polypeptide   Linear polymer of amino acids. Proteins are large polypeptides, and the two terms can be used interchangeably. (Panel 3–1, pp. 112–113) polypeptide backbone Repeating sequence of atoms along the core of the polypeptide chain.
polyribosome mRNA engaged with multiple ribosomes in the act of translation.
polytene chromosome   Giant chromosome in which the DNA has undergone repeated replication and the many copies have stayed together in precise alignment. (Figures 4–50 and 4–51) porin   Channel-forming proteins of the outer membranes of bacteria, mitochondria, and chloroplasts.
position effect variegation Alteration in gene expression resulting from change in the position of the gene in relation to other chromosomal domains, especially heterochromatic domains. When an active gene is placed next to heterochromatin, the inactivating influence of the heterochromatin can spread to affect the gene to a variable degree, giving rise to position effect variegation. (Figure 4–31) positional value   A cell’s internal record of its positional information in a multicellular organism; an intrinsic character that differs according to a cell’s location.
positive feedback   Control mechanism whereby the end product of a reaction or pathway stimulates its own production or activation.
positive selection   In immunology: process of thymocyte maturation in which thymocytes expressing a T cell receptor with appropriate affinity for a self peptide bound to a self MHC protein is signaled to survive and continue development.
post-transcriptional controls   Any control on gene expression that is exerted at a stage after transcription has begun. (Figure 7–54) post-translational Occurring after completion of translation.
preprophase band   Circumferential band of microtubules and actin filaments that forms around a plant cell under the plasma membrane prior to mitosis and cell division. (Figure 17–49) prereplicative complex (preRC) Multiprotein complex that is assembled at origins of replication during late mitosis and early G1 phases of the cell cycle; a prerequisite to license the assembly of a preinitiation complex, and the subsequent initiation of DNA replication. (Figures 17–17 and 17–18) primary cell wall   The first cell wall produced by a developing plant cell; it is thin and flexible, allowing room for cell growth. (Figure 19–63) primary cilium Short, single, nonmotile cilium lacking dynein that arises from a centriole and projects from the surface of many animal cell types. Some signaling proteins are concentrated in the primary cilium. (Figure 15–38) primary Ig repertoire The billions of IgM and IgD immunoglobulin molecules made by the B cells of an adaptive immune system in the absence of antigen stimulation.
primary immune response   Adaptive immune response to an antigen that is made on first encounter with that antigen. (Figure 24–16) primary pathogens Pathogens that can cause overt disease in most healthy people. Some cause acute, life-threatening epidemic infections and spread rapidly between hosts; other potential primary pathogens may persistently infect a single individual for years without causing overt disease, the host often being unaware that they are infected.
primary structure Linear sequence of monomer units in a polymer, such as the amino acid sequence of a protein.
primary tumor   Tumor at the original site at which a cancer first arose. Secondary tumors develop elsewhere by metastasis.
prion disease   Transmissible spongiform encephalopathy— such as Kuru and Creutzfeldt–Jakob disease (CJD) in humans, scrapie in sheep, and bovine spongiform encephalopathy (BSE, or “mad cow disease”) in cows—that is caused and transmitted by an infectious, abnormally folded protein (prion). (Figure 3–33) pro-inflammatory cytokine Any cytokine that stimulates an inflammatory response.