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Absolute refractory periodTime during which an excitable membrane cannot generate an action potential in response to any stimulus.

Accessory reproductive organ Duct through which sperm or eggs are transported, or a gland emptying into such a duct (in the female the breasts are usually included).

Acetylcholine A particular neurotransmitter released by pre- and post-ganglionic parasympathetic neurons, preganglionic sympathetic neurons, somatic neurons, and some CNS neurons.

Acetylcholinesterase Enzyme that breaks down acetylcholine into acetic acid and choline.

Acquired immune deficiency syndrome (AIDS) A progressive immune deficiency caused by infection of CD4 T cells with the human retrovirus HIV.

Acrosome Cytoplasmic vesicle containing digestive enzymes and located at head of a sperm.

Action potential Electric signal propagated by nerve and muscle cells; an all-or-none depolarization of membrane polarity; has a threshold and refractory period and is conducted without decrement.

Activated macrophage Macrophage whose killing ability has been enhanced by cytokines, particularly IL-2 and g-interferon.

Active hyperemia Increased blood flow through a tissue due to increased metabolic activity.

Active transport Energy-requiring system that uses transporters to move ions or molecules across a membrane against an electrochemical difference.

Adaptation Induction or repression of synthesis of proteins in response to stimulus. (Enzyme) An alteration in enzyme activity brought about by an inducer or repressor and involving an altered rate of enzyme synthesis or degradation. (Neural) Decrease in action-potential frequency in a neuron despite constant stimulus. (Evolution) A biological characteristic that favors survival in a particular environment.

Adaptor A synthetic, double-stranded oligonucleotide used to attach sticky ends to a blunt-ended DNA molecule.

ADCC = Antibody-dependent cell-mediated cytotoxicity

Adenovirus A vector that an animal virus derivatives of which have been used to clone genes in mammalian cells.

Adipocyte Cell specialized for triacylglycerol synthesis and storage; fat cell.
AFC = Antibody-forming cell

Afferent Coming into.

Affinity chromatography A chromatography method that makes use of a ligand that binds a specific portion of protein or nucleic acid and which can therefore be used to aid purification of that protein or nucleic acid.

Agonist Chemical messenger that binds to receptor and triggers cell's response; often refers to drug that mimics action of chemical normally in the body.

Agrobacterium tumefaciens The soil bacterium which, when containing the Ti plasmid, is able to form crown galls on a number of dicotyledonous plant species.

AIDS = Acquired immune deficiency syndrome

Allergy Originally defined as altered reactivity on second contact with antigen; now usually refers to a Type I hypersensitivity reaction.

Allele One nucleotide sequence in a given gene different from other sequences of that same gene; can be classified as dominant or recessive.

Allelic exclusion This occurs when the use of a gene from the maternal or paternal chromosome prevents the use of the other. This is seen with antibody and T-cell receptor genes.

Allotype The protein of an allele which may be detectable as an antigen by another member of the same species.

Allograft = Homograft.

Allosteric site Region on the surface of an enzyme (or other protein) that is distinct from the active site and to which activators or inhibitors (effectors) bind; also called the regulatory site. Effector binding induces a conformational change that modulates enzymatic activity

Alpha cell Glucagon-secreting cell of pancreatic islet of Langerhans.

Alpha helix Coiled conformation of polypeptide chain found in many proteins.

Alternative pathway The activation pathways of the complement system involving C3 and factors B, D, P, H and I, which interact in the vicinity of an activator surface to form an alternative pathway C3 convertase. opp. Classical pathway.

Alveolus Thin-walled, air-filled "outpocketing" from terminal air passageways in lungs; cell cluster at end of duct in secretory gland.

Amine hormone Hormone derived from amino acid tyrosine; includes thyroid hormone, epinephrine, norepinephrine, and dopamine.

Amino acids Class of organic compounds in which an amino goup and carboxyl group are linked to a central carbon atom, called the α carbon, to which a variable side chain is bound. Twenty common amino acids are the monomers that polymerize to form proteins. Some of these amino acids can then become modified.

Amnesia Memory loss.

Amniocentesis Prenatal sampling of the amniotic cavity.

Amphipathic molecule Molecule containing polar or ionized groups at one end and nonpolar groups at the other.

Anabolism Cellular synthesis of organic molecules.

Analgesia Removal of pain without loss of consciousness.

Anaphase Mitotic stage during which the sister chromatids separate and move apart (segregate) toward the spindle poles.

Anaphylaxis An antigen-specific immune reaction mediated primarily by IgE which results in vasodilation and constriction of smooth muscles, including those of the bronchus, and which may result in the death of the animal.

Anchorage-dependent Requiring attachment to solid substrate for survival or growth.

Reduction in total blood hemoglobin.

Anemometer An instrument for measuring airflow rate.

Aneuploid Not an exact multiple of the haploid chromosome number (haploid=that
number present in germ cells after meiosis; i.e., each chromosome represented once).

(Neovascularization) The proliferation of new blood vessels. The process transforms a small, usually harmless cluster of abnormal cells (known as an in situ tumor) into a large mass that can spread to other organs.

Angiotensin II Hormone formed by action of angiotensin converting enzyme on angiotensin I: stimulates aldosterone secretion from adrenal cortex, vascular smooth-muscle contraction, and thirst.

Antagonist (Muscle) Whose action opposes intended movement: (Drug) Molecule that competes with another for a receptor and binds to the receptor but does not trigger the cell's response

Antibody Immunoglobulin that is secreted by plasma cell; combines with type of antigen that stimulated its production; directs attack against antigen or cell bearing it.

Antibody-dependent cell-mediated cytotoxicity (ADCC) A cytotoxic reaction in which Fc receptor- bearing killer cells recognize target cells via specific antibodies.

Antibody-forming cells (AFC) Functionally equivalent to plasma cells.

Anticodon Three-nucleotide sequence in tRNA able to base-pair with complementary codon in mRNA during protein synthesis

Antigen Any foreign molecule that reacts with specific receptors on T and T cells so that stimulates a specific immune response.

Antigen-presenting cell (APC) Cell that presents antigen, complexed with MHC proteins on its surface, to T cells.

Antigenic determinants = Epitopes.

Antigen processing The conversion of an antigen into a form in which it can be recognized by lymphocytes.

Antisense RNA The RNA molecules that is the reverse complement of a naturally occurring mRNA, and which can be used to prevent translation of that mRNA in a transformed cell.

Antisense technology The technology of genetic engineering of a gene coding for an antisense RNA

Antiport Cotransport process in which the movement of a molecule or ion across a membrane against its concentration gradient is driven by the movement in the opposite direction of a second ion down its concentration gradient. Transport is mediated by specific membrane-bound proteins called antiporters.

Antiviral proteins Proteins whose synthesis is induced by interferons. They become activated if the cell is infected by virus and limit viral replication.

Aorta Largest artery in body; carries blood from left ventricle of heart to thorax and abdomen.

APC = Antigen-presenting cell

Aphasia Specific language deficit not due to mental retardation or muscular weakness.

Cessation of breathing.

Apoptosis Programmed cell death; a specific suicide process in animal cells that includes fragmentation of nuclear DNA. Apoptosis occurs normally in development, as in the resorption of the tadpole tail during metamorphosis into a frog; or it can be induced, for example by DNA damage that exceeds the capacity of repair mechanisms.

Any variation from normal heartbeat rhythm.

Arteriole Blood vessel between artery and capillary; surrounded by smooth muscle; primary site of vascular resistance.

Artery Thick-walled, elastic vessel that carries blood away from heart to arterioles.

Arthus reaction Inflammation seen in the skin some hours following injection of antigen. It is a manifestation of a Type III hypersensitivity reaction.

Aseptic Free of microbial infection.

Asthma Disease characterized by severe airway constriction and plugging of the airways with mucus.

Aterium Chamber of heart that receives blood from veins and passes it on to ventricle on same side of heart

ATP (Adenosine 5'-triphosphate) A nucleotide containing two high-energy phospho-anhydride bonds whose hydrolysis or transfer is each accompanied by a large free-energy change (ΔG) of ~7 kCal/mole. It is the most important molecule for capturing and transferring free energy in all cells.

ATPase One of a large group of enzymes that catalyze hydrolysis of ATP to yield ADP and inorganic phosphate with release of free energy. Theses enzymes normally are associated with cellular membranes or cytoskeletal structures and the energy released is utilized in some coupled activity such as active transport, muscle contraction, unwinding of DNA, or movement of cilia and flagella.

Atrophy Wasting away; Decrease in size; e.g. Disuse atrophy (Decrease in size of muscle fibers that are not used for a long time.) and Denervation atrophy (Decrease in size of muscle fiber whose nerve supply is destroyed.)

ATP synthase (mitochondrial) Multimeric protein complex bound to inner mitochondrial membranes that catalyzes synthesis of ATP coupled to proton movement down its electrochemical gradient; also called F0F1 complex. Similar enzymes are present in bacterial plasma membranes and the thylakoid membranes of chloroplasts (the CF0CF1 complex).

Autoimmunity Immune recognition and reaction against the individual's own tissue.

A method of detecting radioactively labeled molecules through exposure of an X-ray-sensitive photographic film

Autoreceptors Receptors on a cell affected by a chemical messenger released from the same cell

Autosome Any chromosome other than a sex chromosome.

Auxotroph A mutant microorganism that will grow only if supplied with a nutrient not required by the wild-type.

Avidin A glycoprotein of MW 66 kDa composed of four subunits of 16 kDa each and that has a high affinity for biotin and is used in the detection system for biotinylated probes

Axon Extension from neuron cell body; propagates action potentials away from cell body; also called a nerve fiber.

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