occluding junction Type of cell junction that seals cells together in an
epithelium, forming a barrier through which even small molecules cannot pass. Okazaki fragments Short lengths of DNA produced on the lagging strand
during DNA replication. They are rapidly joined by DNA ligase to form a
continuous DNA strand. oligodendrocyte Type of glial cell in the vertebrate central nervous system
that forms a myelin sheath around axons. oligomer Short polymer, usually consisting (in a cell) of amino acids
(oligopeptides), sugars (oligosaccharides), or nucleotides (oligonucleotides).
(From Greek oligos, few, little.) oligosaccharide Short linear or branched chain of covalently linked sugars O-linked glycosylation Addition of an oligosaccharide chain to a protein
through the OH group of a serine or threonine side chain. 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. oocyte The developing egg. It is usually a large and immobile cell. oogenesis Formation and maturation of oocytes in the ovary. operator Short region of DNA in a bacterial chromosome that controls the
transcription of an adjacent gene. operon In a bacterial chromosome, a group of contiguous genes that are
transcribed into a single mRNA molecule. ORC see origin recognition complex organelle Membrane-enclosed compartment in a eucaryotic cell that has a
distinct structure, macromolecular composition, and function. Examples are
nucleus, mitochondrion, chloroplast, Golgi apparatus. Organizer see Spemann's Organizer origin recognition complex (ORC) Large protein complex that is bound to
the DNA at origins of replication in eucaryotic chromosomes throughout the
cell cycle. osmolarity A term used to describe the concentration of a solute in terms of
the osmotic pressure it can exert. osmosis Net movement of water molecules across a semipermeable membrane
driven by a difference in concentration of solute on either side. The membrane
must be permeable to water but not to the solute molecules. osteoblast Cell that secretes matrix of bone. osteoclast Macrophage-like cell that erodes bone, enabling it to be remodeled
during growth and in response to stresses throughout life. outer membrane Outermost of the two membranes surrounding an organelle;
the membrane adjacent to the cytosol. outer nuclear membrane The outermost of the two nuclear membranes. It is
continuous with the endoplasmic reticulum and is studded with ribosomes on
its cytosolic face. overlap microtubule In the mitotic or meiotic spindle, a microtubule
interdigitating at the equator with the microtubules emanating from the other
pole. ovulation Release of an egg from the ovary. ovum see egg 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. (2 43.) oxidative phosphorylation Process in bacteria and mitochondria in which
ATP formation is driven by the transfer of electrons from food molecules to
molecular oxygen. Involves the intermediate generation of a proton gradient
(pH gradient) across a membrane and chemiosmotic coupling.
