glossary G-7differentiation See cell differentiation.
diffusion Net movement of like molecules (or ions) down
their concentration gradient.
digestion The breakdown of food particles into nutrient mol-
ecules small enough to be absorbed.
digestive system Organ system with specialized regions
where food is ingested, digested, and absorbed and undi-
gested residues are stored, then eliminated.
diploid (dip-loyd) Having two chromosomes of each type
(that is, pairs of homologous chromosomes). Compare
haploid.
disaccharide (die-sak-uh-ride) [Gk. di, two, and sakcharon,
sugar] A type of simple carbohydrate, of the class called oli-
gosaccharides; two monosaccharides covalently bonded.
disease Condition that develops when the body’s defenses
cannot prevent a pathogen’s activities from interfering with
normal body functions.
disease vector Something (such as an insect) that carries a
pathogen from an infected person or contaminated material
to new hosts.
disjunction The separation of each homologue from its part-
ner during anaphase I of meiosis.
distal tubule The tubular section of a nephron most distant
from the Bowman’s capsule; a major site of water and sodium
reabsorption.
DNA Deoxyribonucleic acid (dee-ox-ee-rye-bow-new-clay-ik)
For all cells (and many viruses), the molecule of inheri-
tance. A category of nucleic acids, each usually consisting
of two nucleotide strands twisted together helically and
held together by hydrogen bonds. The nucleotide sequence
encodes the instructions for assembling proteins, and, ulti-
mately, a new individual.
DNA chip A microarray of thousands of DNA sequences that
are stamped onto a glass plate; can help identify mutations
and diagnose diseases by pinpointing which genes are silent
and which are being expressed in a body tissue.DNA clone An identical copy of foreign DNA that was
inserted into plasmids (typically, bacteria).
DNA fingerprint A DNA profile used in criminal
investigations.
DNA polymerase (poe-lim-uh-rase) Enzyme that assembles
a new strand on a parent DNA strand during replication; also
takes part in DNA repair.
DNA profile Of each individual, a unique array of DNA
sequences inherited from each parent.
DNA repair Following an alteration in the base sequence of a
DNA strand, a process that restores the original sequence, as
carried out by DNA polymerases and other enzymes.
DNA replication The process by which the hereditary mate-
rial in a cell is duplicated for distribution to daughter nuclei.exchange corresponding segments at the break points. Cross-
ing over breaks up old combinations of alleles and puts new
ones together in chromosomes.
cutaneous membrane A dry, sturdy epithelial membrane;
the skin.
cyclic AMP Cyclic adenosine monophosphate. A nucleo-
tide that has roles in intercellular communication, as when
it serves as a second messenger (a cytoplasmic mediator of a
cell’s response to signaling molecules).
cytokine Any of the chemicals released by white blood cells
that help muster or strengthen defense responses.
cytokinesis (sigh-toe-kih-nee-sis) [Gk. kinesis, motion] Cyto-
plasmic division; the splitting of a parent cell into two daugh-
ter cells.
cytoplasm (sigh-toe-plaz-um) [Gk. plassein, to mold] All cel-
lular parts, particles, and semifluid substances enclosed by
the plasma membrane except for the nucleus.
cytosine (sigh-toe-seen) A pyrimidine; one of the nitrogen-
containing bases in nucleotides.
cytoskeleton A cell’s internal “skeleton.” Its microtubules
and other components structurally support the cell and orga-
nize and move its internal components.
cytosol The jellylike fluid portion of the cytoplasm.
cytotoxic T cell Type of T lymphocyte that directly kills
infected body cells and tumor cells.
deforestation The removal of all trees from a large tract of
land, such as the Amazon Basin or the Pacific Northwest.
deletion At the cellular level, loss of a segment from a chro-
mosome. In a DNA molecule, loss of one to several base pairs.
demographics A population’s vital statistics.
dendrite (den-drite) [Gk. dendron, tree] A short, slender
extension from the cell body of a neuron that receives
messages.
dendritic cell A type of white blood cell that alerts the adap-
tive immune system when an antigen is present in tissue fluid
of the skin or body linings.
dense connective tissue A type of fibrous connective tissue
with more collagen fibers than loose connective tissue; it is
strong but not very flexible.
dermis The layer of skin underlying the epidermis, consist-
ing mostly of dense connective tissue.
desertification (dez-urt-ih-fih-kay-shun) The conversion of
grasslands, rain-fed cropland, or irrigated cropland to desert-
like conditions, with a drop in agricultural productivity of
10 percent or more.
diaphragm (die-uh-fram) [Gk. diaphragma, to partition]
Muscular partition between the thoracic and abdominal
cavities, the contraction and relaxation of which contributes
to breathing.
diastole Relaxation phase of the cardiac cycle.
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