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.
(dawl-ton)
The atomic mass unit; a measure of mass for atoms and subatomic particles.
The atomic mass unit; a measure of mass for atoms and subatomic particles.
Darwinian
fitness
A measure
of the relative contribution of an individual to the gene pool of the next
generation.
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.
deciduous
[L. decidere, to
fall off]
Refers to plants that shed their leaves at a certain season.
Refers to plants that shed their leaves at a certain season.
decomposers
Saprotrophic
fungi and bacteria that absorb nutrients from nonliving organic material such
as corpses, fallen plant material, and the wastes of living organisms, and
convert them into inorganic forms.
dehydration
reaction
A
chemical reaction in which two molecules covalently bond to one another with
the removal of a water molecule.
deletion
(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.
demography
The study
of statistics relating to births and deaths in populations.
denaturation
For
proteins, a process in which a protein unravels and loses its native
conformation, thereby becoming biologically inactive. For DNA, the separation
of the two strands of the double helix. Denaturation occurs under extreme
conditions of pH, salt concentration, and temperature.
dendrite
(den-dryt)
[Gk. dendron, tree]
One of usually numerous, short, highly branched processes of a neuron that conveys nerve impulses toward the cell body.
One of usually numerous, short, highly branched processes of a neuron that conveys nerve impulses toward the cell body.
denitrification
The
process by which certain bacteria living in poorly aerated soils break down
nitrates, using the oxygen for their own respiration and releasing nitrogen
back into the atmosphere.
density
The
number of individuals per unit area or volume.
density-dependent
factor
Any
factor influencing population regulation that has a greater impact as
population density increases.
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-independent
factors
Any
factor influencing population regulation that acts to reduce population by the
same percentage, regardless of size.
deoxyribonucleic
acid (DNA)
(dee-oks-ee-
ry-boh-noo-klay-ik)
A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell's proteins.
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.
dependent
variable
In an
experiment, the dependent variable is the factor that responds when another
factor is manipulated.
depolarization
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.
deposit-feeder
A
heterotroph, such as an earthworm, that eats its way through detritus,
salvaging bits and pieces of decaying organic matter.
dermal
tissue system
The
protective covering of plants; generally a single layer of tightly packed
epidermal cells covering young plant organs formed by primary growth.
dermis
[Gk. derma, skin]
The inner layer of the skin, beneath the epidermis.
The inner layer of the skin, beneath the epidermis.
desmosome
(dez-muh-some)
[Gk. desmos, bond + soma, body]
A type of intercellular junction in animal cells that functions as an anchor.
A type of intercellular junction in animal cells that functions as an anchor.
determinate
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 animals, in which the organism stops growing after it
reaches a certain size.
determination
The
progressive restriction of developmental potential, causing the possible fate
of each cell to become more limited as the embryo develops.
detritus
(deh-try-tis)
Dead organic matter.
Dead organic matter.
detritivores
[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).
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).
deuterostome
(doo-ter-oh-stomes)
[Gk. deuteros, second + stoma, mouth]
One of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial, indeterminate cleavage, enterocoelous formation of the coelom, and development of the anus from the blastopore.
One of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial, indeterminate cleavage, enterocoelous formation of the coelom, and development of the anus from the blastopore.
development
The
progressive production of the phenotypic characteristics of a multicellular
organism, beginning with the fertilization of an egg.
diaphragm
[Gk. diaphrassein, to
barricade]
A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs.
A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs.
diastole
(dy-ass-toh-lee)
The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
diastolic
pressure
The
pressure in an artery during the ventricular relaxation phase of the heart
cycle.
dicot
(dy-kot)
A subdivision of flowering plants whose members possess two embryonic seed leaves, or cotyledons.
A subdivision of flowering plants whose members possess two embryonic seed leaves, or cotyledons.
dicotyledon
(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.
A member of the class of flowering plants having two seed leaves, or cotyledons, among other distinguishing features; often abbreviated as dicot.
differentiation
See cellular differentiation.
diffusion
[L. diffundere, to
pour out]
The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
digestion
[L. digestio, separating
out, dividing]
The process of breaking down food into molecules small enough for the body to absorb.
The process of breaking down food into molecules small enough for the body to absorb.
dihybrid
(dy-hy-brid)
A hybrid individual that is heterozygous for two genes or two characters.
A hybrid individual that is heterozygous for two genes or two characters.
dihybrid
cross
A
breeding experiment in which parental varieties differing in two traits are
mated.
dikaryon
dy-kah-ree-on)
A mycelium of certain septate fungi that possesses two separate haploid nuclei per cell.
A mycelium of certain septate fungi that possesses two separate haploid nuclei per cell.
dioecious
(dy-ee-shus)
(dye-ee-shus) [Gk. di, two + oikos, house]
Referring to a plant species that has staminate and carpellate flowers on separate plants.
Referring to a plant species that has staminate and carpellate flowers on separate plants.
dimorphism
Displaying
two separate growth forms.
diploid
cell
(dip-loyd)
[Gk. di, double, two + ploion, vessel]
A cell containing two sets of chromosomes (2n), one set inherited from each parent.
A cell containing two sets of chromosomes (2n), one set inherited from each parent.
directed
molecular evolution
A
laboratory version of evolution at the molecular level that can produce
"designer molecules." A large starting population of molecules
(typically nucleic acids) that varies randomly in base sequence and shape is
subjected to replication with variation, followed by selection. After several
cycles of replication and selection, the population of molecules will evolve
toward one containing a high proportion of molecules well adapted to the
selection criterion applied.
directional
selection
Natural
selection that favors individuals on one end of the phenotypic range.
disaccharide
(dy-sak-ur-ide)
[Gk. di, two + sakcharon, sugar]
A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
diurnal
[L. diurnus, of
the day]
Applied to organisms that are active during the daylight hours.
Applied to organisms that are active during the daylight hours.
dispersion
The
distribution of individuals within geographical population boundaries.
diversifying
selection
[L. diurnus, of
the day]
Natural selection that favors extreme over intermediate phenotypes.
Natural selection that favors extreme over intermediate phenotypes.
division
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.
DNA
Abbreviation
of deoxyribonucleic acid.
DNA
ligase
(ly-gaze)
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.
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
The
addition of methyl groups (–CH3) to bases of DNA after DNA
synthesis; may serve as a long-term control of gene expression.
DNA
polymerase
An enzyme
that catalyzes the elongation of new DNA at a replication fork by the addition
of nucleotides to the existing chain.
DNA probe
A
chemically synthesized, radioactively labeled segment of nucleic acid used to
find a gene of interest by hydrogen-bonding to a complementary sequence.
domain
A
taxonomic category above the kingdom level; the three domains are Archaea,
Bacteria, and Eukarya.
dominance
hierarchy
A linear
"pecking order" of animals, where position dictates characteristic
social behaviors.
dominant
allele
In a
heterozygote, the allele that is fully expressed in the phenotype.
dormancy
[L. dormire, to
sleep]
A period during which growth ceases and metabolic activity is greatly reduced; dormancy is broken when certain requirements, for example, of temperature, moisture, or day length, are met.
A period during which growth ceases and metabolic activity is greatly reduced; dormancy is broken when certain requirements, for example, of temperature, moisture, or day length, are met.
dorsal
[L. dorsum, the
back]
Pertaining to or situated near the back; opposite of ventral.
Pertaining to or situated near the back; opposite of ventral.
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 form
of native DNA, referring to its two adjacent polynucleotide strands wound into
a spiral shape.
Down
syndrome
A human
genetic disease resulting from having an extra chromosome 21, characterized by
mental retardation and heart and respiratory defects.
duodenum
(doo-oh-dee-num)
[L. duodeni, twelve each - from its length, about 12 fingers'
breadth]
The first section of the small intestine, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall.
The first section of the small intestine, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall.
duplication
An
aberration in chromosome structure resulting from an error in meiosis or
mutagens; duplication of a portion of a chromosome resulting from fusion with a
fragment from a homologous chromosome.
dynein
(dy-nin)
A large contractile protein forming the sidearms of microtubule doublets in cilia and flagella.
A large contractile protein forming the sidearms of microtubule doublets in cilia and flagella.
