Life Science Dictionary D


Daily torpor: A Daily decrease in metabolic activity and body temperature during times of inactivity for some small mammals and birds.
Dalton: A measure of mass for atoms and subatomic particles.
Darwin(d): A unit for the rate of change of morphology.
Darwinian medicine: The application of evolutionary principles to medicine.
Data: Recorded observations.
Day-neutral plant: A plant whose flowering is not affected by photoperiod.
Decapod: A member of the group of crustaceans that includes lobsters, crayfish, crabs, and shrimps.
Decomposer: Any of saprobic fungi and prokaryotes that absorb nutrients from non-living organic material such as corpses, falling plant material, and the wastes of living organisms, and covert them into inorganic forms.
Deductive reasoning: A type of logic in which specific results are predicted from a general premise.
Deep-sea hydrothermal vent:  A dark, hot, oxygen deficient environment associated with volcanic activity. The food producers are chemoautotrophs prokaryotes.
De-etiolation: The changes a plant shoot undergoes in response to sunlight, also know as greening.
Defective interfering virus: A virus that has lost some function and that depends on co-infection with intact virus for transmission.
Dehydration reaction: A chemical reaction in which two molecules covalently bond to each other with the removal of a water molecule.
Deletion: (1) A deficiency in a chromosome resulting from a lots of a fragment through breakage. (2) A mutational loss of one or more nucleotide pairs from a gene.
Deme: A discrete local population.
Demographic transition: A shift from zero population growth in which birth rates and death rates are high to zero population growth characterized instead by low brith and death rates.
Demography:  The study of statistics relating to births and deaths in populations.
Denaturation: In proteins, a process in which a protein unravels and loses its native conformation, thereby becoming inactive. In DNA, the separation of the two strands of the double helix due to extreme conditions of pH, salt concentration, and temperature.
Dendogram: A phylogenetic tree in which the branch lengths are constrain to all be equidistant from the root; also known as ultrameric tree.
Dendrite: One of usually numerous short, highly branched processes of a neuron that convey nerve impulses toward the cell body.
Dendritic cell: An antigen-presenting cell, located mainly in the lymphatic tissues and skin, That is particularly efficient in presenting antigen to naive helper T cells, thereby initiating a primary immune response.
Density: The number of individuals per unit area or volume.
Density dependent: Referring to any characteristic that varies according to an increase in population density.
Density dependent inhibition: The phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another.
Density dependent selection: Selection that occurs when relative fitness depends on population density.
Density independent: Referring to any characteristic that is not affected by population density.
Deoxyribonucleic acid (DNA): A double stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell’s proteins.
Deoxyribose: The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.
Depolarization: An electric state in an excitable cell whereby the inside of the cell is made less negative relative to the outside than at the resting membrane potential.
Derivatives: New cells that are displayed from an apical meristem and continue to divide until the cells they produce become specialized.
Derived characteristic: A trait found in an organism that was not present in the common ancestor of a group of organisms being studied.
Dermal tissue system: The outer protective cover of plants.
Descent with modification: Darwin’s term for evolution.
Desert: A terrestrial biome characterized by very low precipitation.
Desmosome: A type of intercellular junction in animal cells that functions as an anchor.
Determinative cleavage: A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.
Determinate growth: A type of growth characteristic of most animals and some plant organs, in which growth stops after a certain size is reached.
Determination: The progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops.
Detritivore: A consumer that derives its energy from nonliving organic material; a decomposer.
Detritus: Dead organic matter.
Deuteromycete: Traditional classification for a fungus with no sexual stage. When a sexual stage is discover, the species is assigned to a phylum.
Deuterostome: Member of the two large groups of bilaterian animals including echinoderms, hemichordates, and chordates. In this group the initial embryonic opening becomes the anus.
Developmental constrains: Limits on what kinds of organism can develop.
Diabetes mellitus: An endocrine disorder marked by inability to maintain glucose homeostasis. Type I form results from autoimmune destruction of insulin-secreting cells. Type II form most commonly results from reduced responsiveness of target cells to insulin.
Diacylglycerol (DAG): A second messenger produced by the cleavage of  a certain kind of phospholipid in the plasma membrane.
Diapause: A resting stage that allows organisms to survive harsh conditions.
Diaphragm: A sheet of muscle that form the bottom wall of the thoracic cavity in mammal, active in ventilating the lungs.
Diapsid: Member of an amniote clade distinguish by a pair of holes on each side of the skull, including lepidosaurs and archosaurs.
Diastole: The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
Diastolic pressure: Blood pressure that remains between heart contractions.
Diatom: An unicellular photosynthetic alga with a unique glassy cell wall containing silica.
Dicots: A term traditionally used to refer to flowering plants that have two embryonic seed leaves, or cotyledons. Dicots do not form a clade.
Differential gene expression: The expression of different set of genes by cells with the same genome.
Differentiation: The process by which a cell becomes more and more specialized in its function and morphology through the regulation of gene expression and biochemical activities.
Diffusion: Spreading due to the cumulative effect of small random movements. The spontaneous tendency of a substance to move down its concentration gradient, from a more concentrated to a less concentrated area.
Diffusion approximation: A mathematical approximation that describes diffusion using a differential equation.
Digestion: The process of breaking down food into molecules small enough for the body to absorb.
Dihybrid: An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents that are doubly homozygous for different alleles, are dihybrids
Dikaryotic: Referring to a fungal mycelium with two haploid nuclei per cell, one from each parent.
Dimer: Two molecules that are bound together.
Dinoflagellate: Member of a phylum of eukaryotes in the alveolate kingdom. All are single-celled organisms and many are photosynthetic.
Dinosaur: Member of an extremely diverse group of ancient reptiles varying in body shape, size, and habitat.
Dioecious: A term typically used to describe an angiosperm species in which carpellate and staminate flowers are on separate plants.
Diploblastic: Having two germ layers.
Diploid: Carrying two copies of each chromosome.
Diplomonad: A protist that have modified mitochondria, two-equal size nuclei, and multiple flagella.
Directional selection: Selection favoring one allele over another or favoring increased values of a quantitative trait. It is equivalent to positive selection.
Disaccharide: A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
Discicristate: A kingdom of eukaryotes. It includes kinetoplastids (e.g. trypanosome) and euglenoids.
Dispersal: The distribution of individuals within geographic population boundaries.
Dispersion: The pattern of spacing among individuals within geographic population boundaries.
Disruptive selection: Natural selection favoring extreme values of a trait.
Dissociation curve: A chart showing the relative amounts of oxygen bound to hemoglobin when the pigment is exposed to solutions varying in their partial pressure of dissolved oxygen, pH, or other characteristics.
Distal tubule: In the vertebrate kidney, the portion of a nephron that helps refine filtrate and empty it into a collecting duct.
Disturbance: A force, like fires or storms, that changes a biological community and usually removes organisms from it.
Distyly: A polymorphism with two different arrangements of anther and stigma that promotes outcrossing.
Disulfide bridge: A strong covalent bond formed when the sulfur of one cysteine monomer bonds to the sulphur of another cysteine monomer.
Divergence: the evolution of increasing difference between lineages in one or more characters.
Diversify: to increase in the number of species usually accompanied by divergence.
Dizygotic twins: Twins formed from separated zygotes and therefore related in the same way as siblings.
DNA fingerprint: An individual’s unique collection of DNA restriction fragments, detected by electrophoresis and nucleic acid probes.
DNA ligation: Chemical ligation of two DNA strands. It is used in DNA repair, replication, and other molecular processes.
DNA microarray assay: A method to detect and measure the expression of thousands of genes at one time by hybridization with various samples of cDNA.
DNA polymerase: An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
Domain: (1) A taxonomic category above the kingdom level. The three domains of life are Archaea, Bacteria, and Eukarya. (2) An independent folding part of a protein.
Dominance: If the heterozygote is precisely intermediate between the two homozygotes, there is no dominance. Any deviation from this additive model is described as dominance.
Dominance deviation: The difference between the trait value of a genotype and the value expected with no dominance.
Dominant allele: An allele is completely dominant with respect to a certain phenotype if produces that phenotype when present in either one or two copies.
Dominant species: Those species in a community that have the highest abundance or highest biomass. These species exert a powerful control over the occurrence and distribution of other species.
Dopamine: A biogenic amine closely related to epinephrine and norepinephrine.
Dormancy: A condition typified by extremely low metabolic rate and a suspension of growth and development.
Dorsal: on or relating to the back of an animal, plant, or organ.
Dorsal lip: The dorsal side of the blastopore.
Dosage compensation: A mechanism that ensures that sex-linked genes are expressed at the appropriate levels in both males and females.
Double circulation: A circulation scheme with separate pulmonary and systemic circuits which ensures vigorous blood flow to all organs.
Double fertilization: A mechanism of fertilization in angiosperms in which two sperm cells unite with two cells in the embryo sac to form the zygote and endosperm.
Double helix: The from of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
Down syndrome: A human genetic disease caused by the presence of an extra chromosome 21; characterized by mental retardation, and heart and respiratory defects.
Duchenne muscular dystrophy: A human genetic disease caused by a sex-linked recessive allele; characterized by a progressive weakening and a loss of muscle tissue.
Duodenum: The firsts section of the small intestine, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall.
Duplication: An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
Dynamic stability hypothesis: The idea that long food chains are less stable than short chains.
Dynein: A large contractile protein forming the side-arms of microtubule doublets in cilia and flagella