Oil • A triglyceride that is liquid at room temperature. (Contrast with fat.)

Okazaki fragments • Newly formed DNA making up the lagging strand in DNA replication. DNA ligase links Okazaki fragments together to give a continuous strand.

Olfactory [L. olfacere: to smell] • Having to do with the sense of smell.

Oligomer [Gr.: oligo: a few + meros: units] • A compound molecule of intermediate size, made up of two to a few monomers. (Contrast with monomer, polymer.)

Oligosaccharins • Plant hormones, derived from the plant cell wall, that trigger defenses against pathogens.

Ommatidium [Gr. omma: eye] • One of the units which, collected into groups of up to 20,000, make up the compound eye of arthropods.

Omnivore [L. omnis: everything + vorare: to devour] • An organism that eats both animal and plant material. (Contrast with carnivore, detritivore, herbivore.)

Oncogene [Gr. onkos: mass, tumor + genes: born] • Genes that greatly stimulate cell division, giving rise to tumors.

Oocyte (oh´ eh site) [Gr. oon: egg + kytos: container] • The cell that gives rise to eggs in animals.

Oogenesis (oh´ eh jen e sis) [Gr. oon: egg + genesis: source] • Female gametogenesis, leading to production of the egg.

Oogonium (oh´ eh go´ nee um) • In some algae and fungi, a cell in which an egg is produced.

Operator • The region of an operon that acts as the binding site for the repressor.

Operon • A genetic unit of transcription, typically consisting of several structural genes that are transcribed together; the operon contains at least two control regions: the promoter and the operator.

Opportunity cost • The sum of the benefits an animal forfeits by not being able to perform some other behavior during the time when it is performing a given behavior.

Opsin (op´ sin) [Gr. opsis: sight] • The protein portion of the visual pigment rhodopsin. (See rhodopsin.)

Optic chiasm [Gr. chiasma: cross] • Structure on the lower surface of the vertebrate brain where the two optic nerves come together.

Optical isomers • Two isomers that are mirror images of one another.

Optimality models • Models developed to determine the structures or behaviors that best solve particular problems faced by organisms.

Ordovician • A period in the Paleozoic era in Earth's geologic history (490-443 mya).

Organ [Gk. organon: tool] • A body part, such as the heart, liver, brain, root, or leaf. Organs are composed of different tissues integrated to perform a distinct function. Organs are in turn often integrated into systems, such as the digestive or reproductive system.

Organ identity genes • Plant genes that specify the various parts of the flower. See homeotic genes.

Organ of Corti • Structure in the inner ear that transforms mechanical forces produced from pressure waves ("sound waves") into action potentials that are sensed as sound.

Organelles (or gan els´) • Organized structures that are found in or on cells. Examples: ribosomes, nuclei, mitochrondria, chloroplasts, cilia, and contractile vacuoles.

Organic • Pertaining to any aspect of living matter, e.g., to its evolution, structure, or chemistry. The term is also applied to any chemical compound that contains carbon.

Organism • Any living being.

Organizer • Region of an early embryo that directs the development of nearby regions. In amphibian early gastrulas, the dorsal lip of the blastopore is the organizer.

Organogenesis • The formation of organs and organ systems during development.

Origin of replication • A DNA sequence at which helicase unwinds the DNA double helix and DNA polymerase binds to initiate DNA replication.

Osmolarity • The concentration of osmotically active particles in a solution.

Osmoregulation • Regulation of the chemical composition of the body fluids of an organism.

Osmoreceptor • A neuron that converts changes in the osmotic potential of interstial fluids into action potentials.

Osmosis (oz mo´ sis) [Gr. osmos: to push] • The movement of water across a differentially permeable membrane, from one region to another region where the water potential is more negative.

Ossicle (oss´ ick ul) [L. os: bone] • The calcified construction unit of echinoderm skeletons.

Osteoblasts (oss´ tee oh blast) [Gk. osteon: bone + blastos: sprout] • Cells that lay down the protein matrix of bone.

Osteoclasts (oss´ tee oh clast) [Gk. osteon: bone + klastos: broken] • Cells that dissolve bone.

Otolith (oh´ tuh lith) [Gk. otikos: ear + lithos: stone[ Structures in the vertebrate vestibular apparatus that mechanically stimulate hair cells when the head moves or changes position.

Oval window • The flexible membrane that, when moved by the bones of the middle ear, produces pressure waves in the inner ear

Ovary (oh´ var ee) [L. ovum: egg] • Any female organ, in plants or animals, that produces an egg.

Oviduct [L. ovum: egg + ducere: to lead] • In mammals, the tube serving to transport eggs to the uterus or to outside of the body.

Oviparous (oh vip´ uh rus) • Reproduction in which eggs are released by the female and development is external to the mother's body. (Contrast with viviparous.)

Ovulation • The release of an egg from an ovary.

Ovule (oh´ vule) • In plants, a structure that contains a gametophyte and, within the gametophyte, an egg; when it matures, an ovule becomes a seed.

Ovum (oh´ vum) [L. ovum: egg] • The egg, the female sex cell.

Oxidation (ox i day´ shun) • Relative loss of electrons in a chemical reaction; either outright removal to form an ion, or the sharing of electrons with substances having a greater affinity for them, such as oxygen. Most oxidation, including biological ones, are associated with the liberation of energy. (Contrast with reduction.)

Oxidative phosphorylation • ATP formation in the mitochondrion, associated with flow of electrons through the respiratory chain.

Oxidizing agent • A substance that can accept electrons from another. The oxidizing agent becomes reduced; its partner becomes oxidized.