Glossary ❮ 321diplomonads A phylum that is associated with the
archezoan eukaryotes.
directional selection Occurs when members of a
population at one end of a spectrum are selected
against and/or those at the other end are selected
for.
disaccharide A sugar consisting of two monosaccha-
rides bound together. Common disaccharides
include sucrose, maltose, and lactose.
dispersive DNA replication A theory that suggests
that every daughter strand contains someparental
DNA, but it is dispersed among pieces of DNA
not of parental origin.
disruptive selection Selection is disruptive when
individuals at the two extremes of a spectrum of
variation do better than the more common forms
in the middle.
distributionDescribes the way populations are
dispersed over a geographic area.
divergent evolution Two related species evolve in a
way that makes them less similar, sometimes caus-
ing speciation.
division The classification category that replaces the
phylum in plant classification.
DNA methylation The addition of CH 3 groups to
the bases of DNA, rendering DNA inactive.
DNA polymeraseThe main enzyme in DNA repli-
cation that attaches to primer proteins and adds
nucleotides to the growing DNA chain in a
5 ′-to-3′direction.
DNA replication The process by which DNA is
copied. This process occurs during the S phase of
the cell cycle to ensure that every cell produced
during mitosis or meiosis receives the proper
amount of DNA.
dominance hierarchy A ranking of power among
the members of a group of individuals.
double helixThe shape of DNA—two strands held
together by hydrogen bonds.
Down syndrome A classic aneuploid syndrome
affecting one of every 700 children born in the
United States. It most often involves a trisomy of
chromosome 21, and leads to mental retardation,
heart defects, short stature, and characteristic facial
features.
Duchenne muscular dystrophy Sex-linked dis-
order caused by the absence of an essential
muscle protein that leads to progressive weakening
of the muscles combined with a loss of muscle
coordination.
ecosystemAll the individuals of a community and
the environment in which it exists.
ectodermOuter germ layer that gives rise to the ner-
vous system, skin, hair, and nails.
ectothermic animal Animal whose basic metabolic
rates increase in response to increases in temperature.
Edwards syndrome The presence of trisomy 18,
which occurs in one out of every 10,000 live births
and affects almost every organ of the body.
electron transport chain (ETC) The chain of mole-
cules, located in the mitochondria, that passes elec-
trons along during the process of chemiosmosis to
regenerate NAD+to form ATP. Each time an elec-
tron passes to another member of the chain, the
energy level of the system drops.
elementThe simplest form of matter.
embryologyThe study of embryonic development.
emigration rateRate at which individuals relocate
out ofa given population.
endergonic reaction A reaction that requires input
of energy to occur. A +B+energy→C.
endocytosis Process by which substances are
brought into cells by enclosure into a membrane-
created vesicle that surrounds the substance and
escorts it into the cell.
endoderm Inner germ layer that gives rise to the
inner lining of the gut, digestive system, liver,
thyroid, lungs, and bladder.
endodermis Cells that line the innermost layer of
the cortex in plants that give rise to the casparian
strip.
endometrium Inner wall of the uterus to which the
embryo attaches.
endopeptidases Enzymes that initiate the digestion
of proteins by hydrolyzing all the polypeptides
into small amino acid groups.
endosymbiotic theoryProposes that groups of
prokaryotes associated in symbiotic relationships
to form eukaryotes (mitochondria and chloroplasts).
endothermic animal Animal whose body tem-
perature is relatively unaffected by external
temperature.
enhancer DNA region, also known as a “regulator,”
that is located thousands of bases away from the
promoter that influences transcription by interact-
ing with specific transcription factors.http://www.ebook3000.com