Encyclopedia of Biology

35,000 living species, and about half that number in
the fossil record. Most gastropods travel by using a
large flattened muscular foot and are univalve (one-
piece shell), although a few have no shell. Gastropods
have a defined head with a mouth and one or a pair of
sensory tentacles. Examples of gastropods are snails
and slugs.

gastrovascular cavity A body cavity in certain
lower invertebrates such as cnidarians and flatworms
that serves for both digestion and circulation. It has a
single opening that serves as both mouth and anus.
Since lower invertebrates do not have a circulatory sys-
tem, it also functions to distribute nutrients to cells that
line the cavity.

gastrula An animal embryo in an early stage of
development, between blastula and embryonic axes,
forming the characteristic three cell layers of endoderm,
mesoderm, and ectoderm, and which will give rise to
all of the major tissue systems of the adult animal.
See alsoEMBRYO.

gastrulation The rearrangement of the germ layers
by the blastoderm during animal development to the
new positions in the embryo that will produce the three
primary germ layers of endoderm, mesoderm, and ecto-
derm.

gated ion channel A specific ion channel that opens
and closes to allow the cell to alter its membrane
potential. An ion channel is a membrane protein that
forms an aqueous pore so that charged ions can cross
through the membrane. There are several types of ion
channels. For example, a ligand-gated ion channel is
where gating is controlled by binding of a chemical sig-
nal (the ligand) to a specific binding site on the channel
protein. Other ion channels are voltage gated and
mechanically gated.

gel electrophoresis The analytical laboratory pro-
cess to separate molecules according to their size. The
sample is put on an end of a slab of polymer gel, a

lyophilic colloid that has coagulated to a jelly. An elec-

tric field is applied through the gel, which separates the

molecules; small molecules pass easily and move

toward the other end faster than larger ones. Eventual-

ly all sizes get sorted, since molecules with similar elec-

tric charges and density will migrate together at the

same rate. There are several types of gel composition,

and various chemicals can be added to help separation.

gene Structurally, a basic unit of hereditary material;

an ordered SEQUENCEof NUCLEOTIDEbases that encodes

one polypeptide chain (via mRNA). The gene includes,

however, regions preceding and following the coding

region (leader and trailer) as well as (in EUKARYOTES)

intervening sequences (INTRONs) between individual

gene 139

Exon Intron Exon

Gene

The gene is the functional and physical unit of heredity passed

from parent to offspring. Genes are pieces of DNA, and most

genes contain the information for making a specific protein.

(Courtesy of Darryl Leja, NHGRI, National Institutes of Health)