urogenital structures; and the hypomere, which differ-
entiate into limbs, peritoneum, gonads, heart, blood
vessels, and mesenteries.
mesophyll Plant tissue that forms the inner or mid-
dle cells of the leaf and lies between the upper and
lower epidermis. It contains two types of chlorenchyma
cells (cells that contain chlorophyll): the long and verti-
cally arranged palisade cells, the upper layer, which are
on top of the round and loosely packed spongy cells in
the lower layer, where most of the gas exchange occurs
and where photosystem II is most active. It is in both
the palisade and spongy cells that photosynthesis takes
place by way of chloroplasts.
mesotrophic lake Any lake with a moderate nutri-
ent supply.
Mesozoic era Ageological time that extends from
the end of the Paleozoic era (230 million years ago) to
the beginning of the Cenozoic era (about 65 million
years ago). Subdivided into the Triassic, Jurassic, and
Cretaceous periods, which includes the age of the
dinosaurs.
See alsoGEOLOGICAL TIME.
messenger RNA (mRNA) An RNA molecule that
transfers the coding information for protein synthesis
from the chromosomes to the ribosomes. Fragments of
ribonucleic acid serve as templates for protein
synthesis by carrying genetic information from a strand
of DNA to ribosomes for translation into a protein.
The information from a particular gene or group of
genes is transferred from a strand of DNA by con-
structing a complementary strand of RNA through
transcription. Transfer RNA (tRNA), composed of
three nucleotide segments attached to specific amino
acids, correctly match with a template strand of
mRNA, lining up the correct order of amino acids and
bonding them, via translation in the ribosome with
rRNA (ribosomal RNA), to form a protein.
See also DEOXYRIBONUCLEIC ACID; RIBONUCLEIC
ACID.
met- A qualifying prefix indicating the oxidized form
of the parent protein, e.g., methemoglobin.metabolism The entire physical and chemical pro-
cesses involved in the maintenance and reproduction of
life in which nutrients are broken down to generate
energy and to give simpler molecules (CATABOLISM),
which by themselves may be used to form more com-
plex molecules (ANABOLISM).
In the case of heterotrophic organisms, the energy
evolving from catabolic processes is made available for
use by the organism.
In medicinal chemistry, the term metabolismrefers
to the biotransformation of xenobiotics and particular-
ly DRUGs.
See alsoBIOTRANSFORMATION;XENOBIOTIC.metabolite Any intermediate or product resulting
from METABOLISM.metalloenzyme An ENZYMEthat, in the active state,
contains one or more metal ions that are essential for
its biological function.metallo-immunoassay A technique in which
ANTIGEN-ANTIBODY recognition is used, with attach-
ment of a metal ion or metal complex to the antibody.
The specific absorption or (radioactive) emission of the
metal is then used as a probe for the location of the
recognition sites.
See alsoIMAGING;RADIONUCLIDE.metallothionein Asmall, cysteine-rich protein that
binds heavy metal ions, such as zinc, cadmium, and
copper in the form of CLUSTERs.metamorphosis The change and reorganization of
the tissues and body shape during the development
of an animal from larva to adult. Metamorphosis can
be complete or incomplete, i.e., lacking a pupal stage.
Simple or gradual metamorphosis is an incompletemetamorphosis 217