d (1) dalton unit; (2) distal; (3) dorsal.

d- (1) dextrorotatory; (2) deoxy-.

D (1) aspartic acid; (2) Devonian Period.

Da Dalton unit.

dacryagogue /DACK-ree-uh-goag/ n. A substance causing lacrimation.

dacryelcosis /dack-ree-el-KOH-suhs/ n. Ulceration of the lacrimal sac.

dacryocystitis /dack-ree-oh-sist-TIGHT-uhs/ n. Inflammation of a lacrimal sac, a condition that can result in obstruction of the affected nasolacrimal duct.

dacryorrhea /dack-ree-oh-REE-uh/ n. Excessive lacrimation.

dactyl /DAKT-uhl/ n. A digit (finger or toe).

dactyloid /DAKT-uhl-oid/ adj. Finger-shaped.

dalton (Da) n. A unit of mass (also known as an atomic mass unit, amu), equal to the mass of a hydrogen atom (1.67 x 10^-24 g). When measured in grams, it is equal to the reciprocal of Avagadro's number.

dam n. The female parent; used in connection with domestic animals.

daughter cells The cells produced by division of a single parent cell.

dark reactions In photosynthesis, steps that can proceed in darkness by means of chemical energy produced by other steps ("light reactions") that depend on the presence of light.

dd Dideoxy-.

DEAE (also DEAE-C) Diethylaminoethyl cellulose.

deaminization /duh-am-in-eye-ZAY-shun, dee-/ n. Chemical decomposition in which a compound such as an amino acid or alkaloid loses its amino group and forms ammonia.

decaliter /DEK-uh-LEET-er/ n. A volume of ten liters.

deciliter /DES-uh-LEET-er/ n. One-tenth of a liter.

deciduous /duh-SID-joo-uhs/ adj. Falling out or falling off (as teeth, horns, or leaves).

decimorgan /DES-uh-MORE-guhn/ See: Morgan unit.

decomposers See: saprophyte.

decubation /deck-you-BAY-shun/ n. The act of lying down.

decubital /duh-KYOOB-uh-tuhl/ adj. Related to bedsores.

decubitus /duh-KYOOB-uh-tuhs/ n. A bedsore.

defective virus A virus that can reproduce only in the presence of another, so-called helper virus, that carries the genes necessary for the replication of the defective virus.

defloration /dehf-luh-RAY-shuhn/ n. Rupture of the hymen.

dehydrate /duh-HIGH-drate, dee-/ v. To reduce or entirely remove the water content of a substance or body.

dehydration synthesis /duh-HIGH-dray-shun, dee-/ The linking of monomers into a polymer by removing a molecule of water from each.

dehydrogenase /duh-high-DRAW-juhn-aze, dee-/ n. An enzyme that transfers one or more hydrides from a substrate to an acceptor, usually NAD+/NADP+ or a flavin cofactor such as FMN or FAD+.

dejecta /duh-JEK-tuh, dee-/ n. Feces.

deletion /duh-LEE-shun, dee-/ n. (1) the loss of a segment of genetic material from a chromosome; (2) the chromosome segment deleted.

deletion map /duh-LEE-shun, dee-/ A description of a chromosome that uses deletions as markers for specific areas of that chromosome.

deliquesce /del-luh-KWESS/ v. To liquify.

deme /DEEM/ n. A small, geographically localized population, usually thought of as panmictic.

denaturation /duh-nay-cher-RAY-shun, dee-/ The unraveling or separation of a protein or other large molecule that occurs under certain conditions. Denaturation makes a protein lose its functionality. Common factors that can cause denaturation are changes in pH, salinity, or temperature.

dendrite /DEND-right/ n. A short, branched process that conveys nerve impulses to the body of a neuron.

dendritic cell /den-DRIT-ik/ A type of leukocyte that functions as an antigen-presenting cell, activating T lymphocytes. PICTURE

Dendrobatidae /den-droh-BAT-uh-die, -dee/ n. The anuran family comprised of the poison dart frogs. MORE INFORMATION

dendrochronology /den-droh-kruh-NAWL-uh-jee/ n. The use of tree growth rings to date environmental events.

dendrology /den-DRAWL-uh-jee/ n. The study of trees.

dentary /DENT-er-ee/ n. One of the pair of bones present in the lower jaw of most vertebrates.

dentin (or dentine) /DENT-in, den-TEEN/ n. The bony portion of a tooth surrounding the pulp.

dentulous /DENCH-uh-luhs/ (also dentate /DEN-tate/ or dentated /DEN-tate-uhd/) adj. Toothed, having teeth. Compare: edentulous.

deoxyribonucleic acid (DNA) /dee-AWKS-ee-RIBE-oh-n(y)oo-KLAY-uhk/ The molecule in living organisms that contains inherited information. DNA is a double-stranded helix (PICTURE). The two strands of the helix are easily separated and can then serve as a template for the synthesis of additional, complementary strands. This actually occurs during DNA replication.

deoxyribonucleotide See: nucleotide.

deoxyribose /dee-AWKS-ee-RIGH-bohs, -bohz/ The five-carbon monosaccharide component of DNA. MOLECULAR STRUCTURE

deoxyriboside /dee-AWKS-ee-RIGH-boh-side/ See: nucleoside.

depilate /DEP-uh-late/ v. To remove hair.

deposition /DEP-uh-ZISH-shun/ Accumulation by sedimentation, chemical precipitation, or evaporation of a suspending solute.

derived trait A synapomorphy.

dermad /DERM-ad/ adv. Toward the skin.

dermal /DERM-uhl/ adj. Relating to the skin.

dermal tissue system (also epidermis) /ep-uh-DERM-uhs/ A layer protecting the young parts of a plant; it is one cell thick.

dermatology /derm-uh-TAWL-uh-jee/ n. The study of the skin and of the treatment of its diseases.

dermatophytosis /der-MAWD-uh-fye-TOE-suhs/ (commonly known as ringworm) n. A type of skin infection, of widespread occurrence, caused by certain fungi feeding on keratin.

dermatology /derm-uh-TAWL-uh-jee/ n. The study of the skin and of the treatment of its diseases.

dermotropic /DERM-uh-TRAWP-ik, DERM-uh-TROPE-ik/ adj. Attracted to or entering through skin.

desmosome /DEZ-muh-soam/ n. An animal cell junction structure that serves to attach adjacent cells.

desiccated /DEH-sick-kate-uhd/ adj. In a dried state.

desiccation /deh-sick-KAY-shun/ n. A process of drying; in an organism: moisture loss.

Derotremata /der-uh-TREM-uh-duh, -TREEM-/ n. A group of salamanders in which gill slits, but no external gills, are present in the adult; includes the giant salamanders' family of Cryptobranchidae and the amphiumas.

detritus /deh-TRITE-uhs/ n. Material produced by the decomposition of dead organisms.

detrivore /DET-truh-vore/ n. An organism feeding on detritus.

development /duh-VEL-up-ment/ n. The process by which a mature multicellular organism grows from a single cell.

developmental biology The study of the processes by which an organism changes from a single cell into a mature, multicellular individual.

determinate growth Growth that occurs only during the initial stage of the life of an organism, or until a given organ is completely formed. Compare: indeterminate growth.

Devonian Forest — The first forests, composed primarily of ferns, horsetails, and lycophytes — appeared in the Devonian.

dextrocardia /decks-troh-CARD-ee-yuh/ n. Presence of the heart on the right side of the body.

dextrorotatory /decks-troh-ROTE-tuh-tore-ee/ adj. Rotating a plane of polarized light clockwise (said of a chemical compound). Compare: levorotatory

dextrose n. Another name for glucose.

dextroversion (also dextrorotation or dextrotorsion) n. A turning to the right. Compare: levoversion

DG sn-1,2-diacylglycerol.

DHAP Dihydroxyacetone phosphate.

diagenesis /die-uh-JEN-uh-suhs/ (pl diageneses /die-uh-JEN-uh-seez/) n. Change in fossils, or in sedimentary minerals, occurring after their deposition. Diagenesis can result from any physical process (erosion, chemical change, distortion under pressure, bacterial infiltration, etc.).

diagnosis /die-ag-NOE-suhs, die-uhg-/ n. Recognition of the nature of a disease from symptoms.

diaphragm /DIE-uh-fram/ n. (1) any thin membrane; (2) A muscular wall separating the abdomen from the thoracic cavity; it contracts with each breath (inspiration) and relaxes with each exhalation (expiration). PICTURE

diastasis /die-AST-uh-suhs/ n. (1) the final, rest stage of diastole; (2) an injury involving separation, particularly of an epiphysis from its parent bone.

diastole /die-AST-uh-LEE/ n. The portion of the cardiac cycle during which the heart expands and fills with blood. Compare: systole.

dibasic /die-BASE-ick/ adj. Having two hydrogen atoms that can be replaced by basic radicals.

dichroic /die-KROH-ick/ (also dichromatic /die-kroh-MAT-ick/) adj. (1) having two colors; (2) existing in two color varieties.

dicot /DIE-cawt/ n. Short for dicotyledon

dicotyledon /die-cawt-uhl-EE-duhn/ n. An angiosperm with two cotyledons. COMPARISON OF MONOCOT AND DICOT SPROUTS

Digestive System (Full-size Image)

diethylaminoethyl cellulose (DEAE or DEAE-C) A resin used in diffusion purification and separation.

diffusion /duh-FYOO-shuhn, diff-/ n. The tendency of a substance to move in the direction toward which it is less concentrated.

digestion n. The process by which food is broken down into a form that can be absorbed by the body.

dihybrid cross /die-HIGH-brid/ A mating in which the parents differ with respect to two genetically determined traits.

dihydroxyacetone phosphate (DHAP) /die-high-DRAWK-see-ASS-uh-tone/ A compound involved in a wide variety of biochemical reactions. Its main role is in glycolysis.

dikaryotic /die-kar-ee-AWT-ick/ Having two nuclei per cell.

dimer /DIGH-mer/ n. A chemical compound composed of two simpler molecules; a polymer composed of two monomers.

dimeric /digh-MARE-ick/ adj. In chemistry, composed of two parts.

dimethyl sulfate (DMS) /die-METH-uhl/ A colorless oily liquid with an onionish aroma. Commonly used as a reagent for methylation of amines, phenols, and thiols.

dimethyl sulfoxide (DMSO) A colorless liquid that dissolves both polar and nonpolar compounds. It is itself miscible in many organic solvents, and in water. It is a polar aprotic solvent, meaning that it can dissolve ions dissolving like a protic solvents, yet lacks an acidic hydrogen. Widely used as a solvent in chemical reactions.

dL (or dl) Deciliter.

dimethylformamide (DMF) /DIE-meth-uhl-FORM-uh-mide/ A common solvent; a colorless liquid, miscible with water and most organic liquids.

dioecious /die-EE-shuhs/ adj. Having male (staminate) flowers on separate plants from those bearing female (carpellate) flowers.

diotic /die-AWT-ick/ adj. Affecting, or pertaining to both ears.

diploblastic /DIP-low-BLAST-ik/ adj. Developing from only two germ layers (ectoderm and endoderm).

diploid /DIP-loid/ adj. (1) in the case of a single-celled eukaryotic organism, having two complete sets of chromosomes; (2) in the case of a multicellular eukaryotic organism, having two complete sets of chromosomes in each somatic cell. See also: haploid and tetraploid.

diplophase /DIP-low-faze/ n. The phase in the life cycle of a diploid organism, which begins with the formation of the zygote and ends with meiosis.

diplotene /DIP-low-teen/ n. A stage of meiotic prophase. ADDITIONAL DETAILS

directed mutagenesis /myoot-uh-JEN-uh-suhs/ Alteration of DNA at a specific site in order to study the change's effects.

disaccharides (also disaccharoses) n. A group name for complex sugars composed of two bonded monosaccharides. Examples of common disaccharides are sucrose, lactose, and maltose. STRUCTURE OF COMMON DISACCHARIDES

discodactylous /DISK-oh-DAK-tuhl-uhs/ adj. Having suction cups on the toes.

discoid /DISK-oid/ adj. Disk-shaped.

discoplacental /DISK-oh-plu-SENT-uhl/ adj. Having a discoid placenta.

discopodous /duh-SKAWP-puh-duhs, dis-/ adj. Having discoid feet.

dispersal /duh-SPER-suhl/ n. The process in which, or the degree to which, organisms spread out from a source.

dissect /duh-SECT or DIGH-sect/ v. To cut apart an organism so as to reveal its separate parts and their connections.

dissection /digh-SEX-shun, duh-/ n. The act of dissecting.

distal /DIS-tuhl/ adj. (1) further from the center, center line, or axis; (2) with reference to an appendage: further from the trunk.

diurnal /die-ERN-uhl/ adj. Occurring or active during the day. Compare: nocturnal.

divergent evolution (also divergence) /duh-VERJ-uhnt, die-/ adj. A way of describing evolution, in which two or more new types of organisms arise from a common ancestor by accumulating traits that differentiate them from each other. MORE INFORMATION

DMF Dimethylformamide.

DMS Dimethyl sulfate.

DMSO Dimethyl sulfoxide.

DNA See: Deoxyribonucleic acid.

DNA polymerase /puh-LIM-er-aze/ n. An enzyme that catalyzes the synthesis of a complementary strand of DNA on a single-stranded DNA template. ANIMATION

DNA-RNA hybrid /HIGH-brid/ (also DNA-RNA chimera /KIME-er-uh/) A duplex molecule composed of an RNA chain hydrogen bonded to a complementary DNA chain.

DNA probe See: probe.

DNA repair genes Genes encoding proteins that can correct errors in DNA sequences.

DNA vector A virus, plasmid, or other vehicle, used to introduce DNA into a cell.

DNP 2,4-dinitrophenol.

dodecandrous adj. Having twelve stamens.

dolichol n. Any of various long-chain, usually unsaturated, organic compounds composed of isoprene units ending in an α-saturated isoprenoid group.

domain n. (1) a discrete portion of a protein with its own function. The combination of domains in a single protein determines its overall function; (2) in taxonomy, the type of category with highest rank (higher than kingdom). CHART OF RELATIVE TAXONOMIC RANKS

dominant adj. An allele expressed even when only one copy of it is present. See also: gene, genome.

dopa L-3,4-dihydroxyphenylalanine.

dormancy /DORE-muhn-see/ n. A period of interrupted growth and metabolic inactivity.

dormant /DORE-muhnt, -mant/ adj. In a state of dormancy.

dorsal /DORE-suhl/ adj. (1) thoracic; (2) of or pertaining to the back.

dosage effect /DOE-suhj/ The effect of varying gene copy number; increasing/reducing the number of copies of a gene within a genome can increase/reduce the number of copies of gene product produced, causing changes in physiology and development.

double circulatory system A blood circulation in which the pulmonary circulation is separate from the systemic circulation (as in mammals, birds, and crocodilians).

double fertilization The unique mode of fertilization seen only in flowering plants where two sperm cells fertilize two separate cells within the embryo sac. One cell contains the egg and the other contains two polar nuclei.

dropsy /DRAWP-see/ n. An archaic term for swelling of soft tissue caused by accumulation of water.

Drosophila /druh-SAWF-uh-luh/ A genus of flies commonly used in genetics and cytology experiments; commonly known as fruit flies. PICTURE

dryopithecine /DRY-oh-PITH-uh-seen/ n. Any of various Miocene apes, formerly widespread in Eurasia. MORE INFORMATION

dugong /DOO-gong/ n. A large herbivorous marine mammal (Dugong dugon) belonging to the order Sirenia, which also includes the manatees. Average adult length: 2.7 meters (8.9 ft). PICTURE | GEOGRAPHIC RANGE

duodenum /doo-AWD-uh-nuhm/ n. The upper, short (~25 cm) portion of the small intestine. It connects the pylorus with the jejunum. PICTURE 1 | PICTURE 2

dura mater (also dura) /D(Y)ER-uh MADE-er/ The tough, fibrous outer membranous sheath of the brain and spinal cord.

duplex DNA /DOOP-lex/ Two single polynucleotide DNA chains joined along their lengths to form a double helix.

dTDP Thymidine diphosphate.

dTMP Thymidine monophosphate.

dTTP Thymidine triphosphate.

dyad /DIE-ad/ n. A replicated chromosome — a pair of sister chromatids.

dyne /DINE/ n. The amount of force that gives an unencumbered mass of one gram an acceleration of one centimeter per second per second.

dysneuria /dis-NUHR-ee-yuh/ n. Impaired nerve function.

dyspepsia /dis-PEP-see-yuh/ n. Indigestion.

dysplasia /dis-PLAYZ-yuh/ n. Abnormal tissue development.

dyspnea /disp-NEE-yuh/ n. A desire or hunger for air; manifested in strained, usually painful, breathing.

dzo (or zho or dzopkyo or khainag) n. A hybrid between yak (Bos grunniens) and cattle (Bos taurus). MORE INFORMATION