organism. In the case of site-directed mutagenesis, the
substitution or modification of a single amino acid at a
defined location in a protein is performed by changing
one or more BASE PAIRSin the DNA using recombinant
DNA technology.
mutation A heritable change in the NUCLEOTIDE
SEQUENCEof genomic DNA (or RNA in RNA virus-
es), or in the number of GENEs or chromosomes in a
cell, that can occur spontaneously or be brought
about by chemical mutagens or by radiation (induced
mutation).
mutualism A state where two different species ben-
efit from their association. Two types exist: symbiotic
and nonsymbiotic mutualism. Examples include
lichens (algae and fungi), mycorrhizae and rooting
plants, yucca plant pollination by the yucca moth, and
bees and flowers. In symbiotic mutualism, both indi-
viduals interact physically, and their relationship is
biologically essential for survival, e.g., the fungus–
alga relationship. The more common nonsymbiotic
mutualism is when individuals live independent lives
but cannot survive without each other, e.g., bees and
flowering plants.
mutual prodrug The association in a unique
molecule of two, usually synergistic, DRUGs attached to
each other, one drug being the carrier for the other and
vice versa.
mycelium Threadlike tubes, filaments, or hyphae;
the roots of mushrooms; the thalus or vegetative part
of a fungus. Bacteria (Actinomycetales) also produce
branched mycelium.
mycorrhizae A symbiotic (mutualism) connection
between plant roots and the mycelia of some fungi
species. The fungus provides water and mineral nutri-
ents to the plant, and the plant provides energy to the
fungus. Ectomycorrhizae form between tree species
and basidiomycete fungi, and the fungus provides a
sheath around the root that it penetrates. Endomycor-
mycorrhizae 237derivative
chromosome
20Deletion DuplicationInsertionTranslocationInversionchromosome
4chromosome
4chromosome
20chromosome
20chromosome
4derivative
chromosome
4chromosome
20A permanent structural alteration in DNA. In most cases, DNA
changes either have no effect or cause harm, but occasionally a
mutation can improve an organism’s chance of surviving and
passing the beneficial change on to its descendants.(Courtesy
of Darryl Leja, NHGRI, National Institutes of Health)