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.

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.

target cell A cell that a particular hormone effects by its direct action (either passing through the membrane or binding to a surface receptor).



tarsals The bones that make up the ankle joint.

taxis The behavior when an animal turns and moves toward or away from an external stimulus (pl.: taxes).

taxon Term applied group of organisms comprising a given taxonomic category

taxonomy A systematic method of classifying plants and animals. Classification of organisms based on degrees of similarity purportedly representing evolutionary (phylogenetic) relatedness.

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.

tectonic plates Segments of the lithosphere that comprise the surface of the Earth much the way a turtle shell is composed of its plates.

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.

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.

template strand The strand of DNA that is transcribed to make RNA.



temporal lobe The lobe of the cerebral cortex that is responsible for processing auditory signals.

tendons Bundles of connective tissue that link muscle to bone. Fibrous connective tissue that connects muscle to bone. .



terminal buds Buds located at the end of a plant shoot.

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.



termination codon One of three three-base sequences that initiate termination of the protein synthesis process.

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.

test cross Genetic crossing of an organism with known genotype (one that exhibits the recessivephenotype) with an individual expressing the dominant phenotype but of unknown heritage.

tetrad The four chromatids in each cluster during synapsis; formed by the two sister chromatids in each of the two homologous chromosomes.

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.

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.

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.

thermacidophiles A group of archaebacteria that are able to tolerate high temperatures and acidic pH.

thermiogenesis The generation of heat by raising the body's metabolic rate; controlled by the hypothalamus.

thermoregulation The regulation of body temperature.

thigmotropism Plants' response to contact with a solid object; e.g., tendrils' twining around a pole. Plant response to touch.

thoracic cavity The chest cavity in which the heart and lungs are located.

thorax In many arthropods, one of three regions formed by the fusion of the segments (others are the head and abdomen).

thorns Stems modified to protect the plant.

thoroughfare channels Shortcuts within the capillary network that allow blood to bypass a capillary bed.



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.


thymine One of the pyrimidine bases in DNA, thymine is replaced by uracil in RNA.

thyroid-stimulating hormone A hormone produced by the anterior pituitary that stimulates the production and release of thyroid hormones.

tight junctions Junctions between the plasma membranes of adjacent cells in animals that form a barrier, preventing materials from passing between the cells.

tissues Groups of similar cells organized to carry out one or more speciÞc functions. Groups of cells performing a function in a multicellular organism.

toxins Term applied to poisons in living systems.

trace fossil Any indication of prehistoric organic activity, such as tracks, trails, burrows, or nests.

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.

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.



transcription The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule.

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.



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.

transforming factor GrifÞth's name for the unknown material leading to transformation; later found to be DNA.

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.

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.



translocation 1)The movement of a segment from one chromosome to another without altering the number of chromosomes. 2) the movement of þuids through the phloem from one part of a plant to another, with the direction of movement depending on the pressure gradients between source and sink regions.



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.



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.

trichomes Extensions from the epidermis of the plant that provide shade and protection for the plant.

trichocysts Barbed, thread-like organelles of ciliated protozoans that can be discharged for defense or to capture prey.

trilobites A group of benthonic, detritus-feeding, extinct marine invertebrate animals (phylum Arthropoda), having skeletons of an organic compound called chitin. Trilobites appear in abundance early in the Cambrian period and were dominant animals in the Burgess Shale fauna, before finally becoming extinct at the end of the Permian period.



triplet Three-base sequence of mRNA that codes for a specific amino acid or termination codon.

trisomy A condition where a cell has an extra chromosome.

trophoblast The outer layer of cells of a blastocyst that adhere to the endometrium during implantation.

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.

tropic hormone Hormone made by one gland that causes another gland to secrete a hormone.

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.

true-breeding Occurs when self-fertilization gives rise to the same traits in all offspring, generation after generation. Now interpreted as equivalent to homozygous.

trypanosomes A type of roundworm, responsible for human disease associated with eating raw or undercooked pork.

tubal ligation A contraceptive procedure in women in which the oviducts are cut, preventing the ova from reaching the uterus.

tubal pregnancy Occurs when the morula remains in the oviduct and does not descend into the uterus.

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.

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.

tubers Swollen underground stems in plants that store food, such as the irish potato.

tubular secretion The process in which ions and other waste products are transported into the distal tubules of the nephron.

tubulins The protein subunits from which microtubules are assembled.

tumor suppressor genes Genes that normally keep cell division under control, preventing the cell from responding to internal and external commands to divide.

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.

turgor pressure Pressure caused by the cytoplasm pressing against the cell wall.

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.