

Biology Glossary - D

 (1) Dihydrouridine (see tRNA).
(2) Aspartic acid (see aminoacids).

D loop (mol. biol.)
(1) (displacement loop) A single-stranded loop formed when a short ssDNA molecule pairs with a complementary region of one strand of a dsDNA molecule, displacing the corresponding region of the homologous strand (the D loop). For example, under certain (non-physiological) conditions in vitro a negatively supercoiled ccc dsDNA molecule can spontaneously take up a short complementary single (linear) strand to give rise to a D loop in a reaction driven by the energy of supercoiling; the resulting ‘joint molecule’ is more relaxed than the original supercoiled DNA. D-loop formation
is promoted e.g. by the RECA protein; it may occur e.g. during recombination and may be involved in the priming of certain types of DNA replication.
 (2) The loop of the ‘D arm’ in a tRNA molecule (see TRNA).

D period
See Helmstetter–Cooper Model.

D value
 (1) (D10 value; decimal reduction time) The time required, at a given temperature, to reduce the number of viable cells or spores of a given microorganism to 10% of the initial number; it is usually quoted in minutes. The temperature (°C) at which the D value is determined may be indicated by a subscript, e.g., D112.

(2) The percentage of digestible organic matter in the ‘dry matter’ (DM) – DM (in g/kg) being determined by oven-drying of feed samples and correction for e.g. loss ofvolatile fatty acids.
The atomic mass unit; a measure of mass for atoms and subatomic particles.
Darwinian fitness
daughter cell
A cell that is the offspring of a cell that has undergone mitosis or meiosis. The term "daughter" does not indicate the sex of the cell.
day-neutral plant
A plant whose flowering is not affected by photoperiod.
[L. decidere, to fall off]
Refers to plants that shed their leaves at a certain season.
dehydration reaction
(1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of a nucleotide from a gene.
The study of statistics relating to births and deaths in populations.
(den-dryt) [Gk. dendron, tree]
The number of individuals per unit area or volume.
density-dependent factor
density-dependent inhibition
density-independent factors
deoxyribonucleic acid (DNA)
(dee-oks-ee- ry-boh-noo-klay-ik)
dependent variable
An electrical 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. A neuron membrane is depolarized if a stimulus decreases its voltage from the resting potential of –70 mV in the direction of zero voltage.
dermal tissue system
[Gk. derma, skin]
The inner layer of the skin, beneath the epidermis.
(dez-muh-some) [Gk. desmos, bond + soma, body]
determinate cleavage
determinate growth
Dead organic matter.
[L. detritus, worn down, worn away + voro, to devour]
Organisms that live on dead and discarded organic matter; include large scavengers, smaller animals such as earthworms and some insects, as well as decomposers (fungi and bacteria).
(doo-ter-oh-stomes) [Gk. deuteros, second + stoma, mouth]
[Gk. diaphrassein, to barricade]
diastolic pressure
(dye-cottle-ee-don) [Gk. di, double, two + kotyledon, a cup-shaped hollow]
A member of the class of flowering plants having two seed leaves, or cotyledons, among other distinguishing features; often abbreviated as dicot.
See cellular differentiation.
[L. diffundere, to pour out]
[L. digestio, separating out, dividing]
dihybrid cross
(dy-ee-shus) (dye-ee-shus) [Gk. di, two + oikos, house]
Displaying two separate growth forms.
diploid cell
(dip-loyd) [Gk. di, double, two + ploion, vessel]
directed molecular evolution
directional selection
(dy-sak-ur-ide) [Gk. di, two + sakcharon, sugar]
[L. diurnus, of the day]
Applied to organisms that are active during the daylight hours.
The distribution of individuals within geographical population boundaries.
diversifying selection
[L. diurnus, of the day]
Natural selection that favors extreme over intermediate phenotypes.
A taxonomic grouping of related, similar classes; a high- level category below kingdom and above class. Division is generally used in the classification of prokaryotes, algae, fungi, and plants, whereas an equivalent category, phylum, is used in the classification of protozoa and animals.
Abbreviation of deoxyribonucleic acid.
DNA ligase
width="400" valign="TOP"> A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain.
DNA methylation
DNA polymerase
DNA probe
A taxonomic category above the kingdom level; the three domains are Archaea, Bacteria, and Eukarya.
dominance hierarchy
dominant allele
[L. dormire, to sleep]
[L. dorsum, the back]
Pertaining to or situated near the back; opposite of ventral.
double circulation
double fertilization
double helix
Down syndrome
(doo-oh-dee-num) [L. duodeni, twelve each - from its length, about 12 fingers' breadth]



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