Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

O-GlcNAcylation The reversible, post-translational, covalent modification of an intracellular protein by the addition to the hydroxyl group of the side chain of a serine or threonine of the sugar N-acetylglucosamine (GlcNAc). O-GlcNAcylation can alter the activity of the modified protein.

Okazaki fragments Short (<1000 bases), single-stranded DNA fragments that are formed during synthesis of the lagging strand in DNA replication and are rapidly joined by DNA ligase to form a continuous DNA strand. (Figure 5-29)

oligopeptide A small to medium-size linear polymer composed of amino acids connected by peptide bonds. The terms peptide and oligopeptide are often used interchangeably.

O-linked oligosaccharide Oligosaccharide chain that is attached to the side-chain hydroxyl group in a serine or threonine residue in a glycoprotein. See also N-linked oligosaccharides.

oncogene A gene whose product is involved either in transforming cells in culture or in inducing cancer in animals. Generally is a mutant form of a normal gene (proto-oncogene) for a protein involved in the control of cell growth or division. (Figure 24-11)

oncogene addiction Describes the observation that some cancers, despite containing numerous genetic abnormalities, depend on only a few genetic alteration to maintain their malignant phenotype. It is said that these cancers are “addicted” to certain oncogenic mutations.

oncoprotein A protein encoded by an oncogene that causes abnormal cell proliferation; may be a mutant unregulated form of a normal protein or a normal protein that is produced in excess or in the wrong time or place in an organism.

oocyte The metazoan egg cell, containing one set of chromosomes from the maternal parent.

open reading frame (ORF) Region of sequenced DNA that is not interrupted by stop codons in one of the triplet reading frames. An ORF that begins with a start codon (usually AUG) and extends for 100 or more codons is called a long open reading frame and has a high probability of encoding a protein.

operator Short DNA sequence in a bacterial or bacteriophage genome that binds a repressor protein and controls transcription of an adjacent gene. (Figure 7-3)

G-19

operon In bacterial DNA, a cluster of contiguous genes transcribed from one promoter that gives rise to an mRNA containing coding sequences for multiple proteins. (Figure 5-13a)

optogenetics A technique in which channelrhodopsins are expressed in electrically excitable cells, allowing their membrane potential to be manipulated using light.

organelle Any membrane-limited subcellular structure found in eukaryotic cells. (Figures 1-12, 1-17, 1-19, and 1-20)

osmosis Net movement of water across a semipermeable membrane (permeable to water but not to solute) from a solution of lesser to one of greater solute concentration. (Figure 11-6)

outer mitochondrial membrane The smooth outside boundary of the mitochondrion.

oxidation Loss of electrons from an atom or molecule as occurs when a hydrogen atom is removed from a molecule or oxygen is added; opposite of reduction.

oxidation potential The voltage change when an atom or molecule loses an electron; a measure of the tendency of a molecule to loose an electron. For a given oxidation reaction, the oxidation potential has the same magnitude but opposite sign as the reduction potential for the reverse (reduction) reaction.

oxidative phosphorylation The phosphorylation of ADP to form ATP driven by the transfer of electrons to oxygen (O₂) in bacteria and mitochondria. Involves generation of a proton-motive force during electron transport and its subsequent use to power ATP synthesis.