ABAXIAL. (Of a leaf surface) facing away from stem. Compare ADAXIAL.
ABDOMEN. (1) Vertebrate body region containing viscera (e.g. intestine, liver, kidneys) other than heart and lungs; bounded anteriorly in mammals but not other classes by a diaphragm. (2) Posterior arthropod trunk segments, exhibiting,in insects, but not in crustaceans.
ABDUCENS NERVE. Sixth vertebrate CRANIAL NERVE. Mixed, but mainly motor, supplying external rectus eye-muscle.
ABERRANT CHROMOSOME BEHAVIOUR. Departures from normal mitotic and meiotic chromosome behaviour, often with a recognized genetic basis. Includes (1) achiasmate meiosis, where chiasmata fail to form (e.g. in Drosophila spermatogenesis; see SUPPRESSOR MUT A TI 0 N); (2) amitosis, where a dumb-bell-like constriction separates into two the apparently ‘interphasellike’, but often highly polyploid, ciliate macronucleus prior to fission of the cell; (3) chromosome extrusion or loss, as with X-chromosomes in egg maturation of some parthenogenetic aphids (see s EXDETERMINATION); and . in Drosophila where gynandromorphs may result; but notably in some midges (e.g. Miastor, Heteropeza) where paedogenetic larvae produce embryos whose somatic cells contain far fewer chromosomes than GERM LINE cells, owing to selective elimination during 1 cleavage (see WEISMANN). In some scale insects, males and females develop from fertilized eggs, but males are haploid because the entire paternal chromosome set is discarded at cleavage (see HETEROCHROMATIN, PARASEXUALITY, GYNOGENESIS); (4) meiotic drive, where a mutation causes the chromosome on which it occurs to be represented disproportionately often in gametes produced by meiosis, as with the segregation distorter (SD) locus of Drosophila; mutants homozygous for the SD allele are effectively sterile; (5) premeiotic chromosome doubling (see Aut0mixis); (6) ENDOMITOSIS, where chromosomes replicate and separate but the nucleus and cell do not divide; (7) POLYTENY, where DNA replication occurs but the strands remain together to form thick, giant chromosomes.
ABIOTIC. Environmental features, such as climatic and EDAPHIC factors, that do not derive directly from the presence of other organisms. See BIOTIC.
ABOMASSUM 2 ABOMASSUM. The ‘true’ stomach of RUMINANTS. +L s ABSCISIC ACID (ABSCISIN, DORMIN). Inhibitory plant GROWTH SUBSTANCE (a sesquiterpene). Present in a variety of plant organs , leaves, buds, fruits, seeds and tubers. Promotes senescence and abscission of leaves; induces dormancy in buds and seeds. Antagonizes influences of growth-promoting substances. Believed to act by inhibiting nucleic acid and protein synthesis.
ABSCISSION LAYER. Layer at base of leaf stalk in woody dicotyledons and gymnophytes, in which the parenchyma cells become separated \ *from one another through dissolution of the middle lamella before leaf-fall.
ABSORPTION SPECKS Graph of light absorption versus wavelength of incident light. Shows how much light (measured as quanta) is absorbed by a pigment (e.g. plant pigments) at each wave length. CO~paIX ACTION SPECTRUM.‘
ABYSSAL. Inhabiting deep water, roughly below 1000 metres.
ACANTHODII. Class of primitive, usually minnow-sized, fossil fish . abundant in early Devonian freshwater deposits. Earliest known gnathostomes. Bony skeletal tissue. Fins supported by very stout spine; several accessory pairs of fins common. Row of spines between pectoral and pelvic fms. Heterocercal tail. Relationships with osteichthyan fishes uncertain, but probably not directly ancestral. See PLACODERMI..
ACANTHOPTERYGII. Spmy-rayed fish. Largest superorder of (teleost) fishes. Spiny rays in their fins consist of solid pieces of bone (and not numerous’small bony pieces); are unbranched and pointed at their tips. Radial bones of each ray are sutured or fused, preventing relative lateral movement. Often have short, deep bodies,.and relatively large fins, making these fish very manoeuvrable. See TELEOSTEI.
ACARI (OCARINA). Order of ARACHNIDA including mites and ticks. -External segmentation much reduced or absent. Larvae usually with three pairs of legs, nymphs land adults with four pairs. Of considerable economic and social importance as many are ectoparasites and vectors of pathogens. :.-
ACCESSORY BUD. A bud generally situated above or on either side of main axillary bud.
ACCESSORY CHROMOSOME. SUPER NUMERARY CHROMOSOME.
ACCESSORY NERVE. Eleventh cranial nerve of tetrapod vertebrates, unusual in originating from both brain stem and spinal cord. A mixed nerve, whose major motor output Js to muscles of throat, neck and viscera.
ACCESSORY PIGMENT. Pigment that captures light energy and transfers it to chlorophyll a, e.g. chlorophyll b, carotenoids, phycobiliproteins.
ACCOMMODATION. Changing the focus of the eye. In man and a few other mammals occurs by changing the curvature of the lens; at rest lens is focused for distant objects and is focused for near objects by becoming more convex with contraction of the ciliary muscles in the CILIARY BODY.
See EYE ,OCULOMOTOR NERVE.
ACELLULAR. Term sometimes applied to organisms or their parts in which no nucleus has sole charge of a specialized part of the cytoplasm, as in unicellular organisms. Applicable to coenocytic organisms (e.g. many fungi), and to tissues forming a SYNCYTIUM. Sometimes preferred to ‘unicellular’. See MULTICELLULARTY.
ACENTRIC. (Of chromosomes) chromatids or their fragments lacking ally CENTROMERES.
ACETABULUM. Cup-like hollow on each side of hip girdle into which head of femur (thigh bone) fits, forming hip joint in tetrapod vertebrates. See PELVIC GIRDLE.
ACETIFICATION. See FERMENTATION.
ACETYLCHOLINE (Ach). NEUROTRANSMITTER of many interneural, neuromuscular and other chdinergic effector synapses. Relays electrical signal in chemical form, with transduction back to electrical signal at the postsynaptic membrane. Initiates depolarization of postsynaptic membranes to which it binds; but hyperpolarizes vew-gw in leased there in quanta1 fashion in response to calcium ion uptake on arrival of an ACTION POTENTIAL. It diffuses across the synaptic cleft and binds to receptor sites on the postsynaptic membrane, whereupon these ion channels open and allow appropriately sized positive ions to, enter cell, initiating membrane depolarization. Hydrolysis to choline and the postsynaptic membrane ropriately brief (see SUM MATION). Vertebrate ACh postsynaptic receptors are distinguished as nicotinic or muscarinic on the results of alkaloid administration. receptors- GAnglia, neuromuscular junctions and possibly some brain and spinal cord regions) are blocked by curare, muscarinic (peripheral autonomic interneural synapses) by atropine. ACh is found in some protozoans.Compare ADRENERGIC. j
ACETYLCHOLINESTERASE. See CHOLINESTERASE.
ACETYL COENZYMEA. See COENZYME A.
ACHENE. Simple, dry, one-seeded fruit formed from a single carpel, without any special method of opening to liberate seed; seed coat is not adherent to the pericarp; may be smooth-walled (e.g. buttercup), feathery (e.g- traveller’s joy), spiny (e.g. corn buttercup), or winged (when termed a samara) as in sycamore and maple.
ACHIASMATE. Of meioses lacking chiasmata. One form of ABERRANT CHROMOSOME BEHA VIOUR. See SUPPRESSORMUTATION.
ACHLAMYDEOUS. (Of flowers) lacking petals and sepals; e.g. willow.
ACID DYES. Dyes consisting of an acidic organic compound (anion) which is the actively staining part, combined with an inorganic cation, e.g. eosin. Stain particularly cytoplasm and collagen. See BASIC DYES.
ACXD HYDROLASE. Any hydrolytic enzyme whose optimum pH of activity is in the acidic range. Many different examples occur in LYSOSOMES. Pepsin is an acid prOteaSe.
ACID PHOSPHATASE. One of several acid hydrolases located in CYSOMES and concentrated in the trans-most cisternae of the GOLGI APPARATUS.
ACID RAIN. Rainfall (precipitation) with a pH less than 5 - 6. Rain dissolves carbon dioxide, forming carbonic acid, giving it a normal pH of 5.6, but lower pHs result as it dissolves atmospheric pollutants such as oxides of nitrogen and sulphur dioxide. Some acid rain results from effects of atmospheric ozone production, some natural and some attributable to human activity. Its most serious consequence is the release of cations from the soil resulting in leaching. In the case of Mg++ ions this leads to chlorosis of leaves and poor plant growth, even death.
ACINAR CELLS. See ACINI.
ACINAR GLAND. (Zool.) Multicellular gland (e.g. seminal vesicle) with fla&-like secretory portions.
ACINI. (Zool.) Cells lining tubules of pancreas and secreting digestive juices. Their secretory vesicles (zymogen granules) concentrate the Enzymes and fuse with the apical portion of the plasmalemma under , the StiInUlUS Of ACETYLCHOLINE Or CHOLECYSTOKININ , releSlSing their contents into the lumen of the duct. Much used in the study of secretion.
ACOELOMATE. Having no COELOM. Refers to some lower animal phyla, e.g. coelenterates, platyhelminths, nemerteans and nematodes.
ACOUSTICNERVE. See VESTIBULO~OCHLEAR NERVE.
ACOUSTICO-LATERALIS SYSTEM. See LATERAL LINE SYSTEM.
ACQUIRED CHARACTERISTICS, INHERITANCE OF. See LAMARCKISM. I ACTIN
ACOWRED IMMUNE RESPONSE. Secondary antibody response to presence of antigen and differing from the initial response (which may precede it by a matter of years) in that it appears more quickly, achieves a higher antibody titre (concentration) in the blood and in that the principal IMMUNOGLOBULIN species present is IgG rather thanIgM.See ANTIBODY , IMMUNITY.
ACRANIA. See CEPHALOCHORDATA. .
ACRASIOMYCOTINA (ACRASIALES) . Cellular slime moulds. Those MYXOMYCOTA which may exist as separate amoebae (myxamoebae), and retain their original identities within the pseudoplasmodium (slug) formed by swarming.
ACROCENTRIC. Of chromosomes and chromatids in which the CENTROMERE iscloseto oneend.
ACROMION. Point of attachment of clavicle to scapula in mammals and mammal-like reptiles. A bone process.
ACROPETAL. (Bot.) Development of organs in succession towards apex, the oldest at base, youngest at tip (e.g. leaves on a shoot). Also used in reference to direction of transport of substances within a plant, i.e. towards the apex. Compare BASIPETAL.
ACROSOME. Specialized penetrating vesicular organelle, formed from GOLGI APPARATUS and part of the nuclear envelope at the tip of a spermatozoon. It contains HY A L u R 0 N 1 D A s E, several lytic enzymes and acid hydrolases released when the sperm cell membrane fuses at several points with the acrosome during the acrosome reaction, dissolving the jelly around the egg so that the sperm can penetrate it. Some sperm discharge an acrbsomalprocess composed of rapidly polymerizing
ACTH (ADRENOCORTICOTROPIC HORMONE, CORTICOTROPIN). A polypeptide of 39 amino acids secreted by r lobe of the pituitary, involved in the growth and secretory activity of adrenal cortex. Has a minor positive effect on aldosterone secretion, but an important role in glucocorticoid secretion. Both stress and low blood glucocorticoid levels cause release from the hypothalamus of corticotropin releasing factor (CRF) which initiates ACTH release. See ADRENAL GLAND CORTISOL
ACTIN which punctures the egg membranes prior to fusion with ovum (e.g. in some echinoderms).
ACTIN. Diagnostic eukaryotic protein, absent from prokaryotes. Filamentous actin (F-actin) is composed of globular protein monomers (cG-actin molecules) polvmerized to form long fibrous molecules, two of which coil round one other in the thin actin filaments of muscle ’ and other eukaryotic cells, where they are termed microfilaments. Each G-actin molecule-hinds one calcium ion and one AT P or A D P molecule, when it polymerizes to form F-actin with ATP
ACTINOMORPHIC ~Like MICROTUBULES, the opposite ends of actin filaments grow and depolymerize at different rates and play a vital role in CYTOSKELETON structure. Stress fibres are bundles of actin filaments and other proteins at the lower surfaces of cells in culture dishes and , will contract if exposed to ATP in vitro. Microfilaments are involved in the building of F I L o P o D I A, microspikes and MICROVILLI where, as in stress fibres, they form paracrystalline bundles. Filaments of actin and MYOSIN are capable of contracting together as ACTOMYOSIN in both muscle and non-muscle cells, e.g. in the contractile ring of dividing cells, in belt DESMOSOMES and in CYTOPLASMIC STREAMING.
ACTINOMORPHIC. (Of flowers) regular; capable of bisection vertically in two (or more) planes into similar. halves, e.g. buttercup. Such flowers are also said to exhibit RADIAL SYMMETRY.
ACTINOMYCETE. Member af an order (Actinomycotales) of Grampositive bacteria with cells arranged in hypha-like filaments. Mostly saprotrophs, some parasites. Source of streptomycin.
ACTINOMYCIN D. Antibiotic derived from species of the bacterial genus Streptomyces. Binds to DNA between two G-C base pairs and prevents movement of RNA polymerase, so preventing transcription in both prokaryotes and eukaryotes. Penetrates into intact cells. See ANTIBIOTIC.
ACTINOPTERYGII. Ray-finned fishes. Generally regarded as subclass of Osteichthyes, and includes all common fish except sharks, skates and rays. Earliest forms (chondrosteans) represented in the Devonian by the palaeoniscoids and today by e.g. Polypterus; later forms (holosteans) were predominantly Mesozoic fishes but represented today by e.g. Lepisosteus (gar pikes); teleosts are the dominant fish of the modern world and represent the subclass in almost every part of the globe accessible to fish. Internal nostrils absent; SC + LES ganoid in : primitive forms, but reduced or even absent in teleosts. The paired , fins are webs of skin braced by horny rays (like ribs of a fan), each a row of slender scales, there being no fleshy fin lobes except in the most primitive forms. A swim bladder is present and the skeleton is bony. Internal groupings given here probably represent GRADES : ratherthan CLADES. See TELEOSTEI, ACANTHOPTERYGII.
ACTINOZOA (ANTHOZOA). Sea anemones, corals, sea pens, etc. A class of Coelenterata (Subphylum Cnidaria). The body is a polyp, there being no medusoid stage in the life cycle. Polyp more complexly organized L than that of other coelenterates; coelenteron divided by vertical 1 mesenteries. May have external calcareous skeleton as in well-known icorals, but some forms have internal skeleton of spicules in mesogloea.
ACTION POTENTIAL. Localized reversal and then restoration of electrical potential between the inside and outside of a nerve or muscle cell (or fibre) which marks the position of an impulse as it travels alongthecell. See IMPULSE, ACTIVATION.
ACTION SPECTRUM. Plot of the quanta of different wavelengths required for a photochemical response against the wavelength of light used. Its reciprocal indicates photochemical efficiency.
ACTIVATED SLUDGE. Material consisting largely of bacteria and protozoa, used in and produced by one method of sewage disposal. Sewage is mixed with some activated sludge and agitated with air; organisms . of the sludge multiply and purify the’sewage, and when it is allowed to settle they separate out as a greatly increased amount of activated sludge. Part of this is added to new sewage and part disposed of.
ACTIVATION. (Of eggs). When the membrane of the sperm ACROSOME fuses with the egg plasma membrane, anzactivation reaction passes to-the surface involving an A duration than in nerve or muscle. It logical development and may be achieved merely by pricking of some eggs (e.g. frog).
ACTIVATION ENERGY. Free energy of activation is the amount of energy needed to bring all the molecules in 1 mole of a substance at a given temperature to the transition state (when there is high probability that a chemical bond will be made or broken) at the top of an activation bar-rier. Its biological significance is that enzym+es accelerate reactions by lowering their energies of activation, the principal factor 1 permitting such complicated chemistry to occur at relatively low temperatures.
ACTIVE SITE. Part of an enzyme molecule in its natural hydrated state which, by its three-dimensional conformation and charge distribution, confers upon the enzyme its substrate specificity. It binds to a substrate molecule, forming a transient enzyme-substrate complex. Enzymes may have more than one active site and so catalyse more than one reaction. Competitive inhibitors of an enzyme reaction bind reversibly to the active site-and reduce its availability for normal substrate. Active sites may only take on their appropriate conformation after the enzyme has combined at some other site with an appropriate modulator molecule. Some active sites require metal ions as prosthetic groups (e.g. human carboxypeptidase requires a zinc atom). See ENZYME.
ACTIVE TRANSPORT. The energy-dependent carriage of a substance across a cell membrane, accumulating it on the other side in opposition to chemical or electrochemical gradients (i.e. ‘uphill’). The process involves ‘pumps’ composed of protein molecules in the membrane (often traversing it) which carry out the transport. Requires an , appropriate energy supply, commonly ATP, or a gradient of4 protons
ACTOMYOSIN cross the membrane it self usually generated by redox, photochemical or ATP-hydrolysing reactions. Collapse of this gradient drives proton-linked symports or antiports (see TRANSPORT PROTEINS).Alternatively, a membrane potential arising from ion asymmetry ; across the membrane may drive specific ions through special transport systems. Probably all cells engage in active transport. See SODIUM PUMP, ELECTRON TRANSPORT SYSTEM, FACILITATED DIFFUSION.
ACTOMYOSIN. Complex formed when the pure proteins ACTIN and MYOSIN are mixed, resulting in increased viscosity of the solution. Actomyosin under- presemzeAT nesmm ions (Mg++), when A TP hvdrolvu Completion of this results in reaggregation of the two proteins. Live muscle cells have an. absolute reQuirefnent for calcium ions (Ca++) before myosin and actin filaments will interact, and when Ca++ is removed the actin and myosin dissociate. Such interactions form the basis of ’ many biological force-generating events, notably during MUSCLE CONTRACTION, CYTOPLASMIC STREAMING, CELL LOCOMOTION and blood clot contraction.
ADAPTATION. (1) Evolutionary. Some property of an organism is normally regarded as an adaptation (i.e. fits the organism in its environment) if (a) it occurs commonly in the population, and (b) the cause of its commonness was NATURAL SELECTION in its favour. Adaptations are not, therefore, ‘fortuitous benefits, theimplication being that they have a genetic basis, since selection operates only upon genetic differences between individuals: Alternatively, we often in practice identify an adaptation by its effects rather than itscauses. Learned abilities which improve an individual’s,~~~~~~~ or inclusive fitness, but without clear genetic causation, are cases in point. See ‘TELEOLOGY. (2) Physiological. A change in an organism, resulting from exposure to certain environmental conditions, allowing it to respond-more effectively to them. (3) Sensory. A change in excitability of a sense organ through continuous stimulation, increasingly in tense stimuli being required to produce the same response.
ADAPTIVE ENZYME. Inducible enzyme. See ENZYME.
ADAPTIVE IMMUNE RESPONSE. Response,’ ultimately by B-CELLS, to The presence of foreign antigen, in which large quantities of antigen specific antibody appear in the blood while MEMORY CELLS with : antigen-specific binding sites persist with capability of rapid clonal expansion on subsequent triggering by the antigen.
ADAPTIVE RADIATION. Evolutionary diversification from a single ancestral (prototype) population of descendant populations into more and morenumerous ADAPTIVE ZONES and ecological NICHES. May involveboth ANAGENESIS and CLADOGENESIS.
ADAPTWE ZONE. A more or less distinctive set .of ecdogical niches established and occupied by an evolutionary lineage with time.
ADXIAL:. (O/f a,leaf surface) facing the stem. Compare ABAXIAL. '
ADENINE. A purine base of D-NA, RNA, some nucleotides and their derivatives.
ADENOHYPOPHYSIS. See PITUITARYGLAND.
ADENSINE DIPHOSPHATE. See ADP. _ "
ADENOSINE MONOPHOSPHATE. See AMP. .
ADEOSINE TRIPHOSPHAT See ATP.
ADENOVIRUS. One kind of DNA tumour virus of animals: See VIRUS. _
ADENYL CYCLASE (ADENYLATE cYcLASE). A plasma .membrane-bound enzyme converting ,A TP to cyclic AMP (see AMP). Many peptide hormones and local chemical signals operate through activation of this enzyme.
ADH. See ANTIDIURETICHO&MONE.
ADH~ION. Cells of a multicellular animal must be able to recognize and adhere to each-other in order to group together as tissues. It is notyetclear how this happens,but INTERCELLULAR JUNCTIONS are implicated. Involved in MORPHOGE~NESIS and MULTICELLULARITY.
ADIPOSE TISSUE. A connective tissue. (1) Brown adipose tissue (brown fat) comprises cells whose granular cytoplasm is due to high concentration of cytochromes and whose function appears to be release of heat in the neonatal mammal. Distributed around neck and between scapulae in these and hibernating mammals but not otherwise extensively in adults. Richly innervated and vascularized. (2) White j adipose tissue is distributed,widely in animal bodies, comprising large cells (fat cells) each with single large fat droplet inside a thin rim of cytoplasm. This depot fat is composed largely of triglyceride. JD -uR-&ENeA LreIlNeEas,eG LoUt CtaAtGtyO Na,cGidRsO WaTnH@HlolcReMrOoNl E avIial daActCivTaHtiiotnl koif mi-ntrinsic lizprobably via cyclic AMP (see AMP, SECOND MESSENGER). Its nerve supply is less than that of brown adipose tissue.
ADP (ADENOSINE DIPHOSPHATE). A nucleoside diphosphate found universally inside cells. Phosphorylated to ATP during energy-yielding ~ catabolic reactions and produced in turn when ATP itself is hydrolysed:
ADRENAL GLAND (SUPRARENAL G., EPINEPHRIC G.). Endocrine gland of most tetrapod vertebrates lying paired on either side of the.mid-line,
ADRENALINE one atop each kidney. Each is a composite of an outer cortex derived from coelomic mesoderm, making up the bulk of the gland, and an inner medulla derived from neural crest cells of the ectoderm. Rarely found as a composite gland in fish. Cortex comprises three zones, the outermost secreting aldosterone which promotes water retention by kidneys by increasing renal potassium excretion and sodium retention; and other glucocorticoids under The medulla comprises sinuses. These mimic effects of the sympathetic nervous system (see AUTONOMIC NERVOUS SYSTEM), releasebeingunderhypothalamic control via the splanchnic nerve. They promote liver and muscle glycogenolysis via cyclic AMP (See AMP), lipolysis in’ ADIPOSE TISSUE, vasodilation in skeletal and heart muscle and brain, and vasoconstriction in skin and gut. They relax bronchi and bronchioles and increase rate and power of heart beat, raising blood pressure. All adrenal hormones are known as ‘stress’ hormones, those of the cortex responding to internal physiological stress such as low blood temperature or volume, while medullary hormones are released in response to stress situations (often auditory or visual) outside the body. See L-DoPA.
ADRENALINE. (In USA, EPINEPHRINE.) Hormone derivative of amino acid tyrosine secreted by chromaffin cells of ADRENAL GLAND and to a lesser extent by sympathetic nerve endings.
ADRENERGIC. Of a motor nerve fibre secreting at its end noradrenaline (norepinephrine) or, less commonly, adrenaline. Characteristic of postganglionic sympathetic nerve endings. Compare CHOLINERGIC.ADRENO CORTICOTROPIC HORMONE ( A C T H ) . See ACTH,ADRENAL GLAND.
ADVENTITIOUS. Arising in an abnormal position; of roots, developing from part of the plant other than roots (e.g. from stem or leaf :cutting); of buds, developing from part of the plant other than a leaf 1 axil (e.g. from a root). ,
AECIOSPORE. Binucleate spore of rust fungi produced in a cup-shaped structure, the aecium (pl. aecia).
AERENCHYMA. Secondary spongy tissue of some aquatic plants, with intercellular air spaces formed by the activity of a CORK cambium or phellogen. May develop in a lesser way from the lenticels of land plants such as willow (Salix), and poplar (Pop&r) if partially submerged. Seems to function mainly in a flotation capacity rather than as a respiratory aid.
AEROBIC. Requiring free (gaseous or dissolved) oxygen. In most cases the oxygen. is utilized in aerobic respiration, but a few enzymes (oxygenases) insert oxygen atoms directly into organic substrates. See RESPIRATION. AEROBIC RESPIRATION. See AEROBIC,RESPIRATION AESTIVATIN. (Bot.) Arrangement of parts in a flower-bud. (Zool.) DORMANCY during summer or dry season as e.g., in lungfish (dipIlOanS),See HIBERNATION.
AETHELIUM. A sessile, rounded or pillow-shaped fruitification formed by a massing of the whole plasmodium in the‘Myxomycota.
AFFERENT. Leading towards, as of arteries leading to vertebrate gills or of nerve fibres (sensory) conducting an input towards the central NERVOUS SyStem. OppOSite Of EFFERENT.
A-FORM HELIX. Less common right-handed double helical form of DNA (compare B-FORM and Z-FORM HELICES), and; under some conditions, the most stable form of double-stranded DNA.
AFTER-RIPENING. Dormancy exhibited by certain seeds (e.g. hawthorn, apple) which, although embryo is apparently fully developed, will not germinate immediately seed is formed. Even when removed from seed coat and provided with favourable conditions, the embryo has to undergo certain chemical and physical changes before it can grow. Possibly associated with delay in production of required growth substances, or with gradual breakdown of growth inhibitors. See *DORMANT.
AGAMOSPECIES. See SPECIES.
AGAMOSPERMY. Any plant APOMIXIS in which embryos and seeds are. formed but without prior sexual fusion. Excludes vegetative reproduction (vegetative apomixis). Occurs widely in higher plants, both ferns and flowering plants. Unknown in gymnophytes. See PSEUDOGAMY.
AGAMOSPORY. Asexual formation of an embryo and the* subsequent development of a seed.
AGAR. Mucilage obtained from cell walls of certain red algae. Mixture of polysaccharides, some sulphated, forming gel with water and melting at a higher temperature than that at which it solidifies. Used as a solidifying base for culture media in microbiology.
AGAROSE. Polysaccharide used as gel in dolumn chromatography and inelectrophoresis.See SOUTHERN BLOT TECHNIQUE.
AGEING (SENESCENCEj. Progressive deterioration in function of cells, tissues, organs, etc., related to the period of time since that function commenced. By dividing indefinitely, bacteria and many protozoans
AGLUTlNATlQN 1 2 avoid ageing; higher plants often seem capable of unlimited vegetative propagation. Regeneration-and renewal in many simple invertebrates seem t0 permit escape from senescence. GERM LI NBS Of sexual metazoa are potentially immortal (see WEISMANN). Expressed as disintegration of somatic tissue, ageing may be due to gradual accumulation of somatic mutations or to late expression of genes not subject to strong selection. Some evidence suggests loss of DNA METHYLATION maybe involved.In the pOpulatiOn cOnteXt ,itmaybe due to inbreeding or to some other factor reducing genetic variation.
AGGLUTINATION. Sticking together or clumping; as of bacteria (an effect of antibodies), or through mismatch of AGGLUTINOGENS of red blood cells and plasma AGGLUTININS in blood transfusions. See LECTIN. -
AGGLUTININS (SOANTIBODIES). Plasma and cell-surface proteins that , by interacting with AGGLUTINOGENS (antigens) on foreign cells can cause cell clumping ( AGGLUTINA TION).
AGGLUTINOGEN. Proteins acting as ceil-surface antigens of red blood Cells and interacting with A G G L u T I N I N S to cause red cell clumping and possible -blockage of blood vessels. Genetically determined, and thebasisof BLOOD-GROUPS.
AGGREGATE FRUIT. Fruit which develops from several separate carpels of a single flower (e.g. magnolia, raspberry, strawberry).
AGNATHA. Class of Subphylum Vertebrata (sometimes also a superclass, other vertebrates forming Superclass Gnathostomata). Modern forms (cyclostomes) include lampreys (Subclass Monorhina) and hagfishes (Subclass Diplorhina), but fossil forms included anaspids, osteostracans and heterostracans. Jawless vertebrates. Buccal chamber acts as muscular pump sucking water in, serving for-filter-feeding in lamprey larvae as well as ventilating gills --an advance over ciliary mechanisms. Paired appendages almost unknown. Earliest forms (heterostracans) appear in the late Cambrian.
AGONISTIC BEHAVIOUR. Intraspecific behaviour normally interpreted as attacking, threatening, submissive or fleeing. Actual physical injury tends to be rare in most apparently aggressive encounters.
AGROBACTERIUM. Bacterial genus noted for crown gall tumour-inducing ability. Oncogenic strains are host to a tumour-inducing (Ti) PLASM ID which can be transmitted between species. A segment (T) of the Ti plasmid is transmitted to the plant host cell and is the immediate agent of tumour induction. See ON COGEN E.
AHNFELTAN. A complex phycocolloid substance occurring in the cell walls of some red algae (Rhodophyta). AIRBLADDER. See GAS BLADDER,
ALEURONE LAYER AIR SACS. (1) Expanded bronchi in abdomen and thorax of birds, initially in five pairs but one or more pairs fusing to form thin-walled passive sacs with limited. vascularization. Ramify throughout the body and within bones. Connected to lung by small tubes whose relative diameters are probably, crucially important in establishing a unidirectional passage of air from lung to sacs and back to lung. The avian ventilation system lacks a tidal rhythm characteristic of mammals. (2) Expansions of insect tracheae into thin-walled diverticulae whose compression and expansion assist V E NT I LA T IO N.
AKINETE. Vegetative cell which becomes transformed into a thickwalled, resistant spore. Formed by certain Cyanobacteria and some algae (e.g. some Chlorophyta).
ALBINISM. Failure to develop pigment, particularly melanin, in skin, hair and iris. Resulting albinos light-skinned with white hair and ‘pink’ eyes due to reflection from choroid capillaries behind retina. In mammals, including humans, usually due to homozygous autosomal recessive gene resulting in failure to produce enzyme tyrosine monooxygenase.
ALBUMEN. Egg-white of birds and some reptiles comprising mostly solution of ALB u MIN with other proteins and fibres of the glycoprotein ovomucoid. Contains the dense rope-like CHALA ZA and with yolk supplies protein and vitamins to embryo, but is also major source of water and minerals.
ALBUMIN. Group of several small proteins produced by the liver, forming up to half of human plasma protein content, with major responsibility for transport of free fatty acids, for blood viscosity and 0 s MO T I C P 0 TENT IA L. If present in low concentration oedema may result, as in kwashiorkor.
ALBUMINOUS CELLS. Ray and parenchyma cells in gymnophyte phloem, closely associated morphologically and physiologically with sieve cells.
ALCYONARIA. Order of coelenterates within the Class Actinozoa. Sea pens, soft corals, etc. Have eight pinnate tentacles and eight mesenteries. Polyps colonial, with continuity of body wall and enteron. Skeleton, often of calcareous spicules, within mesogloea and occasionally externally.
ALDOSTERONE. Hormone of ADRENAL cortex. See OSMOREGULATION.
ALEURONE GRAINS. Membrane-bound granules of storage protein occurring in the outermost cell layer of the endosperm of wheat and other grains.
ALEURONE LAYER. Metabolically active cells of outer cereal endosperm
ALEUROPLAST (in contrast to metabolically inactive cells of most of the endosperm) - . containing akurone grains, several hydrolytic enzymes and reserves of phytin (releasing inorganic phosphate and inositol on digestion by phytase). During germination, aleurone cells secrete a-amylase into . the endosperrn, initiating its digestion. Recent work suggests that the .’ synthesis of enzymes by aleurone cells may not, be as specifically ihduced by gibberellins from the embryo axis as was once thought, although these growth substances are certainly implicated in the control of endosperm digestion. ALEUROPLAST. Colourless plastid (leucoplast) storing protein; found in many seeds, e.g. brazil nuts.
ALGAE. Informal term covering many simple photosynthetic plants, including prokaryotic forms (CYANOBACTERIA, PROCEOPHYTA), although the majority ire eukaryotic. The algal plant body (TH ALL US) mai be unicellular or multicellular, filamentous, or flattened and ribbon-like, with relatively complex internal organization in the higher forms, e.g. some of the brown algae (Phaeophyta). Algae are either aquatic (marine or freshwater) or of damp situations, such as damp walls, rock faces, tree trunks, moss hummocks, or soil. Algal sexual reproduction differs from that of other chlorophyllous plants; when unicellular, the entire organism may function as a gamete;-when multicellular, gametes may be formed in unicellular or mulZicellular gametangia, each cell- of the latter being fertile and producing a gamete. These character&i&s ‘distinguish algae from higher plants.-The formal taxon ‘algae’ has been abandoned in recent classifications, component grolips being considered sufficiently distinctive to merit divisional status, dependent upon similarities and differences between pigments, assimilatory products, flagella, cell wall chemistry and aspects of cell ultrastructure. Eukaryotic algae include the following divisions: Bacillariophyta, Chlorophyta, Charophyta, Eugleno, phyta, Chrysophyta, Xanthophyta, Prymtiesiophyta, Pyrrophyta (Dinophyta), Eusti&natophyta, Cryptophyta, Rhohophyta and Phaeophyta.
ALGIN. A complex phycocolloid occurring in the cell. walls and intercellular spaces of brown algae (Phaeophyta), and commercially marketed.
ALIMENTARY CANAL. The gut; a hollow sac with one opening (an enteron) or a tube (said to be ‘entire’ since “it opens at both mouth and anus) in whose lumen food is digested, and across whose walls the digestion products are absorbed. The epithelium lining the lumen is endodermil iri origin, but the bulk of the organ system in higher forms is mesodermal, and is muscularized and vascularized. There are usually many associated glands.
ALKALINE PHOSPMATASE. Broad specificity enzyme, hydrolysing Pnany phosphoric acid esters pith an optimum activity in the basic pH range. Breaks down pyrophosphate in vertebrate blood plasma, enabling bone mineralization.
ALKALOIDS. Group ,of clinically important basic nitrogenous organic compounds produced by a few families of dicotyledonous plants, e.g. Solonaceae, Papaveraceae; possibly end-products of nitrogen metabolism, e.g. atropine, caffeine, cocaine, morphine, nicotine, quinine, strychnine.
ALKYLATIN~ AGENT. A substance introducing alkyl groups (e.g. -CH,,--C,HS, etc.) into either hydrocarbon chains or aromatic rings. Alkylation of DNA residues important in regulating transcription. See DNA REPAIR MECHANISMS; DNAMETHYLATION. '
ALLANTOIS. ‘Stalk of endoderm and mesoderm which grows out ventrally from the posterior end of embryonic gut in AMNIOTES, expanding in reptiles and birds into a large sac underlying and for tiuch of its surface attached to the CHORION. May represent precocious development of ancestral amphibian bladder. One of the three EXTRAEMBRYONIC MEMBRANES. A richly vascularized organ of gaseous exchange within cleidoic eggs, also functioning as a bladder to store embryo’s nitrogenous waste. In higher primates and rodents, persists into later life as the urinary bladder.
ALLELES (ALLELOMORPHS). Representatives of the same gene LOCUS, and as such said to be alleles of (allelomorphic to) one another, a relational property dependent upon the prior concept of gene locus. Identical and non-identical alleles occur, being represented singly in haploid cells. Classically, alleles were ascribed to the same gene on the basis of two criteria: (i) failure to recombine with one another at meiosis, as if occupying the same locus, and (ii) failure, when mutant, to exhibit COMPLEMENTATI~N when present together in a diploid. Alleles of the same ‘gene differ by’ MUTATION at one or more nucleotide sites within the same length of DNA, and backmutation from one to another may occur. There may be many alleles of a gene’in a population, but normally only two in the same diploid Cell. SeeMULTIPLE ALLELISM.
ALLELE COMPLEMENTATION. Interaction between individually defective mutant alleles of the same gene to give a phenotype more functional than either could produce by itself. Due to interaction (hybridization) of protein products. A source of confusion in the
delineation Of CISTRONS. See COMPLEMENTATION.
ALLELOPATHY. Inhibition of one species of plant by chemicals produced by another plant (e.g. by Salvia Zeucoph~ZZa - purple sage).
ALLEN’S R.ULE. States that the extremities (tail, ears, feet, bill)
AL’LERGIC REACTION END0THER EM I c animals tend t0 be I&tkly Smaller in Cooler regions Of a SpeCkSI%nge. See BERGMANN'S RULE.
ALLERGIC REACTION. Release of histamine and other mediators of ANAHYLAXIS, producing symptoms of asthma, hay fever and hives. Membrane receptors of mast cells and basophilic leucocytes k-- bind Ig antibodies which in turn bind antigen (allergen) and trigger histamine release. Often controllable by antihistamines. Disposition to allergic reaction is termed an allergy.ALLOANTIBODY. Antibody introduced into an individual but produced in a different member of the same species.
ALLOANTIGEN (ISOANTIGEN). Antigen stimulating antibody response in genetically different members of the same species.
ALLOCHRONIC. Of species or species populations that are either sympatric at different times of the year or otherwise have non-overlapping breeding seasons (e.g. different flowering seasons in anthophytes). See ALLOPATRY,SYMPATRIC.
ALLOCHTHONOUS. Originating somewhere other than where found.
ALLOGAMY. (Bot.) Cross-fertilization.
ALLOGENEIC (ALLOGENIC). With different genetic constitutions.. Often refers t o intraspecific genetic variations. See IN F R A s P EC 1 F I C VARIATION.
ALLOGRAFT (HOMOGRAFT). Graft between individuals of the same species but of different genotypes (allogeneic). See Au To GR A F T, ISOGRAFT,XENOGRAFT.
ALLOGROOMING. Grooming of one individual by another of the same species (a conspecific).
ALLOMETRY. Study of relationships between size and shape. Organisms do not grow isometrically; rather proportions change as size changes.Thus juvenile mammals have relatively large heads, while limb proportions of arthropods alter in successive moults. Summarized by the
exponential equation y = bx”, where y = size of structure at some . stage, b = a constant for the structure, x = body size at the stage considered and a = allometric constant (unity for isometric growth). The analysis is open to multivariate generalization. See HETEROCHRONY.
ALLOPATRIC. Geographical distribution of different species, or subspecies or populations within a species, in which they do not occur together but have mutually exclusive distributions. Populations occupying different vertical zones in the same geographical area may still befullyallopatric see ALLOCHRONY,SYMPATRIC.
ALLOPOLYPLOID. Typically, a TETRAPLOID organism derived by ALLOTETRAPLOID chromosome.doubling from a hybrid between diploid species whose chromosomes have diverged so much that little or no synapsis occurs between them at meiosis, so that only biv_alents are formed (e.g. New World cottons, Gossypium spp.). This clearly distinguishes the term frdm AUTOPOLYPLOID, btit some polyploids do not fall readily into either category. Allopolyploids may back-cross with one or other diploid parent stock; hence. allotetraploids, which are generally themselves fully fertile (since they form bivalents at meiosis), behave in effect as new reproduc&ely isolated species. Hqwever, if the original diploid progenitors were closely related species, or even cotyjes of the same species, then MULTIVALENTS may arise in meioses, which then resemble meioses in typical autopolyploids. Nevertheless, as a result of their greater fertility classical allopolyploids have been more significant. in evolution than have classical autopolyploids. Many
new plant j species have arisen this way. Cultivated tiheat (Triticuti aestivum) is an allohexaploid, combining doubltig in a triploid hybrid between an allotetraploid and.a diploid. ’
ALL-OR-NONE RESPONSE. Ability of certain excitable tissues, under standardized conditions, to respond to stimuli of whatever intensity in just two ways: (a) no response (stimulus sub-threshold), or (b) a fullsize response (stimulus at or above threshold). ACTION POTENTIALS of nerve and muscle mknibranes are charticteriied by all-or-none behaviour. Where thresholds of differeot units in a response differ, as in the many motor fibres of the sciatic nerve, or the various MOTOR
UNITS of an entire muscle, an increase in stimulus intensity may bring progressively mqre units to respqnd. In muscle, this constitutes spatial SUMMATION. Nerve signals cannot use such amplitude variations.
ALLOSTERIC. Of those molecules (typically proteins) whose threedimensional configurations alter in response to their environmental situation,’ no&ally registered by a change in molecule function.
Often the key to regulation of critical biochemical pathways, serving as a feedback monitoring device in cybernetic circuits both inside and outside cells (see REGULATORY ENZYMES). At least as significant is allosteric control of GENE EXPRESSION b3 regulatdry proteins. Among non-enzyme proteins, the haemoglobih mdlecule is allosteric under different blood pH values, with marked effects upon its oxygen saturation curve (see BOHR EFFECT). For’ allosteric inhibition and induced fit of enzymes, see ENZYME.
ALLOTETRAPLOID. An ALLOPOLYPLOID derived by doubling the set of chromosomes resulting from fusion between haploid gametes from more or_less distantly related parental species. In classical cases, there is no meiotic SYNOPSIS between the chromosomes of different origin, and more or less complete fertility is achieved. Far more common in plants than animals, probably through comparative rarity of vegeta tive ALLOTOPIC habit and/or parthenogenesis in the latter, in which it is difficult to rule out autopolyploidy as the source. See POLYPLOIDY.
ALLOTOPIC. Of closely related sympatric populations, whose distributions are such that both occupy the same geographical range, but each occurs in a different habitat within that range.
ALLOTYPE. Genetic. variant within a LOCUS of a given species population, such as allelic forms within a
BLOOD GROUP SYSTEM or variants of heavy chain’” constant regions of ANTIBODY molecules. See IDIOTYPE,ISOTYPE.
ALLOZYMES. Forms of an enzyme that$ are encoded by different allelic genes ALPHA-ACTININ (a-ACTININ). An accessory protein of, muscle, anchoring actin filaments at the Z-disc and cross-linking adjacent sarcomeres; also cross-links actin in many other cells to contribute to the CYTOSKELETON.
ALPHA BLOCKER. Drug-blocking ADRENERGIC alpha receptors, preventing activity of the sympathetic neurotransmitter NORADRENALINE.
ALPHA HELIX. (Of proteins) a common secondary structure, in which the chain of amino acids is coiled around its long axis. Not all proteins adopt this conformation, it depending upon the molecule’s primary structure. When adopted there are about amino acids perturn (corresponding to 0.54 nm along the axis), amino acid R-groups pointing outwards. Hydrogen bonds between successive turns stabilize the helix. The &helix may alternate with other secondary structures of the molecule such as P-sheets or ‘random’ sections. See PROTEIN.
ALPHA RECEPTOR. ADRENERGIC membrane receptor site binding NORADRENALINE
in preference to ADRENALINE. May be excitatory or inhibitory, depending on the tissue. As with beta receptors, effects are mediated through an adenylate cyclase molecule adjacent in the membrane. The commonest receptors on postsynaptic membranes of postganglionic cells of sympathetic system. See CHOLINERGIC,AUTONOMIC NERVOUS SYSTEM, LPHA BLOCK ER.
ALPHA-RICHNESS. Number of species present in a small, local, homogeneous area. See ALTERNATION OF GENERATIONS. Either (1) me&genesis, a life cycle alternating between a generation reproducing sexually and another reproducing asexually, the two often differing morphologically; or (2) the alternation within a life cycle of two distinct cytological generations, one being haploid and the other diploid. See L I FE c Y c LE.
Metagenesis occurs in a few animals, e.g.cnidaria and parasitic flatworms, where both generations are normally diploid. The ALVEOLUS alternation of distinct cytological generations is clearest in plants such as ferns and some algae, where the two generations (gametophyte and sporophyte) are independent and either identical in appearance (alternation of isomorphic generations) or quite dissimilar (alternation of heteromorphic generations). In mosses and liverworts the dominant (vegetative) plant is the gametophyte while the sporophyte (the capsule) is more or less nutritionally dependent on the gametophyte. In flowering plants, the male (micro-) and female (macro-) gametophytes are reduced to microscopic proportions, the male gametophyte being shed as the pollen grain and the female gametophyte (embryo sac) being retained on the sporophyte in the ovule. A clearcut alternation of physically distinct plants is avoided here, although alternating cytological phases are still discernible. In vascular plants generally, the sporophyte generation is the vegetative plant itself, be it a fern, herb, shrub or tree.
ALTRICIAL. Animals born naked, blind and immobile (e.g. rat ‘and mouse pups, many young birds). See NIDICOLOUS.
ALTRUISM. Behaviour benefiting another individual at the expense -of the agent. Widespread and apparently at odds with Darwinian theory, which predicts that any genetic component of such behaviour should be selected against. Theories of altruism in biology tend to be concerned with cost-benefit analysis, as dictated by the logic of natural selection. One component of Darwinian FITNESS may be the care a parent bestows upon its offspring, although this is not usually considered altruism, HAMILTON'S RULE indicates the scope for evolutionary spread of genetic determinants of altruistic character traits, compatibly with Darwinian theory, and explains the evolution of parental care, while showing that reciprocal altruism can evolve even in the absence of relatedness between participants (e.g. members of different species). MU L T I c E L L u L A R I T Y may afford opportunities for sacrifice of somatic cells (e.g. leucocytes) for a genetically related germ line harbouring the potentially immortal UNITS OF SELECTION. See ARMSRACE.
ALVEOLUS. (1) Minute air-filled sac, grouped together as alveolar sacs to form the termini of bronchioles in vertebrate lungs. Their thin walls are composed of squamous epithelial and surfactant-producing cells. A rich capillary network attached to the alveoli supplies blood for gaseous exchange across the huge total alveolar surface. A surfactant (lecithin) layer reduces surface tension, keeping alveoli open from birth onwards,.and provides an aqueous medium to dissolve gases. Macrophages in the alveolar walls remove dust and debris. (2) Expanded sac of secretory epithelium forming internal termini of ducts of many glands, e.g. mammary glands. (3) Bony sockets into which teeth fit in mandibles and maxillae of jawed vertebrates, lying in the alveolar process of the jaws.