Online Biology Dictionary - D

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/ A substance causing the secretion of tears.

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

dacryocystitis /dack-ree-oh-sist-TIGHT-uhs/ Inflammation of the lacrimal sac.

dacryorrhea /dack-ree-oh-REE-uh/ Excessive tearing of the eyes.

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

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

dalton unit (Da) 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 The female parent (used in connection with domestic animals).

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

dd Dideoxy-.

DEAE (also DEAE-C) Diethylaminoethyl cellulose.

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

deciduous /duh-SID-joo-uhs/ Refers to something that falls out or off, as teeth, horns, or leaves.

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

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

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

decubitus /duh-KYOOB-uh-tuhs/ 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/ Rupture of the hymen.

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

dehydrogenase /dee-high-DRAW-juhn-aze/ 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-/ Feces.

deletion /duh-LEE-shun, dee-/ (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/ To liquify.

deme /DEEM/ 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/ A short, branched process of a neuron that conveys nerve impulses to the body of the cell.

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

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

DNA double helix

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. 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/ To remove hair.

dermal /DERM-uhl/ 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.

dermotropic /DERM-uh-TRAWP-ik, DERM-uh-TROPE-ik/ Attracted to or entering through skin <~ bacteria>.

desmosome /DEZ-muh-soam/ A cell structure functioning in cell-to-cell adhesion.

developmental biology The study of the processes by which an organism changes from a zygote into its mature, multicellular structure.

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

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

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

dextrose Another name for glucose.

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

DG sn-1,2-diacylglycerol.

DHAP Dihydroxyacetone phosphate.

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

diastasis /die-AST-uh-suhs/ (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/ The portion of the cardiac cycle during which the heart expands and fills with blood. Compare: systole.

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

dicot /DIE-cawt/ Short for dicotyledon

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

Digestive System (click to enlarge)

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

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

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

dihydroxyacetone phosphate (DHAP) A compound involved in a wide variety of biochemical reactions. Its main role is in glycolysis.

dimer A chemical compound composed of two simpler molecules; a polymer composed of two monomers.

dimeric In chemistry, composed of two parts.

dimethyl sulfate (DMS) 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.

dimethylformamide (DMF) A common solvent; colorless liquid, miscible with water and most organic liquids.

dioecious /dye-EE-shuhs/ Having male (staminate) flowers on separate plants from those bearing female (carpelate) flower.

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

diploid /DIP-loid/ (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/ 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/ 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.

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

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

diurnal Occurring or active during the day. Compare: nocturnal.

divergence A way of describing evolution, in which two or more new types of organisms arise from a common ancestor by accumulating distinctive traits that differentiate them from each other. In molecular biology, where this mode of evolution is usually taken for granted, the word divergence is used for the percentage difference between related nucleotide or amino acid sequences. MORE INFORMATION

DMF Dimethylformamide.

DMS Dimethyl sulfate.

DMSO Dimethyl sulfoxide.

DNA See: Deoxyribonucleic acid.

DNA polymerase (green) synthesizing DNA

DNA polymerase An enzyme that catalyzes the synthesis of a complementary strand of DNA on a single-stranded DNA template (see figure right).

DNA-RNA hybrid (also DNA-RNA chimera) 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 Having twelve stamens.

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

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

dopa L-3,4-dihydroxyphenylalanine.

dorsal (1) thoracic; (2) of or pertaining to the back.

dosage effect 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.

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

duodenum /doo-AWD-uh-nuhm/ 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 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/ A replicated chromosome -- a pair of sister chromatids.

dyne /DINE/ 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/ Impaired nerve function.

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

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

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

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