Online Biology Dictionary - A

a (1) ampere; (2) anterior; (3) anode; (4) area; (5) artery.

A (1) adenine; (2) alanine.

Å Angstrom unit.

aardvark A nocturnally-active African animal that feeds on ants and termites. The single representative (Orycteropus afer) of the mammalian order Tubulidentata. Occurs throughout sub-Saharan Africa in suitable habitat (distribution). Also known as the Cape anteater, it is considered unrelated to the South American anteaters. PHOTO1 PHOTO2 SKULL

Ab Antibody.

abaxial (1) away, or facing away, from the axis or center line; (2) on the lower side (e.g. of a leaf). Compare: adaxial.

abdominal /ab-DOM-uh-nuhl/ Pertaining to the abdomen.

abdomen /AB-doe-men/ (1) in quadrupeds, the region, not including the back, between the chest and pelvis (the belly); the cavity enclosed within this region and lined by the peritoneum; (2) in arthropods, the posterior segment of the body, attached to the rear of the thorax.

abdominogenital /ab-DOM-un-no-JEN-uh-tuhl/ Of, or relating to, the abdomen and genital organs.

abdominopelvic cavity /ab-DOM-un-no-PEL-vik/ The large body cavity lying below the diaphragm and above the pelvis. It contains the digestive organs, liver, pancreas, urinary organs, various glands, and, in women, the uterus. For purposes of discussion, it is divided into the abdominal and pelvic cavities, although there is membrane forming a physical separation between the two.

abiotic /a-bigh-AWT-ik/ Marked by the absence of life.

abscess /AB-sess/ A localized concentration of pus, anywhere in the body.

abscisic acid /ab-SIS-ik/ A plant hormone inhibiting growth (helps plants withstand adverse conditions).

absorption The taking in of water, along with its dissolved contents, through a cell membrane. Compare: ingestion.

abundance of an mRNA The mean number of copies of a particular mRNA molecule per cell.

abundant mRNAs Those mRNA molecules present at high copy number within a cell.

abyssal zone /uh-BIS-uhl/ The cold, lightless zone in the depths of the sea where pressures are intense.

Ac-CoA Acetyl-coenzyme A.

acanthoid Thorny, spiny, sharp.

Ac-CoA Acetyl-coenzyme A.

ACAT Acyl-CoA:cholesterol acyl transferase.

acaulescent Stemless.

acarine Relating to ticks and mites.

acarology The study of ticks and mites.

acceptor splicing site Boundary between an intron's 3' end and the adjacent exon's 5' end -- also known as the right (or 3') splicing junction.

acclimation /AK-luh-MAY-shun/ Adjustment of an organism to a change in some environmental factor.

acentric fragment (also acentric chromosome A centromereless portion of a broken eukaryotic chromosome.

acetic acid (also ethanoic acid) The weak organic acid in vinegar. When pure and water-free (glacial acetic acid) it is colorless, hygroscopic, and freezes at 16.7°C (62°F). The acetyl group, a derivative of acetic acid, is basic in the biochemistry of nearly all organisms. When bonded to coenzyme A it is essential to carbohydrate and fat metabolism. Acetic acid is produced by zymotic bacteria, in particular by members of the genera Acetobacter and Clostridium. Such bacteria are nearly ubiquitous and produce the sour smell and taste of spoiling food.

acetyl-coenzyme A (Ac-CoA) A molecule participating in many metabolic processes. Its most familiar function is in the Krebs cycle, which it initiates by transferring its two-carbon acetyl group to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).

accipitral Of or relating to a hawks.

acephalia Headlessness.

acetabulum The socket of the hip joint.

acetamide Amide of acetic acid.

acetylcholine (ACh) An ester of acetic acid and choline that functions as a neurotransmitter.

acheilia Congenital absence of one or both lips.

Acheulean (also Acheulian) /uh-CHEW-lee-uhn/ An archaeological tool industry characteristic of certain early humans, particularly Homo erectus. MORE ABOUT ACHEULEAN TOOLS

aciculate Having or composed of needles or spines.

acid Any substance that reduces a solution's hydrogen ion concentration. An acid can react with a base to produce a salt. In aqueous solution acids taste sour, redden litmus paper, and release hydrogen gas by reacting with metals such as iron or zinc. They are hydrogen-containing molecules or ions capable of giving up a proton to a base (or accepting an unshared pair of electrons from a base). An acid solution has a pH of less than 7. See: hydrogen ion concentration.

Acinonyx jubatus /uh-SIN-uh-nix JOO-but-uhs/ Scientific name for the cheetah. GEOGRAPHIC DISTRIBUTION | ABOUT THE CHEETAH'S DIET

Lake sturgeon Acipenser fulvescens (click to enlarge)

Acipenseriformes The order of fish including the sturgeons and paddlefishes.

acoelomate /ay-SEEL-uh-MATE/ An solid-bodied organism lacking a cavity between the gut and body wall.

acoumeter /uh-KOO-muh-der, AH-koo-MEED-er/ An instrument for testing hearing.

ACP Acyl carrier protein.

acquired mutation See: somatic cell mutation.

Acrididae /uh-KRID-uh-dee/ A family of grasshoppers.

acrocentric chromosome /AK-row-CENT-rik/ A chromosome in which the centromere is located near the chromosome's end.

acrogenous Referring to plants that grow from the apical cells (at the apex of the stem).

acromion /uh-KROH-mee-ahn/ The projecting process of the scapula that forms the hard point of the shoulder. PICTURE

acromiohumeral /uh-KROH-mee-o-HUM-er-uhl/ Pertaining to the acromion and humerus.

acrosome /AK-ruh-sohm/ A vesicle at the tip of a sperm cell that allows it to penetrate the egg.

The acrosome reaction Used with permission. Courtesy Chris Patton, Stanford Sea Urchin Embryology Lab

actin /AK-tin, -tuhn/ A family of proteins making up 5-10% of the total protein of the typical eukaryotic cell. Actin, which is 5-7 nm in diameter, is the material of which microfilaments, which perform a contractile function in the cytoskeleton, are composed. It is also present in the contractile thin filaments of sarcomeres in muscle cells.

Actinophrys /ak-tuh-NAWF-ruhs/ A genus of protozoans with radial spines. PICTURE

Actinopterygii /ak-tuh-nawp-ter-RIDGE-ee-eye/ Ray-finned fishes (fishes with fins supported by radiating spines).

action potential A rapid change in the electrical potential of a plasma membrane that occurs when the cell has been activated by a stimulus. It is caused by a selective closing and opening of voltage-sensitive ion channels.

activation energy The amount of energy (E_a) required to convert a stable molecule into a reactive one. It is the energy required to produce the unstable condition in which the energy state of the bonds of the reactants is raised to a level corresponding to the unstable transition state that precedes a chemical reaction.

active immunity Immunity that arises in response to exposure to an antigen. See: passive immunity

active site The region of an enzyme where it binds to its substrate.

active transport The transport by a carrier protein of a molecule through a plasma-membrane against its concentration or electrochemical gradient.

aculeolate /ak-yuh-LEE-uh-luht/ Having tiny prickles.

acuminate /uh-KYOOM-uh-nuht, -nate/ Tapering or pointed.

acusection /ak-yoo-SEK-shun/ To cut with an electrosurgical needle.

acutifoliate /uh-KYOOT-uh-FOAL-ee-uht/ Having sharp leaves.

acyl-CoA:cholesterol acyl transferase (ACAT) /ASS-uhl-, ASS-eel-/ A member of the class of enzymes known as acyltransferases. ACAT transfers amino-acyl groups from one molecule to another.

acyltransferase /ASS-uhl-TRANS-fer-aze/ A transferase enzyme that acts on acyl groups.

adaptation /ad-ap-TAY-shun/ (1) a genetic process whereby a population or at type of organism is thought to have become better suited to its environment; (2) a trait considered to be the product of such a process.

adaptive peak /uh-DAPT-iv/ A hypothetical state of a population in which the combination of alleles present in the population are better suited to the environment than would be any other combination.

adaptive radiation /uh-DAPT-iv raid-ee-AY-shun/ A hypothetical process whereby an originally generalized form gives rise to a number of specialized types. MORE INFORMATION

adaptive zone /uh-DAPT-iv/ A set of ecological niches that may be occupied by various distinct types of organisms exploiting the identical resources in a like manner.

adaxial /ad-AX-ee-uhl/ (1) toward, or facing toward, the axis or center line; (2) on the upper side (e.g. of a leaf). Compare: abaxial.

additive genetic effects (also additive effects) Effects of alleles at different loci that are equal, when combined, to the sum of their individual effects. See also: anticipation, complex trait.

adducent /ad-D(Y)OO-suhnt/ Leading or conducting toward.

adenine (A) /ADD-uh-neen/ A purine base found in nucleic acids. In DNA it is one member of the base pair AT (adenine-thymine). See also: nucleotide.

adenocarcinoma /AD-uh-no-kar-sin-NO-muh/ A malignant neoplasm of a glandular epithelium.

adenoma /ad-uh-NO-muh/ Any neoplasm of a glandular epithelium.

adenosine diphosphate (ADP) /uh-DEN-oh-seen die-FOSS-fate/ The product of ATP dephosphorylation by ATPases. ADP is converted into ATP by ATP synthases.

ATP (click to enlarge)

adenosine triphosphate (ATP) /uh-DEN-oh-seen try-FOSS-fate/ A nucleotide with multiple functions. In its phosphate bonds it stores energy produced by photosynthesis and cellular respiration. This energy is consumed by various metabolic processes when ATP is hydrolyzed into ADP.

adenylate cyclase /ad-DEN-uh-late SIGH-klaze/ The enzyme that catalyzes the conversion of cAMP to ATP.

ADH Alcohol dehydrogenase.

AdoMet S-adenosyl methionine.

adrenal /uh-DREE-nuhl/ (1) next to the kidney; (2) the adrenal gland; (3) of or pertaining to the adrenal gland.

adrenal gland /uh-DREE-nuhl/ A gland covering the upper portion of each kidney.

adrenosterone /uh-DREE-no-stuh-RONE/ An androgen secreted by the adrenal gland.

adventitious /ad-ven-TISH-uhs/ Arising or occurring in some location other than the usual one.

aerobic respiration /air-ROBE-ik/ See: respiration.

affected relative pair Pair of genetically related individuals that both have a trait (i.e., that are both affected by the gene causing that trait). Examples are affected sibling, cousin, and avuncular pairs. See also: avuncular relationship.

afferent /AF-fer-uhnt/ Leading toward (opposite of efferent).

Ag (1) antigen; (2) chem. symbol for silver.

A/G ratio Albumin/globulin ratio; ratio of albumin to globulin in blood serum (normal ratio: ~1.55).

agamospermy // Reproduction in which the embryo develops without fertilization. MORE INFORMATION

agglutination /ag-GAM-uh-sperm-ee, AG-guh-muh-sperm-ee/ A uniting or adhering; within a biological context: a clumping of dispersed cells or microorganisms.

agglutinable /uh-GLOOT-uh-nuh-buhl/ Capable of undergoing agglutination.

aggregation technique /ag-ruh-GAY-shun/ A procedure in which two separate embryos are pushed together at the 8-cell stage of development to produce a single embryo.

agnathans /ag-NAY-thuns/ Jawless fishes, such as lampreys and hagfishes.

agonistic behavior /ag-uh-NIST-ik/ Behavior involving a contest over some resource, such as territory or mates.

agriculture /AG-ruh-kuhl-cher/ The science of crop and livestock production.

AIDS Acquired immune deficiency syndrome.

Al Chemical symbol for aluminum.

ala /AL-uh/ (1) alanine; (2) wing or winglike part.

ALA d-aminolevulinic acid.

alanine (A or ala) /AL-uh-neen/ A nonessential amino acid. CODONS | MOLECULAR STRUCTURE | SYNTHESIS

Structure of an alcohol

alcohol /ALK-uh-hall/ Any organic compound with a hydroxyl group (-OH) bound to a carbon atom of an alkyl or substituted alkyl group. The general formula for a simple alcohol is C_nH_2n+1OH.

alcohol dehydrogenases /ALK-uh-hall de-high-DRAW-juh-naze/ A group of dehydrogenases that interconvert alcohols and aldehydes or ketones with the reduction of NAD⁺. In animals, they break down toxic alcohols. In yeast and many other microorganisms they reverse the process, producing alcohols via fermentation.

aldehyde /ALD-uh-hide/ A member of the class of organic compounds with the formula C_nH_2nO and having a terminal carbonyl group.

algesia /al-JEE-zhuh -ze-uh/ Hypersensitivity to pain.

alkali /AL-kuh-LIE/ Any metallic hydroxide other than ammonia that can join with an acid to form a salt (or with an oil to form a soap).

alkaline /AL-kuh-lin, AL-kuh-line/ Like an alkali; basic.

Ethyl Radical

alkyl /AL-kill/ A univalent radical composed of a chain of hydrogen and carbon atoms having the general formula C_nH_2n+1. For example, the ethyl radical shown at right is the alkyl with formula C₂H₅.

allantois /uh-LAN-tuh-wuhs/ One of the four membranes enclosing an embryo. It sequesters the nitrogenous wastes produced by the embryo.

allatum hormones Juvenile hormones.

alleles /uh-LEELS/ Alternative forms of a gene occurring at the same locus.

allele frequency (also gene frequency) /uh-LEEL/ For a particular allele at a specified locus, the proportion of all alleles occurring in a population at that locus.

allelic genes /uh-LEEL-ik/ Alleles.

allelic exclusion /uh-LEEL-ik/ The expression in some particular lymphocyte of just one of the alleles that code for the expressed immunoglobulin.

allogeneic /al-low-juh-NAY-ik/ Having allelic variation between the members of a populution.

allometry /al-LOM-uh-tree/ (or allometric growth) /al-luh-MET-trick/ Growth of a part that differs in rate from that of some other part, or from that of the whole organism. Compare: isometry

allopatric /al-low-PAT-trick/ Of two populations: Occupying different, non-contiguous geographic regions.

allopolyploid /AL-oh-PAWL-ee-ploid/ A polyploid that combines two distinct chromosome sets from different sources. Allopolyploids are produced by hybridization. Compare: autopolyploid.

allosteric control /al-low-STARE-ick/ Regulation of the activity of one site of a protein by interactions at some other site on the protein.

allozymes /AL-low-zimes/ Enzymes produced by the same locus, but differing in molecular structure. They are coded for by distinct alleles.

α-amantin /AL-fuh uh-MAWN-tuhn/ A compound inhibiting transcription of eukaryotic RNA polymerases. RNA polymerase II is inhibited by very low concentrations of α-amantin, while RNA polymerase I is not. It is obtained from the poisonous mushroom Amanita phalloides.

alpha-helix /AL-fuh HEE-licks/ A spiral secondary structure occurring in many proteins. An alpha-helix allows maximum intramolecular bonding between H-N and C=O groups, and therefore water exclusion.

alpha-particle /AL-fuh/ A helium nucleus; composed of two neutrons and two protons and therefore has a double positive charge.

alternation of generations The cyclic alternation, seen in the life cycles of sexual organisms, between a haploid and a diploid form.

alternative splicings Alternative ways of combining a gene's exons to make variant proteins.

altrices /all-TRY-seez/ See: altricial.

altricial /alt-TRISH-uhl/ Slow developing; needing long nursing. Altricial birds or mammals ("altrices") are those with newly hatched or born young being more dependent on their parents (i.e., ones that cannot move about or open their eyes, and that lack down or hair). Compare: precocial.

altruism /AL-troo-iz-uhm/ Unselfish behavior; within a biological context, behavior that assists others to survive and reproduce, but that does not benefit the individual engaging in the behavior.

Alu family /AL-oo/ A set of dispersed repetitive DNA sequences in the genomes of humans and related organisms. Each repeat is about 300 bp long. They are named for the Alu cleavage sites that flank them.

Alu-equivalent family /AL-oo/ A term used to refer to the Alu-related repetitive sequences present in the genomes non-human mammals.

aluminum (Al) /uh-LOOM-uh-nuhm, British: al-loo-MIN-ee-uhm/ Metallic element; atomic number 13, atomic weight 26.9185. PERIODIC TABLE

alveoli (sing alveolus) /al-VEE-oh-lie, -lee/ (1) the minute air cells within the lungs where gas exchange occurs; (2) general: a small hollow, sac, socket, follicle, or depression.

amalgam /uh-MAL-guhm/ A compound containing mercury.

ambilateral /am-buh-LAT-er-uhl/ Occurring on both sides.

aminoacetic acid /uh-MEEN-oh-uh-SEAT-ik/ A synonym for glycine.

amino acid /uh-MEEN-oh/ One of 20 organic molecules serving as the monomers of proteins. An amino acid has both carboxyl and amino groups. The various amino acids differ only in what is attached to the 4th bond of the α carbon. This variable attachment is generally indicated by the letter R. The R group is also called the "side chain." The properties of the side chain determine the unique properties of an amino acid.

amino group /uh-MEEN-oh/ The chemical group --NH₂; the amino group is hydrophilic.

ammonite /AM-muh-nite/ A member of the order Ammonoidea, extinct cephalopods similar to the extant chambered nautilus. Fossil range: Early Devonian–Late Cretaceous. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4 | PICTURE 5 | PICTURE 6

ammonium /uh-MONE-ee-uhm/ The radical NH₄⁺.

amnion /AM-nee-uhn/ The innermost of the four membranes enclosing the embryo of a reptile, bird, or mammal.

amniorrhexis /am-nee-uh-REKS-uhs/ Rupture of the amnion ("breaking of the waters").

amniote /AM-nee-oat/ Vertebrates having an amnion during embryonic development, including reptiles, birds, and mammals.

amniotic egg /am-nee-AWT-ik/ The egg of an amniote. The presence of an amnion prevents drying and allows amniotic eggs to be laid on land.

amphimixis /AMF-uh-MIX-uhs/ Sexual reproduction.

amplexifoliate /am-pleks-uh-FOAL-ee-uht/ Having grasping leaves.

amplification /am-pluh-fuh-KAY-shun/ Increase in the copy number of a DNA fragment. Amplification can occur either in vivo or in vitro. See also: cloning, polymerase chain reaction.

amylase /AM-uh-laze/ An enzyme, present in human saliva, that breaks starch down into sugar.

anabiosis /AN-uh-bigh-OH-suhs/ A state of complete dormancy or suspended animation triggered by dessication (for example, in tardigrades) and ended when water is again present.

anabolic steroids /an-uh-BAWL-ick/ Hormones promoting constructive, as opposed to catabolic, metabolism.

anabolic pathways /an-uh-BAWL-ick/ Metabolic processes that consume energy and build up complex molecules from simple ones.

anabolism /uh-NAB-uh-liz-uhm/ The conversion of ingested materials into the constituents of protoplasm.

anaerobic respiration /an-uh-ROE-ick/ See: respiration.

anagenesis /an-uh-JEN-uh-suhs/ Evolutionary change in a population without branching separation of the population into distinct forms. Compare: cladogenesis.

anal canal /AY-nuhl/ The the lowermost portion of the large intestine, leading from the rectum to the anus. About 3 cm in length, its aperature is controlled by an internal and external sphincter. PICTURE

analogous traits /uh-NAL-uh-guhs/ Traits that are presumed to be similar due to convergence, and not because of their being inherited from a common ancestor.

anastomosis /un-NAST-uh-moe-suhs/ Interconnection between blood vessels or other organic tubular structures such as the veins of a leaf.

anatomy /uh-NAT-uh-mee/ The study of the macroscopic structure of multicellular organisms.

androgen /AND-ruh-jen/ A substance that stimulates the production of male characteristics.

anemia /uh-NEEM-ee-yuh/ A reduction in circulating erythrocytes, hemoglobin, or both.

anemochore /uh-NEM-uh-kore/ An organism with seeds and/or spores suited to distribution by the wind.

angiorrhaphy /ANJ-ee-ORE-uh-fee/ Suturing of vessels.

angiorrhexis /ANJ-ee-ore-REKS-uhs/ Rupture of a blood vessel.

angiosarcoma /ANJ-ee-oh-sar-KOH-muh/ A sarcoma arising in the cells that line blood or lymph vessels. See: hemangiosarcoma, lymphangiosarcoma.

Comparison of gymnosperm with angiosperm (click to enlarge)

angiosperms /ANJ-ee-oh-sperms/ Flowering plants; one of the major categories of the Plant Kingdom. Unlike gymnosperms, angiosperms form their seeds within protective capsules called ovaries.

anisogamy /an-eye-SAWG-uh-mee/ A common type of reproduction in which the two gametes that join to produce the zygote differ greatly in size, with the smaller of the two mobile and seeking out the larger, which is immobile.

ankylodactylia /anv-uh-lo-dak-TILL-ee-uh/ Adhesion of adjacent toes or fingers.

annotation /an-no-TAY-shun/ Providing information that describes features of a DNA sequence.

anorexia /an-uh-RECK-see-yuh/ (1) lack of appetite; (2) anorexia nervosa.

anorexia nervosa /an-uh-RECK-see-yuh nerv-OH-suh)/ A psychiatric eating disorder characterized by an obsessive fear of gaining weight. Affected individuals are far underweight, but cannot recognize the fact themselves. They intentionally starve themselves to control body weight, and often exercise excessively. Often, too, they take diet pills or diuretics. Most are adolescent females, but about 10 percent are male.

antepenultimate /an-tee-pen-ULT-uh-muht/ Third from last.

anteriad /an-TIR-ee-ad/ Toward the front.

anterior /an-TIR-ee-er/ In front.

anterolateral /AN-ter-oh-LAT-er-uhl/ In front, but toward the side.

anteroposterior /AN-ter-oh-POSS-tir-ee-er/ From front to back.

anther /AN-ther/ The pollen sac at the end of a stamen.

anthracosis /an-thruh-KOE-suhs/ A condition of the lungs caused by chronic inhalation of coal dust.

anthropomorphism /an-thrope-uh-MORE-fiz-uhm/ Interpreting what is nonhuman in terms of what is human; attributing human motives to animals.

antibodies (also immunoglobulins, abbreviated Ig) /ANT-uh-bawd-eez/ Proteins produced by plasma cells in response to foreign substances (antigens). An antibody couples specifically with its antigen (and with molecules sufficiently similar to its antigen). Each plasma cell produces one and only one type of immunoglobulin. MORE INFORMATION

anticipation A situation where successive generations have increased severity of a genetic disorder.

anticoding strand /ANT-ee-KODE-ing/ See: sense strand.

antienzyme /ANT-ee-EN-zime/ Something that neutralizes an enzyme.

antigen /ANT-uh-jen/ A foreign substance that stimulates the production of an antibody.

antigen presenting cell (APC) /ANT-uh-jen/ A cell displaying foreign antigen complex with MHC on its surface. T-cells may recognize this complex using their T-cell receptor (TCR).

antisense strand /ANT-ee-SENSE/ See: coding strand.

Anura /an-(Y)ER-uh, uhn-(Y)ER-uh/ The amphibian order that includes the frogs and toads.

anus /AY-nuhs/ The outlet of the intestinal tract, located between the nates. PICTURE

aphasia /uh-FAZE-yuh/ Loss or impairment of the power of speech.

apical /AY-pick-uhl/ Occurring at the end or summit.

apical meristems /AY-pick-uhl MARE-uh-stems/ Embryonic tissue that supplies cells allowing a plant to grow in length; found at the tips of roots and stems.

Apicomplexa A phylum of protists containing numerous organisms that are important causes of disease, in particular the blood parasites of the apicomplexan genera Babesia, Plasmodium, and Theileria, and the coccidian parasites that infect the intestinal tracts of animals.

apnea /AP-nee-uh/ A temporary cessaton of breathing.

apocarpous /AP-uh-KARP-uhs/ In a gynoecium, having the carpels separate.

apomict /AP-oh-mickt/ An organism that normally reproduces via apomixis.

apomictic /ap-oh-MICK-tic/ Reproducing via apomixis.

apomixis (also apomictic reproduction) /ap-oh-MIX-uhs/ Asexual reproduction via agamospermy or vegetative reproduction. MORE INFORMATION

apoptosis /ap-puh(p)-TOE-suhs, ay-/ Programmed cell death. Apoptosis disposes of damaged or unneeded cells.

aposematic /ap-oh-sem-MAT-ik/ Warning, advertising noxious or toxic properties; said of colors, structures, or other features of an organism providing such a warning.

approximal /uh-PROKS-uh-muhl/ Contiguous.

apterous /AP-tuhr-uhs/ Wingless.

arachnid /uh-RACK-nid/ A member of the arthropod class Arachnida, which includes spiders, scorpions, mites, and harvestmen.

Archaea /ARK-ee-yuh, -eye-yuh/ (pronunciation) Archaea is one of the two prokaryote domains, the other being Bacteria. PICTURE (Holobacteria) | PICTURE (Korarchaeota) | MORE INFORMATION

archetype /ARK-uh-type/ (1) primitive type; (2) general form or type representing a category. MORE INFORMATION

archocele /ARK-oh-seel, -uh-seel/ Rectal hernia.

arginine (Arg or R) /ARJ-uh-neen/ A nonessential amino acid. CODONS | MOLECULAR STRUCTURE

Arm bones of a human being (click to enlarge)<small>

arms (1) the forelimbs of a primate; (2) the locomotive limbs of certain invertebrates, such as the rays of a starfish; (3) the two portions of a chromosome lying on either side of the centromere; the shorter of the two is called the p arm, the longer, the q arm.

arrayed library Individual recombinant clones placed in two-dimensional arrays in microtiter dishes. The clones are hosted in cosmid, bacteriophage, YAC, or other cloning vectors. Each clone in the array is identified by its plate and its location on that plate. Arrayed libraries are used for many purposes, in particular, for screening for a specific gene or genomic region of interest. See also: cloning, genomic library.

arrhenotokous (also arrenotokous) /AR-uh-NAWD-uh-kuhs/ Producing male offspring, and only male offspring, from unfertilized eggs.

ARS Autonomously replicating sequence.

arteriogram /are-TEER-ee-uh-gram/ An image produced by arteriography. PICTURE

arteriography /are-teer-ee-AWG-ruh-fee/ A method of visualizing the arteries by exposing film to x-rays passed through a patient who has received an injection of a radioopaque solution.

arteriole /are-TEER-ee-uhl, -ole/ A minute artery connecting with capillaries.

artery /ARE-ter-ee/ A blood vessel through which blood flows away from the heart. Arteries are of two types, pulmonary, carrying venous blood to the lungs, and systemic, carrying oxygen-enriched blood to the body's tissues.

arthritis /are-THRIGH-tuhs/ Inflammation of a joint or joints.

arthrodesis /are-THROW-duh-suhs or ARE-throw-DEE-suhs/ Surgical immobilization of a joint.

arthropod /ARTH-roe-pawd/ An organism belonging to Phylum Arthropoda, which includes all insects, spiders, and crustaceans. An arthropod has jointed appendages and its body is completely covered with an exoskeleton known as a cuticle.

ascospore /ASK-uh-spore/ A meiospore within an ascus.

ascus /ASk-uhs/ A linear sac produced by ascomycete fungi; it usually contains eight ascospores that are the products of one cell undergoing two meiotic divisions followed by one mitotic division. PICTURE

asexual /ass-SEX-yoo-uhl/ Lacking separate sexes; in connection with reproduction: occurring without a fusion of gametes.

asn Asparagine).

asp Aspartic acid).

asparagine (A or asn) /ass-PARE-uh-jeen/ A nonessential amino acid. CODONS | MOLECULAR STRUCTURE

asparagine synthase /ass-PARE-uh-jeen SIN-thaze/ A mammalian enzymeenzyme prokaryotic catalyzing the amination of aspartate to give asparagine. It uses glutamine as a nitrogen source. Aspartate-ammonia ligase is a prokaryotic enzyme that catalyzes the same reaction, but using ammonia as the source of nitrogen. DIAGRAM OF REACTION

aspartate-ammonia ligase /uh-SPART-tate/ See: Asparagine synthase.

aspartic acid (D or asp) /uh-SPART-ik/ A nonessential amino acid. CODONS | MOLECULAR STRUCTURE

assembly Assigning short DNA sequences to their correct positions on a chromosome.

assortative mating A tendency of males of a particular type to mate with females of a particular type; "positive assortative mating" occurs when these two types tend to be alike, and "negative assortative mating" occurs when they tend to be different.

assortment The random combination of chromosomes in gametes.

Red-knobbed Starfish Protoreaster linckii

Asteroidea /ass-ter-ROID-ee-yuh/ The echinoderm class including the starfishes and sea stars.

asthenia Weakness, lack of strength.

astrobiology The study of the possible existence of life in outer space.

ATCase Aspartate carbamoyltransferase.

atelopodia /at-tell-uh-PODE-ee-yuh/ Abnormal or incomplete development of the foot.

atelorachidia /at-tell-uh-rah-KID-ee-yuh/ Incomplete development of the spine.

atm Atmosphere; a unit of pressure equal to 101325 Pa.

ATP See: Adenosine triphosphate.

ATPase /AY-tee-pee-aze/ An enzyme that hydrolyzes ATP to ADP and inorganic phosphate.

atrophy /AT-ruh-phee/ Shrinkage or disappearance of a part or organ.

attic /AT-ik/ The upper portion of the tympanic cavity.

atticitis /at-uh-SIGHT-uhs/ Inflammation of the tympanic attic.

attomole /AT-uh-mole/ 10^-18 mole.

australopithecines /OS-truhl-oh-PITH-uh-seens or os-TRAIL-oh-PITH-uh-seens/ Early hominids discovered in Africa. ABOUT AUSTRALOPITHECINES | Australopithecus afarensis | Australopithecus africanus | Australopithecus anamensis | Australopithecus bahrelghazali | Australopithecus garhi | Paranthropus aethiopicus | Paranthropus boisei | Paranthropus robustus

autecic (also autecious) /awt-TEE-sick, awt-TEE-zhuhs/ Pertaining to or being a parasite infecting a single type of host organism throughout its entire life cycle.

autopolyploid /awt-oh-PAWL-ee-ploid/ A polyploid that is produced without hybridization from a single parental form. Compare: allopolyploid.

autoradiography /awt-oh-raid-ee-AWG-ruh-fee/ A technique used in analyzing DNA fragment sizes. Typically radioactive fragments are segregated according to length by allowing them to migrate through an agarose gel subjected to an polarized electrical field (gel electrophoresis). The gel is then placed on X-ray film to visualize the radioactively labeled fragments.

autosomal dominant /awt-oh-SOAM-uhl/ A gene that is both dominant and located on an autosome.

autosome /AWT-uh-soam/ A chromosome not involved in sex determination. The term is used in opposition to the term sex chromosome.

autotroph /AWT-uh-trofe, -trawf/ An organism that can produce from an inorganic substrate all of the organic molecules it requires; used in opposition to heterotroph.

auxin /AWKS-uhn, -in/ A plant hormone with multiple effects. By causing the appropriate cells to lengthen, it brings about secondary growth, and growth toward light (phototrophic response). Also, with gibberellins, it stimulates fruit production.

Avalonia /av-uh-LONE-ee-yuh/ A continental plate that bordered the Iapetus Ocean In the Early Paleozoic. It later became part of Euramerica, then part of Pangea, and still later, part of Laurasia. Today, it underlies Nova Scotia, Newfoundland, southern Ireland, and southern England, as well as parts of western Europe and New England. SEE MAP

Aves /AY-veez/ The vertebrate class including all birds.

Avogadro's number /av-uh-GAW-droze/ The number of atoms, 6.022 x 10²³, present in one gram-atomic weight of any element.

av shunt (also a-v shunt) Ateriovenous shunt; a direct connection between an artery and a vein that bypasses the capillary system; may be congenital, caused by trauma, or result from a breakdown in adjacent vessel walls. Arteriovenous shunts are also in some cases created intentionally as part of a surgical procedure.

avuncular relationship /uh-VUHNK-yuh-ler/ The genetic relationship that a nephew or niece has to an uncle or aunt.

axial /AX-ee-yuhl/ (1) pertaining to an axis; (2) around an axis; (3) along or parallel to an axis.

axon /AX-on/ A neuron process that carries nerve impulses away from the cell.

azurophil (also azurophile) /azh-YER-uh-fil, -file/ A tissue, microorganism, or a part of a microorganism with an affinity for blue (azure) dye such as the commonly used azure B.

azymic (also azymous) /az-ZIME-ik, -muhs/ Unfermented, unleavened.