ommatidium Asingle unit, or visual section, of a
compound eye such as that found in insects. It resem-
bles a single simple eye that descends directly into the
eye core. Each ommatidium contains a corneal lens,
crystalline cone, and pigment and retinula cells and
functions as a single eye sending an image to the brain.
The insect’s brain must take all the messages from each
ommatidia and piece the image together.
omnivore An animal that eats both plant and animal
material. Humans are omnivores.
See alsoCARNIVORE.
oncogene Anormal cellular gene that, when inap-
propriately expressed or mutated, can transform
eukaryotic cells into tumor cells. A gene that controls
cell growth but also is responsible for directing the
uncontrolled growth of tumor or cancer if it is dam-
aged by means such as an inheritance defect, mutation,
or environmental exposure such as to carcinogens.
There are dozens of oncogenes known, and they func-
tion in a variety of ways, but their commonality is the
overexpression that interferes with the normal regula-
tion of cell growth.
ontogeny The life history of one individual and its
stages as it evolves from zygote to adult. The opposite
of phylogeny, which is the history of a group.
oogamy A process such as the fusion or union
between a small motile or flagellated sperm (gamete)
and large nonmotile egg (gamete).
oogenesis The process and development of producing a
female ovum; developing a diploid cell into a haploid egg.
open circulatory system Atype of circulatory sys-
tem where the internal transport of blood flows
through the body cavity and bathes the organs directly
and not through a system of vessels. Humans have a
closed circulatory system, which is a type of circulatory
system where the blood flows through a system of ves-
sels and the heart. Examples include horseshoe crabs,
lobsters, and insects.operant conditioning A type of associative learning
behavior also called trial-and-error learning, or instru-
mental conditioning. It is a method to modify behavior
(an operant) that utilizes contingencies between the
response and the presentation of the reinforcer. Based
on the 1938 experiments of Burrhus Friederich Skinner
(rats pressing lever for food) and published in his book
The Behavior of Organisms.operon A functional unit consisting of a PROMOTER,
an operator, and a number of structural GENEs, found
mainly in PROKARYOTEs. An example is the operon NIF.
The structural genes commonly code for several func-
tionally related ENZYMEs, and although they are tran-
scribed as one (polycistronic) mRNA, each has its
separate TRANSLATION initiation site. In the typical
operon, the operator region acts as a controlling ele-
ment in switching on or off the synthesis of mRNA.
Also refers to a group or sequence of closely linked
genes that function as a unit in synthesizing enzymes
needed for biosynthesis of a molecule and that is con-
trolled by operator and repressor genes; common in
bacteria and phages. An operator gene is the region of
the chromosome, next to the operon, where a repressor
protein binds to prevent transcription of the operon.
The repressor gene protein binds to an operator adja-
cent to the structural gene, preventing the transcription
of the operon.opsonization The modification of the surface of a
bacterium by coating or deposition of an opsonin (an
antibody or complement), which is a group of proteins
that lyse organisms. An opsonin coats a bacterium in
order to enhance its ability to be eaten (phagocytosis) by
macrophages and other leukocytes; an immune response.optically detected magnetic resonance(ODMR)
A double-resonance technique in which transitions
between spin sublevels are detected by optical means.
Usually these are sublevels of a triplet, and the transi-
tions are induced by microwaves.254 ommatidium