formation, promotes the development of the male duct system in the fetus, and is responsible for secondary sex characteristics such as facial hair growth. |
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| tetrad |
| The four chromatids in each cluster during synapsis; formed by the two sister chromatids in each of the two homologous chromosomes. |
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| thalamus |
| The brain region that serves as a switching center for sensory signals passing from the brain stem to other brain regions; part of the diencephalon. |
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| thecodonts |
| An informal term for a variety of Permian and Triassic reptiles that had teeth set in individual sockets. Small, bipedal thecodontians are the probable ancestors of dinosaurs. |
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| theory |
| A hypothesis that has withstood extensive testing by a variety of methods, and in which a higher degree of certainty may be placed. A theory is NEVER a fact, but instead is an attempt to explain one or more facts. |
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| thermacidophiles |
| A group of archaebacteria that are able to tolerate high temperatures and acidic pH. |
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| thermiogenesis |
| The generation of heat by raising the body's metabolic rate; controlled by the hypothalamus. |
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| thermoregulation |
| The regulation of body temperature. |
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| thigmotropism |
| Plants' response to contact with a solid object; e.g., tendrils' twining around a pole. Plant response to touch. |
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| thoracic cavity |
| The chest cavity in which the heart and lungs are located. |
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| thorax |
| In many arthropods, one of three regions formed by the fusion of the segments (others are the head and abdomen). |
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| thorns |
| Stems modified to protect the plant. |
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| thoroughfare channels |
| Shortcuts within the capillary network that allow blood to bypass a capillary bed. |
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| thylakoids |
| The specialized membrane structures in which photosynthesis takes place. Internal membranes in the chloroplast where the light reaction chemicals are embedded. Collections of thylakoids form the grana. |
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| thymine |
| One of the pyrimidine bases in DNA, thymine is replaced by uracil in RNA. |
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| thyroid-stimulating hormone |
| A hormone produced by the anterior pituitary that stimulates the production and release of thyroid hormones. |
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| tight junctions |
| Junctions between the plasma membranes of adjacent cells in animals that form a barrier, preventing materials from passing between the cells. |
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| tissues |
| Groups of similar cells organized to carry out one or more speciÞc functions. Groups of cells performing a function in a multicellular organism. |
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| toxins |
| Term applied to poisons in living systems. |
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| trace fossil |
| Any indication of prehistoric organic activity, such as tracks, trails, burrows, or nests. |
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| trachea |
| In insects and spiders, a series of tubes that carry air directly to cells for gas exchange; in humans, the air-conducting duct that leads from the pharynx to the lungs. 1 2 |
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| tracheids |
| Long, tapered cells with pitted walls that form a system of tubes in the xylem and carry water and solutes from the roots to the rest of the plant. One type of xylem cells. Tracheids are long and relatively narrow, and transport materials from the roots upward. Tracheids are dead at maturity and have lignin in their secondary walls. 1 2 |
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| transcription |
| The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule. |
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| transfer RNAs (tRNAs) |
| Small, single-stranded RNA molecules that bind to amino acids and deliver them to the proper codon on messenger RNA. The trucks of protein synthesis that carry the specified amino acid to the ribosome. Abbreviated tRNA. |
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| transformation |
| In GrifÞth's experiments with strains of pneumonia bacterium, the process by which hereditary information passed from dead cells of one strain into cells of another strain, causing them to take on the characteristic virulence of the Þrst strain. |
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| transforming factor |
| GrifÞth's name for the unknown material leading to transformation; later found to be DNA. |
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| transgenic |
| An experimentally produced organism in which DNA has been artificially introduced and incorporated into the organism's germ line, usually by injecting the foreign DNA into the nucleus of a fertilized embryo.
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| transition reaction |
| Biochemical process of converting 3-carbon pyruvate into 2-carbon acetyl and attaching it to coenzyme A (CoA) so it can enter Kreb's cycle. Carbon dioxide is also released and NADH is formed (from NAD and H) in this process. |
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| translation |
| The synthesis of protein on a template of messenger RNA; consists of three steps: initiation, elongation, and termination. Making of a polypeptide sequence by translating the genetic code of an mRNA molecule associated with a ribosome. |
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| translocation |
| Breakage and removal of a large segment of DNA from one chromosome, followed by the segment's attachment to a different chromosome.
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| transpiration |
| The loss of water molecules from the leaves of a plant; creates an osmotic gradient; producing tension that pulls water upward from the roots. 1 2 3 |
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| Triassic Period |
| The first period of the Mesozoic Era between 225 and 185 million years ago. Pangaea began to breakup during this time. The ancestors of dinosaurs were present, as were early mammals and mammal-like reptiles. |
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| trichocysts |
| Barbed, thread-like organelles of ciliated protozoans that can be discharged for defense or to capture prey. |
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| trichomes |
| Extensions from the epidermis of the plant that provide shade and protection for the plant. |
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| trilobites |
| A group of benthonic, detritus-feeding, extinct marine invertebrate animals (phylum Arthropoda), having skeletons of an organic compound called chitin. |
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| triplet |
| Three-base sequence of mRNA that codes for a specific amino acid or termination codon. |
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| trisomy |
| Possessing three copies of a particular chromosome instead of the normal two copies.
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| trophoblast |
| The outer layer of cells of a blastocyst that adhere to the endometrium during implantation. |
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| tropic hormone |
| Hormone made by one gland that causes another gland to secrete a hormone. |
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| tropical rain forest biome |
| The most complex and diverse biome; found near the equator in South America and Africa; characterized by thin soils, heavy rainfall, and little þuctuation in temperature. |
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| tropism |
| The movement of plant parts toward or away from a stimulus in the plant's environment. Plant movement in response to an environmental stimulus. |
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| true-breeding |
| Occurs when self-fertilization gives rise to the same traits in all offspring, generation after generation. Now interpreted as equivalent to homozygous. |
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| trypanosomes |
| A type of roundworm, responsible for human disease associated with eating raw or undercooked pork. |
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| tubal ligation |
| A contraceptive procedure in women in which the oviducts are cut, preventing the ova from reaching the uterus. |
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| tubal pregnancy |
| Occurs when the morula remains in the oviduct and does not descend into the uterus. |
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| tube nucleus |
| One of the cells in the male gametophyte in seed plants. The tube nucleus grows through the stigma, style, and into the ovule, clearing the way for the sperm nuclei to enter the embryo sac. |
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| tube-within-a-tube system |
| A type of body plan in animals. The organism has two openings&emdash;one for food and one for the elimination of waste&emdash;and a specialized digestive system. |
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| tubers |
| Swollen underground stems in plants that store food, such as the irish potato. |
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| tubular secretion |
| The process in which ions and other waste products are transported into the distal tubules of the nephron. |
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| tubulins |
| The protein subunits from which microtubules are assembled. |
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| tumor suppressor gene |
| A protective gene that normally limits the growth of tumors. When a tumor supressor is mutated, it may fail to keep a cancer from growing. BRCA1 and p53 are well-known tumor suppressor genes.
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| tumor suppressor genes |
| Genes that normally keep cell division under control, preventing the cell from responding to internal and external commands to divide. |
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| tundra biome |
| Extensive treeless plain across northern Europe, Asia, and North American between the taiga to the south and the permanent ice to the north. Much of the soil remains frozen in permafrost, and grasses and other vegetation support herds of large grazing mammals. |
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| turgor pressure |
| Pressure caused by the cytoplasm pressing against the cell wall. |
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| Turner syndrome |
| In humans, a genetically determined condition in which an individual has only one sex chromosome (an X). Affected individuals are always female and are typically short and infertile. |
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