Phage (bacteriophage) is a bacterial virus.
Phase variation describes an alternation in the type of flagella produced by a bacterium.
Phenotype is the appearance or other characteristics of an organism, resulting from the interaction of its genetic constitution with the environment.
Phosphatase is an enzyme that removes phosphate groups from substrates.
Plasma membrane is the continuous membrane defining the boundary of every cell.
Plasmid is an autonomous self-replicating extrachromosomal circular DNA.
Playback experiment describes the retrieval of DNA that has hybridized with RNA to check that it is nonrepetitive by a further reassociation reaction.
Plectonemic winding describes the intertwining of the two strands in the classical double helix of DNA.
Pleiotropic gene affects more than one (apparently unrelated) characteristic of the phenotype.
Ploidy refers to the number of copies of the chromosome set present in a cell; a haploid has one copy, a diploid has two copies, etc.
Point mutations are changes involving single base pairs.
Polarity refers to the effect of a mutation in one gene in influencing the expression (at transcription or translation) of subsequent genes in the same transcription unit.
Polyadenylation is tha addition of a sequence of polyadenylic acid to the 3’ end of a eukaryotic RNA after its transcription.
Polycistronic mRNA includes conding regions representing more than one gene.
Polymorphism refers to the simultaneous occurrence in the population of genomes showing allelic variations (as seen either in alleles producing different phenotypes or—for example—in changes in DNA affecting the restriction pattern.)
Polyploid cell has more than two sets of the haploid genome.
Polyprotein is a gene product that is cleaved into several independent proteins.
Polysome (polyribosome) is an mRNA associated with a series of ribosomes engaged in translation.
Polytene chromosomes are generated by successive replications of a chromosome set without separation of the replicas.
Position effect refers to a change in the expression of a gene brought about by its translocation to a new site in the genome; for example, a previously active gene may become inactive if placed near heterochromatin.
Kpositive regulator proteins are required for the activation of a transcription unit.
Positive supercoiling describes the coiling of the double helix in space in the same direction as the winding of the two strands of the double helix itself.
Postmeitotic segregation describes the segregation of two strands of a duplex DNA that bear different information (created by heteroduplex formation during meiosis) when a subsequent replication allows the strands to separate.
Primary cells are eukaryotic cels taken into culture directly from the animal.
Primary transcript is the original unmodified RNA product corresponding to a transcription unit.
Primer is a short sequence (often of RNA) that is paired with one strand of DNA and provides a free 3’-OH end at which a DNA polymerase starts synthesis of a deoxyrigonucleotide chain.
Primosome describes the complex of proteins involved in the priming action that initiates synthesis of each Okazaki fragment during discontinuous DNA replication; the primosome may move alon DNA to engage in successive priming events.
Procentriole is an immature centriole, formed in the vicinity of a mature centriole.
Processed pseudogene is an inactive gene copy that lacks introns, contrasted with the interrupted structure of the active gene. Such genes presumably originate by reverse transcription of mRNA and insertion of a duplex copy into the genome.
Processive enzymes continue to act on a particular substrate, that is, do not dissociate between repetitions of the catalytic event.
Prokaryotic organisms (bacteria ) lack nuclei.
Promoter is a region of DNA involved in binding of RNA polymerase to initiate transcription.
-10 sequence is the consensus sequence TATAATG centered about 10 bp before the startpoint of a bacterial gene. It is involed in the initial melting of DNA by RNA polymerase.
-35 sequence is the consensus sequence centered about 35 bp before the startpoint of a bacterial gene. It is involved in initial recognition by RNA polymerase.
Proofreading refers to any mechanism for correcting errors in protein or nuclei acid synthesis that involves scrutiny of individual units after they have been added to the chain.
Prophage is a phage genome covalently integrated as a linear part of the bacterial chromosome.
Proteolytic reactions comprise the hydrolysis of peptide bonds in protein.
proto-oncogenes are the normal counterparts in the eukaryotic genome to the oncogenes carried by some retroviruses. They are given names of the form c-onc.
Provirus is a duplex DNA sequence in the eukaryotic chromosome corresponding to the genome of an RNA retrovirus.
Pseudogenes are inactive but stable components of the genome derived by mutaion of an ancestral active gene.
Puff is an expansion of a band of a polytene chromosome associated with the synthesis of RNA at some locus in the band.
Pulse-chase experiment are performed by incubating cells very briefly with a radioactively labeled precursor (of some pathway or macromolecule); then the fate of the label is followed during a subsequent incubation with a nonlabeled precursor.

Quaternary structure of a protein refers to its multimeric constitution.
Quick-stop dna mutants of E. coli cease replication immediately when the temperature is increased to 42℃.

R loop is the structure formed when an RNA strand hybridizes with its complementary strand in a DNA duplex, thereby displacing the original strand of DNA in the form of a loop extending over the region of hybridization.
Rapid lysis (r) mutants display a change in the pattern of lysis of E. coli at the end of an infection by a T-even phage.
Reading frame is one of three possible ways of reading a nucleotide sequences as a series of triplets.
Reassociation of DNA describes the pairing of complementary single strands of torm a double helix.
Rec A is the product of the recA locus of E. coli; a protein with dual activities, activating proteases and also able to exchange single strands of DNA molecules. The protease-activating activity controls the SOS response; the nucleic acid handling facility is involved in recombinationrepair pathways.
Receptor is a transmembrane protein, located in the plasma membrane, that binds a ligand in a domain on the extracellular side, and as a result has a change in activity of the cytoplasmic domain. (The same term is sometimes used also for the steroid receptors which are transcription factors that are cativated by binding ligands that a steroids or other small molecules.)
Recesssive allele is obscured in the phenotype of a heterozygote by the dominant allele, often due to inactivity or absence of the product of the recessive allele.
Recessive lethal is an allele that is lethal when the cell is homozygous for it.
Reciprocal recombination is the production of new genotypes with the reverse arrangements of alleles according to maternal and paternal origin.
Reciprocal translocation exchanges part of one chromosome with part of another chromosome.
Recombinant progeny have a different genotype from that of either partne.
Recombinant joint is the point at which two recombining molecules of duplex DNA are connected (the edge of the heteroduplex region).
Recombination nodules (nodes) are dense objects present on the synaptonemal complex; could be involved in crossing-over.
Recombination-repair is a mode of filling a gap in one strand of duplex DNA by retrieving a homlogous single strand from another duplex.
Regulatory gene codes for an RNA or protein product whose function is to control the expression of other genes.
Relaxed mutants of E. coli do not display the stringent response to starvation for amino acids (or other nutritional deprivation).
Relaxed replication control refers to the ability of some plasmids to continue replicatin after bacteria cease dividing.
Release (termination) factors respond to termination codons to cause release of the completed polypeptide chain and the ribosme from mRNA.
Renaturation is the reassociaition of deatured complementary single strands of a DNA double helix.
Repeating unit in a tandem cluster is the length of the sequence that is repeated ;appears circular on a restriction map.
Repetition frequency is the (integral) number of copies of a given sequence present in the haploid genome; equals 1 for nonrepetitive DNA, >2 for repetitive DNA.
Repetitive DNA behaves in a reassociation reaction as though many (related or identical) sequences are present in a component, allowing any pair of complementary sequences to reassociate.
Replacement sites in a gene are those at which mutations alter the amino acid that is coded.
Replication-defective virus has lost one or more genes essential for completing the infective cycle.
Replication eye is region in which DNA has been replicated within a longer, unreplicated region.
Replication fork is the point at which strands of parental duplex DNA are separated so that replication can procedd.
Replicative transpostition describes the movement of a transposon by a mechanisim in which first it is replicated, and then one copy is transferred to a new site.
Replicon is a unit of the genome in which DNA is replicated; contains an origin for intitiation of replication.
Replisome is the multiprotin structure that assemble at the bacterial replicating fork to undertake synthesis of DNA. Contains DNA polymerase and other enzymes.