nucleotides, each of which is composed of the
five-carbon sugar deoxyribose, a phosphate
group, and the nitrogenous base adenine,
guanine, cytosine, or thymine. (7.1)
deoxyribose nucleic acid Phoebus Levene’s name
for deoxyribonucleic acid (DNA). (7.1)
descent with modification Darwin’s theory that
natural selection does not demonstrate progress
(or evolution), but merely results from a species’
ability to survive local conditions at a specific
time. (10.2)
desertification the transformation of marginal dry
lands into near-deserts that are unsuitable for
agriculture. (15.3)
dideoxynucleotides variants of each of the four
DNA nucleotides that resemble regular DNA
nucleotides, but lack the 3 ′hydroxyl group. (9.2)
differentiation the process by which certain
portions of a genome are activated or silenced to
enable a cell to take on the specific structure and
function of a given tissue. (4.4, 9.4)
diffusion form of passive transport in which
molecules pass through a membrane from a
region of higher concentration to a region of
lower concentration. (1.3)
diploid describes cells that contain two copies of
every chromosome, characteristic of cells in
organisms in which one set of chromosomes is
inherited from each parent. (11.1)
directional selection selection that favours the
phenotypes at one extreme over another,
resulting in the distribution curve of phenotypes
shifting in the direction of that extreme. (11.3)
directionality the specific orientation of each strand
of DNA, read in the 5 ′to 3 ′direction. The
orientation of one strand in DNA is opposite to
the orientation of the other strand in a double
helix. (7.2)
direct repair immediate recognition and correction
of incorrectly paired nucleotides in DNA
undergoing replication, accomplished by
enzymes such as DNA polymerase. (9.1)
disaccharide sugar that is formed from two
monosaccharides. (1.3)
dispersion distribution of individuals of a
population within its geographical boundaries.
(14.1)
dispersive theory the theory stating that in the
replication of DNA, the parental DNA molecules
are broken into fragments and that both strands
of DNA in each of the daughter molecules are
made up of an assortment of parental and new
DNA. (7.3)
disruptive (diversifying) selection selection that
favours the extremes of a range of phenotypes
rather than intermediate phenotypes. This typeof selection can result in the elimination of
intermediate phenotypes. (11.3)
dissociation of water molecules the formation of H+
and OH−from water molecules. (1.3)
distal tubule the tubule that connects the loop of
Henle to the collecting duct in the kidney. (4.2)
divergence species-forming pathway in which one
or more new species arises from a parent species
that continues to exist. (12.2)
divergent evolution process by which species that
were once similar to an ancestral species become
increasingly different (e.g., speciation, adaptive
radiation). (12.3)
DNA seedeoxyribonucleic acid. (7.1)
DNA amplification the process of generating a large
sample of a target. (9.2)
DNA fingerprint pattern of bands formed by using
gel electrophoresis on DNA fragments. (9.2)
DNA ligase enzyme that splices together Okazaki
fragments during DNA replication on the lagging
strand. DNA ligase catalyzes the formation of
phosphate bonds between nucleotides. (7.3)
DNA particle gun in recombinant DNA technology,
a device that fires DNA-coated microscopic metal
particles directly into plant cells and their nuclei.
It is used as a means of bringing DNA from the
cytoplasm into the cell nucleus. This device was
developed in 1988 by American researcher John
Sanford. (9.3)
DNA polymerase during DNA replication, an
enzyme that slips into the space between two
strands, uses the parent strands as a template,
and adds nucleotides to make complementary
strands. (7.3)
DNA sequence from a single gene or DNA fragment.
(9.2)
dominant allele in a heterozygous pairing, the allele
that is fully expressed in an organism’s
phenotype. (11.1)
dopamine neurotransmitter that elevates mood and
controls skeletal muscles. (5.2)
dynamic equilibrium state of balance achieved
within an environment. Internal control
mechanisms maintain the balance by
continuously opposing outside forces that tend
to change that environment. (4.1)ecological niche the role that members of a
population play in a community. The role
includes the resources members need and how
the members interact with other members of the
population and the community. (13.1)E
574 MHR • Glossary