Da Dalton unit.
dacryagogue /DACK-ree-ə-gōg/ n. A substance causing lacrimation.
dacryelcosis /dack-ree-el-KŌ-səs/ n. Ulceration of the lacrimal sac.
dacryorrhea /dack-ree-ō-REE-ə/ n. Excessive lacrimation.
dactyl /DAKT-əl/ n. A digit (finger or toe).
dactyloid /DAKT-əl-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 1024 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.
DEAE (also DEAE-C) Diethylaminoethyl cellulose.
deaminization /də-am-in-eye-ZAY-shən, dee-/ n. Chemical decomposition in which a compound such as an amino acid or alkaloid loses its amino group and forms ammonia.
decaliter /DEK-ə-LEET-er/ n. A volume of ten liters.
deciliter /DES-ə-LEET-er/ n. One-tenth of a liter.
deciduous /də-SID-joo-əs/ adj. Falling out or falling off (as teeth, horns, or leaves).
decimorgan /DES-ə-MORE-gən/ See: Morgan unit.
decomposers See: saprophyte.
decubation /deck-you-BAY-shən/ n. The act of lying down.
decubital /də-KYOOB-ə-təl/ adj. Related to bedsores.
decubitus /də-KYOOB-ə-təs/ n. A bedsore.
defloration /dehf-lə-RAY-shən/ n. Rupture of the hymen.
deform /də-FORM/ v. To change the shape of (as when one molecule, say an enzyme, acts on another in such a way as to alter its conformation) — deformation. /DEH-form-MAY-shən/
dehydrate /də-HĪ-drate, dee-/ v. To reduce or entirely remove the water content of a substance or body.
dehydration synthesis /də-HĪ-dray-shən, dee-/ The linking of monomers into a polymer by removing a molecule of water from each.
dejecta /də-JEK-tə, dee-/ n. Feces.
deliquesce /del-lə-KWESS/ v. To liquify.
deme /DEEM/ n. A small, geographically localized population, usually thought of as panmictic.
demoid /DEM-oid/ adj. Common, abundant (said of fossils).
denaturation /də-nay-cher-RAY-shən, 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-rīt/ n. A short, branched process that conveys nerve impulses to the body of a neuron.
dendrochronology /den-drō-krə-NAWL-ə-jee/ n. The use of tree growth rings to date environmental events.
dendrology /den-DRAWL-ə-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.
denticles /DENT-ə-kəls/ n. Small, sharp, toothlike scales found on sharks, rays and chimaeras. Unlike ordinary fish scales, denticles have a structure similar to that of real teeth (i.e., an internal pulp cavity surrounded by dentin with an enamel-like coat of vitrodentine).
dentin /DENT-in/ (or dentine /den-TEEN/) n. The bony portion of a tooth surrounding the pulp.
dentulous /DENCH-ə-ləs/ (also dentate /DEN-tate/ or dentated /DEN-tate-əd/) adj. Toothed, having teeth. Compare: edentulous.
deoxyribonucleic acid (DNA) /dee-AWKS-ee-RĪB-ō-n(y)oo-KLAY-ək/ 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.
deoxyriboside /dee-AWKS-ee-RĪ-bō-side/ See: nucleoside.
depilate /DEP-ə-late/ v. To remove hair.
derived trait A synapomorphy.
dermad /DERM-ad/ adv. Toward the skin.
dermal /DERM-əl/ adj. Relating to the skin.
dermal tissue system (also epidermis) /ep-ə-DERM-əs/ A layer protecting the young parts of a plant; it is one cell thick.
dermatology/derm-ə-TAWL-ə-jee/ n. The study of the skin and of the treatment of its diseases.
dermatophytosis /der-MAWD-ə-fī-TŌ-səs/ (commonly known as ringworm) n. A type of skin infection, of widespread occurrence, caused by certain fungi feeding on keratin.
dermographia /DERM-ə-GRAF-ee-yə/ n. A condition of the skin in which pressure or friction leaves a red mark.
dermotropic /DERM-ə-TRAWP-ik, DERM-ə-TRŌP-ik/ adj. Attracted to or entering through skin.
desmosome /DEZ-mə-soam/ n. An animal cell junction structure that serves to attach adjacent cells.
desiccated /DEH-sick-kate-əd/ adj. In a dried state.
desiccation /deh-sick-KAY-shən/ n. A process of drying; in an organism: moisture loss.
Derotremata /der-ə-TREM-ə-də, -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-TRĪT-əs/ n. Material produced by the decomposition of dead organisms.
developmental biology /də-VEL-əp-MENT-əl/ The study of the processes by which an organism changes from a single cell into a mature, multicellular individual.
Devonian Period (D) /də-VŌN-ee-ən, div-/ A geologic period of the Paleozoic Era lasting from 416 to 359.2 mya; named after Devon, a county in sw England, by Adam Sedgwick, who studied the distinctive Old Red Sandstone of Exmoor in that county. The earliest known tetrapods, ammonites, and seed plants all date to this period and it was during the Devonian, too, that terrestrial arthropods became well-established, and bony fish proliferated in the oceans. The first forests, composed mostly of ferns, horsetails, and lycophytes, appeared in the Devonian. During the Devonian Gondwana lay to the south, the continent of Siberia to the north, and Euramerica was forming in between. MAP
dextrocardia /deck-strō-KARD-ee-yə/ n. Presence of the heart oōn the right side of the body.
dextrorotatory /deck-strō-RŌT-tə-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
diagenesis /dī-ə-JEN-ə-səs/ (pl diageneses /dī-ə-JEN-ə-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 /dī-ag-NŌ-səs, dī-əg-/ n. Recognition of the nature of a disease from symptoms.
diaphragm /DĪ-ə-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
diastole /dī-AST-ə-LEE/ n. The portion of the cardiac cycle during which the heart expands and fills with blood. Compare: systole.
dichroic /d-KRŌ-ick/ (also dichromatic /dī-krō-MAT-ick/) adj. (1) having two colors; (2) existing in two color varieties.
dicot /DĪ-cawt/ n. Short for dicotyledon
didactylous /dī-DAK-til-ləs/ adj. Having two digits.
diethylaminoethyl cellulose (DEAE or DEAE-C) A resin used in diffusion purification and separation.
diffusion /də-FYOO-shən, 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. MORE INFORMATION
dihybrid cross /dī-HĪ-brid/ A mating in which the parents differ with respect to two genetically determined traits.
dihydroxyacetone phosphate (DHAP) /dī-hī-DRAWK-see-ASS-ə-tōn/ A compound involved in a wide variety of biochemical reactions. Its main role is in glycolysis.
dikaryotic /dī-kar-ee-AWT-ick/ Having two nuclei per cell.
dilatant /ī-LAY-tənt/ n. Something that causes dilation.
dilation /dī-LAY-shən/ (also dilatation /dil-ə-TAY-shən/) n. Enlargement of a passage, orifice, organ or vessel — dilated /DĪ-lay-ted/ adj.
dimeric /dī-MARE-ick/ adj. In chemistry, composed of two parts.
dimethyl sulfate (DMS) /dī-METH-əl/ 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.
dioecious /dī-EE-shəs/ adj. Having male (staminate) flowers on separate plants from those bearing female (carpellate) flowers.
diotic /dī-AWT-ick/ adj. Affecting, or pertaining to both ears.
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.
diplotene /DIP-lō-teen/ n. A stage of meiotic prophase. ADDITIONAL DETAILS
directed mutagenesis /myoot-ə-JEN-ə-səs/ 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-ō-DAK-təl-əs/ adj. Having suction cups on the toes.
discoid /DISK-oid/ adj. Disk-shaped.
discopodous /də-SKAWP-pə-dəs, dis-/ adj. Having discoid feet.
disease /də-ZEEZ, dih-/ n. A condition of a living organism that in some way impairs its function; a pathological condition; sickness. Each disease has particular set of associated symptoms.
dispersal /də-SPER-səl, dih-/ n. The process in which, or the degree to which, organisms spread out from a source.
dissect /də-SECT or DĪ-sect/ v. To cut apart an organism so as to reveal its separate parts and their connections.
dissection /dī-SEX-shən, də-/ n. The act of dissecting.
distal /DIS-təl/ adj. (1) further from the center, center line, or axis; (2) with reference to an appendage: further from the trunk.
diurnal /dī-ƏRN-əl/ adj. Occurring or active during the day. Compare: nocturnal.
divergent evolution (also divergence) /də-VERJ-ənt, dī-/ 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
DMS Dimethyl sulfate.
DMSO Dimethyl sulfoxide.
DNA See: Deoxyribonucleic acid.
DNA methylation /METH-əl-AY-shən/ n. A biochemical process involving the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring (cytosine and adenine are two of the four bases of DNA). Such modifications can alter gene expression patterns in cells and can be inherited through cell division. Methylation of DNA allows cells to "remember" where they have been or alter gene expression in response to ambient conditions. The general effect of methylation is to reduce the expression of the affected (or "tagged") genes. DNA methylation plays a role in normal organismal development and cellular differentiation in multicellular organisms. Many methylation modifications affecting gene expression are heritable, regulation of this sort is called "epigenetic."
DNA probe See: probe.
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
dormancy /DORE-mən-see/ n. A period of interrupted growth and metabolic inactivity.
dormant /DORE-mənt/ adj. In a state of dormancy.
dorsal /DORE-səl/ adj. (1) thoracic; (2) of or pertaining to the back; (3) of or pertaining to the upper surface of any appendage (e.g. of a finger or of the tongue).
dorsum /DORE-səm/ adj. The dorsal region.
dosage effect /DŌ-səj/ 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 fertilization The unique mode of fertilization seen only in angiosperms, where two sperm cells (male gametes)fertilize two separate cells within the embryo sac. One of them fuses with the female gamete, or egg cell, which after fertilization goes on to develop into the embryo. The other fuses with the two polar nuclei to form the endosperm.
dropsy /DRAWP-see/ n. An archaic term for swelling of soft tissue caused by accumulation of water.
Drosophila /drə-SAWF-ə-lə/ A genus of flies commonly used in genetics and cytology experiments; commonly known as fruit flies. PICTURE
duct /dəkt/ n. Within the body, any narrow tube conveying or enclosing fluids.
dugong /DOO-gawng/ 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
dura mater (also dura) /D(Y)ER-ə-MADE-er/ The tough, fibrous outer membranous sheath of the brain and spinal cord.
dTDP Thymidine diphosphate.
dTMP Thymidine monophosphate.
dTTP Thymidine triphosphate.
dyne /DĪN/ n. The amount of force that gives an unencumbered mass of one gram an acceleration of one centimeter per second per second.
dysneuria /dis-NƏR-ee-yə/ n. Impaired nerve function.
dyspepsia /dis-PEP-see-yə/ n. Indigestion.
dysplasia /dis-PLAYZ-yə/ n. Abnormal tissue development.
dyspnea /disp-NEE-yə/ n. A desire or hunger for air; manifested in strained, usually painful, breathing.
dzo (or zho or dzoyo or khainag) n. A hybrid between yak (Bos grunniens) and zebu cattle (Bos indicus). PICTURE
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