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MOLECULAR BIOLOGY, CELL CULTURE, IMMUNOLOGY, HUMAN PHYSIOLOGY |
D genes
Sets of gene segments lying between the V and J genes in the immunoglobulin
heavy chain genes, and in the T-cell receptor β and δchain genes which
are recombined with V and J genes during ontogeny
Deadaptation Reversible loss of a specific property due to the
absence of the appropriate inducer (not always defined)
Dedifferentiation A term implying irreversible loss of the specialized
properties that a cell would have expressed in vivo. As evidence accumulates
that cultures "dedifferentiate" by a combination of selection
of undifferentiated cells or stromal cells and deadaptation resulting
from the absence of the appropriate inducers, this term is going out
of favor. In is still correctly applied to progressive loss of differentiated
morphology in histological observations of, for example, tumor tissue
Defined medium A culture medium whose exact chemical compositions
are known.
Degeneration In reference to the genetic code, having more than
one codon specifying a particular amino acid.
Denaturation Of nucleic acid molecules: breakdown by chemical
or physical means of the hydrogen bonds involved in base-pairing. Of
proteins: irreversible destruction of conformation by heat
Dendrite Highly branched extension of neuron cell body; receives
synaptic input from other neurons.
Dendritic cells A set of cells present in tissues, which capture
antigens and migrate to the lymph nodes and spleen, where they are particularly
active in presenting the processed antigen to T cells.
Density-gradient centrifugation Separation of molecules and particles
on the basis of buoyant density, by centrifugation in a concentrated
sucrose or cesium chloride solution.
Density limitation of growth Mitotic inhibition correlated with
an increase in cell density
Desmosome Cell junction that holds two cells together; consists
of plasma membranes of adjacent cells linked by fibers yet separated
by a 20-nm extracellular space filled with a cementing substance.
Deoxyribonuclease (DNase) An enzyme that degrades DNA.
Dialysis Process of altering the concentration of substances
in the blood by using concentration differences between plasma and a
bathing solution separated by a semipermeable membrane
Diaphragm Dome-shaped skeletal muscle sheet that separates the
abdominal and thoracic cavities; principal muscle of respiration.
Dideoxynucleotide A modified nucleotide that lacks the 3'-hdroxyl
group and so prevents further chain elongation when incorporated into
a growing polynucleotide. Used in nucleotide sequencing technique.
Differentiation Process usually involving changes in gene expression
by which a precursor cell becomes a distinct structurally and functionally
specialized cell type.
Diploid In eukaryotes, an organism or cell with two chromosome
complements, one derived from each haploid gamete. Each chromosome represented
as a pair, identical in the autosomes and female sex chromosomes and
nonidentical in male sex chromosomes, and corresponding to the chromosome
number and morphology of most somatic cells of the species from which
the cells were derived
Disarmed plasmid A Ti plasmid that has had some or all of the
T-DNA genes removed, so it is no longer able to promote cancerous growth
of plant cells.
DNA sequencing Determination of the order of nucleotides in
a DNA molecule.
Double digestion Cleavage of a DNA molecule with two different
restriction endonucleases, either concurrently of consecutively.
Dynein An ATP powered motor protein in cilia and flagella that
accounts for the movement of these organelles. ATP binding to dynein
and its hydrolysis causes the dynein bridges to sequentially break and
form new bonds with adjacent microtubule doublets.
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