oncogene   An altered gene whose product can act in a dominant fashion to help make a cell cancerous. Typically, an oncogene is a mutant form of a normal gene (proto-oncogene) involved in the control of cell growth or division. (Figure 20–17) open reading frame (ORF) A continuous nucleotide sequence free from stop codons in at least one of the three reading frames (and thus with the potential to code for protein).
opportunistic pathogens   Microbes of the normal flora that can cause disease only if the immune systems are weakened or if they gain access to a normally sterile part of the body.
optogenetics   Use of genetically engineered channelrhodopsin and other light-responsive ion channels and transporters to modulate neuron function and hence analyze the neurons and circuits underlying complex functions, including behaviors in whole animals. (Figure 11–32) organelle   Subcellular compartment or large macromolecular complex, often membrane-enclosed, that has a distinct structure, composition, and function. Examples are nucleus, nucleolus, mitochondrion, Golgi apparatus, and centrosomes. (Figure 1–25)
Organizer Specialized tissue at the dorsal lip of the blastopore in an amphibian embryo; a source of signals that help to orchestrate formation of the embryonic body axis.
origin recognition complex (ORC) Large protein complex that is bound to the DNA at origins of replication in eukaryotic chromosomes throughout the cell cycle. (Figure 5–31) orthologs   Genes or proteins from different species that are similar in sequence because they are descendants of the same gene in the last common ancestor of those species. Compare paralogs. (Figure 1–21) osteoblast   Cell that secretes matrix of bone. (Figure 22–14) osteoclast   Macrophage-like cell that erodes bone, enabling it to be remodeled during growth and in response to stresses throughout life. (Figure 22–16) osteocyte   Nondividing cell in bone that develops from an osteoblast and is embedded in bone matrix. (Figure 22–14) outer membrane Mitochondrial membrane that is in contact with the cytosol.
outer mitochondrial membrane Membrane that separates the organelle from the cytosol.
outer nuclear membrane One of two concentric membranes comprising the nuclear envelope; surrounds the inner nuclear membrane and is continuous with the inner nuclear membrane and the membrane of the endoplasmic reticulum.
OXA complex   Protein translocator in the inner mitochondrial membrane that mediates insertion of inner membrane proteins.
oxidation (verb oxidize) Loss of electrons from an atom, as occurs during the addition of oxygen to a molecule or when a hydrogen is removed. Opposite of reduction. (Figure 2–20) oxidative phosphorylation   Process in bacteria and mitochondria in which ATP formation is driven by the transfer of electrons through the electron transport chain to molecular oxygen. Involves the intermediate generation of a proton gradient (pH gradient) across a membrane and a chemiosmotic coupling of that gradient to the ATP synthase. (Figures 14–12) oxidative phosphorylation   Process in bacteria and mitochondria in which ATP formation is driven by the transfer of electrons through the electron-transport chain to molecular oxygen. Involves the intermediate generation of an electrochemical proton gradient across a membrane and a chemiosmotic coupling of that gradient to the ATP synthase. (Figure 14–10)