inepithelial cells of the renal tubules. This results in
retention of sodium and loss of potassium. Some also
possess varying degrees of glucocorticoid activity. Their
secretion is regulated principally by plasma volume,
serum potassium concentration, and angiotensin II and,
to a lesser extent, by anterior pituitary ACTH (adreno-
corticotropic hormone).
minimum dynamic area The smallest area of habi-
tat necessary to sustain a viable population.
minimum viable population (MVP) The smallest
isolated population having the best chance of surviving
for xyears, regardless of natural catastrophes or future
demographic, environmental, and genetic variables.
Minot, George Richards (1885–1950) American
Pathologist George Richards Minot was born on
December 2, 1885, in Boston, Massachusetts, to James
Jackson Minot, a physician, and Elizabeth Whitney.
He attended HarvardUniversity and received a B.A.
degree in 1908, an M.D. in 1912, and an honorary
degreeof Sc.D. in 1928. In 1915 he was appointed
assistant in medicine at the HarvardMedical School
and the Massachusetts General Hospital. From 1928
to 1948 he was professor of medicine at Harvard and
director of the Thorndike Memorial Laboratory,
Boston City Hospital.
Earlier research revealed that anemia in dogs,
induced by excessive bleeding, is reversed by a diet of
raw liver. In 1926 Minot and William MURPHYfound
that ingestion of a half pound of raw liver a day dra-
matically reversed pernicious anemia in human beings.
He received (with George WHIPPLEand William Mur-
phy) the Nobel Prize for physiology or medicine in
1934 for the introduction of a raw-liver diet in the
treatment of pernicious anemia, which up to that time
was almost always a fatal disease. Today, 10,000 lives a
year are saved in the United States alone because of this
discovery.
With chemist Edwin Cohn, they prepared liver
extracts that, when taken orally, constituted the prima-
rytreatment for pernicious anemia until 1948, when
vitamin B was discovered. Minot died on February 25,
1950, in Brookline, Massachusetts.
Miocene A geological age that extends from the end
of the Oligocene epoch (22.5 million years ago) to the
beginning of the Pliocene epoch (5 million years ago).
See alsoGEOLOGICAL TIME.missense mutation One of four types of point muta-
tions. A point mutation is when a triplet of three
nucleotides (codon) has the base sequence permanently
changed. A missense mutation is when a change in the
base sequence converts a codon from one amino acid
to a codon for a different amino acid. The other three
point mutations are nonsense (codon for a specific
amino acid is converted to a chain-terminating codon),
silent (converts a codon for an amino acid to another
codon that specifies the same amino acid), and
frameshift (nucleotide is deleted or added to the coding
portion of a gene) mutations.Mississippian age The first of the two geologic ages
of the Carboniferous period, extending from about 345
to 310 million years ago. The Pennsylvanian is the sec-
ond (310 to 280 million years ago).
See alsoGEOLOGICAL TIME.mites Mites and ticks belong to the order Acari and
are the most diverse and abundant of all arachnids.
Very small in size, usually less than a millimeter
in length, they are ubiquitous, found in almost
everypart of the world, and account for 30,000
species in at least 50 families. Many are parasitic and
cause disease.mitochondria Cytoplasmic organelles of most
eukaryotic cells, they are surrounded by a double mem-
brane and produce ADENOSINE5’-TRIPHOSPHATEas use-
ful energy for the cell by oxidative PHOSPHORYLATION.
The proteins for the ATP-generating electron transport
ofthe respiration chain are located in the inner mito-
chondrial membrane. Mitochondria contain many
ENZYMEs of the citric acid cycle and for fatty acid ß-
oxidation. They also contain DNA, which encodes
some of their proteins, the remainder being encoded by
nuclear DNA.
See alsoCYTOPLASM;EUKARYOTES.222 minimum dynamic area