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P1 vector A cloning vector based on the P1 bacteriophage, used for cloning relatively large fragments of DNA in E.coli.

Pacemaker  Neurons that set rhythm of biological clocks independent of external cues, and any nerve or muscle cell that has an inherent autorhythmicity and determines activity pattern of other cells.

Palindromic sequence  Nucleotide sequence that is identical to its complementary sequence when each is read in the same direction (e.g., 5’-3’). Many sites recognized by restriction enzymes are palindromes.

Parkinson’s disease  Disease characterized by tremor, rigidity, and delay in initiation of movement due in part to deficit of dopamine in basal ganglia.

Passive diffusion (simple diffusion)  Movement of a molecule across a membrane at a rate proportional to its concentration gradient across the membrane and the permeability of the membrane. Only water, gases (O2, CO2, N2), small hydrophobic molecules, and small uncharged polar molecules such as urea and ethanol can move across biomembranes by simple diffusion. Ref. Active transport and facilitated diffusion.

Papillomaviruses A group of mammalian viruses, derivatives of which have been used as cloning vector.

Paracrine  The action of a cytokine on a cell distinct from that which produced it.

Partial digestion Treatment of a DNA molecule with a restriction endonuclease under such conditions that only a fraction of all the recognition sites are cleaved.

Passage The transfer or subculture of cells from one culture vessel to another. Usually, but not necessarily, implies subdivision of a proliferating cell population enabling propagation of a cell line or cell strain.

Passage number The number of times a culture has been subcultured.

Pathogen  An organism which causes disease.

PCR (Polymerase chain reaction)  Technique for amplifying a specific DNA segment in a complex mixture by multiple cycles of DNA synthesis from short oligonucleotide primers followed by brief heat treatment to separate the complementary strands.

Perforin  Protein secreted by cytotoxic T cells that forms channels in plasma membrane of target cell, which destroys it.

Peritoneum  Membrane lining abdominal and pelvic cavities and covering organs there.

Peroxisome  Small organelle in eukaryotic cells whose functions include degradation of fatty acids and amino acids by means of reactions that generate hydrogen peroxide, which is converted to water and oxygen by catalase.

Phasic  Intermittent.

PFC  = Plaque-forming cell.

Phagocytosis  The process by which cells engulf material and enclose it within a vacuole (phagosome) in the cytoplasm.

Phenotype The aggregate of all the expressed properties of a cell, being the product of the interaction of the genotype with the regulatory environment.

Phenotypic expression A technique designed to maximize the transformation frequency obtained when using a plasmid vector.

Phosphodiester bond  A set of covalent bonds in which two hydroxyl groups form ester linkages to the same phosphate group. Adjacent nucleotides in DNA and RNA are joined by phosphodiester bonds.

Phospholipid bilayer  A symmetrical two-layer structure formed by phospholipids in aqueous solution; the basic structure of all biomembranes. In a bilayer, the polar head groups are exposed to the aqueous medium, while the nonpolar hydrocarbon chains of the fatty acids are in the center.

Phylogeny  The evolutionary history of an organism or group of organisms; often represented in chart form as a phylogenetic “tree”.

Pilus One of the structures present on the surface of a bacterium containing a conjugative plasmid, through which DNA is assumed to pass during conjugation.

Plaque A zone of clearing on a lawn of bacteria caused by lysis of the cells by infecting phage particle.

Plaque-forming cell (PFC)  An antibody-producing cell detected in vitro by its ability to lyse antigen-sensitized erythrocytes in the presence of complement.

Plasma cell  An antibody-producing B cell which has reached the end of its differentiation pathway.

Plasmid A usually circular piece of DNA, primarily independent of the host chromosome, often found in bacterial and some other types of cells.

Plasmid amplification A method involving incubation with an inhibitor of protein synthesis aimed at increasing the copy number of certain types of plasmid in a bacterial culture.

Plasmin  Proteolytic enzyme able to decompose fibrin and thereby to dissolve blood clots.

Plasminogen activator (PA)  Any plasma protein that activates proenzyme plasminogen to plasmin

Plateau  ref. Growth curve

Plating efficiency The percentage of cells seeded at subculture giving rise to colonies. If each colony can say to be derived from one cell, this is synonymous with cloning efficiency. Sometimes used loosely to describe the number of cells surviving after subculture, but this is better termed the “seeding efficiency”

Pluripotent  Referring to a single population of bone marrow stem cell capable of giving rise to multiple types of differentiated cells.

Poikilothermic Body Temperature close to that of the environment and not regulated by metabolism.

Point Mutation  Change of a single nucleotide in DNA, especially in a region coding for protein.

Polyethylene glycol A polymeric compound used to precipitate macromolecules and molecular aggregates.

Polymerase chain reaction  = PCR.

Polypeptide  Linear polymer of amino acids connected by peptide bonds. Proteins are large polypeptides, and the two terms commonly are used interchangeably.

Polyribosome (Polysome)   A complex containing several ribosomes all translating a single messenger RNA.

Population density The number of monolayer cells per unit area of substrate. For cells growing in suspension, this term is identical to the cell concentration.

Population doubling time The interval required for a cell population to double at the middle of the logarithmic phase of growth.

Positional effect refers to the variations in expression levels observed for genes inserted at different positions in a genome.

Primary culture A culture started from cells, tissue, or organs taken directly from an organism and before the first subculture.

Primary lymphoid tissues  Lymphoid organs in which lymphocytes complete their initial maturation steps; they include the fetal liver, adult bone marrow and thymus, and bursa of Fabricius in birds.

Primer A short single-stranded oligonucleotide which, when attached by base-pairing to a single-stranded template molecule, acts as the start point for complementary strand synthesis directed by a DNA polymerase enzyme.

Primary structure  In proteins, the linear arrangement (sequence) of amino acids and the location of covalent (mostly disulfide) bonds within a polypeptide chain.

Probe  Defined RNA or DNA fragment, radioactively or chemically labeled, that is used to locate specific nucleic acid sequences by hybridization.

Prokaryote  Class of organisms, including the eubacteria and archaebacteria, that lack a true membrane-limited nucleus and other organelles.

Prolactin  Peptide hormone secreted by anterior pituitary that stimulates milk secretion by mammary glands.

Prolactin inhibiting hormone (PIH)  Dopamine, which serves as a hypothalamic and posterior pituitary hormone to inhibit prolactin secretion by anterior pituitary.

Promoter the nucleotide sequence, upstream of a gene, that acts as a signal for RNA polymerase binding.

Prophage The integrated form of the DNA molecule of a lysogenic phage.

Prophase  Earliest stage in mitosis during which the chromosomes condense and the centrioles begin moving toward the spindle poles.

Protease An enzyme that degrades protein.

Proteasomes  Organelles which degrade cellular proteins that have been tagged for breakdown by ubiquitination.

Protein A A protein from the bacterium Staphylococcus aureus that binds specifically to immunoglobulin G (i.e. antibody) molecules.

Protein C  Plasma protein that inhibits clotting.

Protein engineering A collection of techniques, including but not exclusively gene mutagenesis, that result in directed alterations being made to protein molecules, often to improve the properties of enzymes used in industrial processes.

Protein kinase C  Enzyme that phosphorylates certain intracellular proteins when activated by diacylglycerol.

Proteoglycan  A group of glycoproteins that contain a core protein to which is attached one or more glycosaminoglycans. They are found in nearly all extracellular matrices, and some are attached to the plasma membrane.

Proton-motive force  The energy equivalent of the proton concentration gradient and electric potential gradient across a membrane. In chloroplasts and mitochondria, these gradients are generated by electron transport, maintained by the thylakoid or inner mitochondrial membrane, and drive ATP synthesis by ATP synthase.

Proto-oncogene  A normal cellular gene that encodes a protein usually involved in regulation of cell growth or proliferation and that can be mutated into a cancer-promoting oncogene, either by changing the protein-coding segment or by altering the regulation of the protein.

Protoplast A cell from which the cell wall has been completely removed.

Pseudoalleles  Tandem variants of an gene; they do not occupy a homologous position on the chromosome (e.g. C4)

Pseudodiploid Numerically diploid chromosome number but with chromosomal aberrations.

Puberty  Attainment of sexual maturity when conception becomes possible. As commonly used, refers to 3 to 5 years of sexual development that culminates in sexual maturity.

Pulse-chase  A type of experiment in which a radioactive small molecule is added to a cell for a brief period (the pulse) during which it is incorporated into macromolecules. The fate of the newly-synthesized, radioactive, molecules is examined during the subsequent period when the radioactive small molecule is removed from the cell medium and an excess of the same small molecule, but unlabeled, is added (the chase).

Purine base  Double-ring, nitrogen-containing subunit of nucleotide. Adenine or guanine.

Pyrimidine base  Single-ring, nitrogen-containing subunit of nucleotide. Cytosine, thymine, or uracil.

Pyruvic acid  Three-carbon intermediate in glycolysis that, in absence of oxygen, forms lactic acid or, in presence of oxygen, enters Krebs cycle. Anionic form, pyruvate is  formed when pyruvic acid loses a hydrogen ion.

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