Plasmid: Autonomously replicating, extrachromosomal circular DNA molecules. 



distinct from the normal bacterial genome and nonessential for cell survival under 

 nonselective conditions. Some plasmids are capable of integrating into the host 

 genome. A number of artificially constructed plasmids are used as cloning vectors. 



Polygenic disorders: Genetic disorders resulting from the combined action of alleles 

 of more than one gene (e.g., heart disease, diabetes, and some cancers). Although 

 such disorders are inherited, they depend on the simultaneous presence of several 

 alleles, thus the hereditary patterns are usually more complex than those of 

 single-gene disorders. Compare sini;le-f>eiu' disorders. 



Polymerase, DNA or RNA: Enzymes that catalyze the synthesis of nucleic acids on 

 preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA 

 from deoxyribonucleotides. 



Poly.nerase chain reaction (PCR): A method for amplifying a DNA sequence using 

 the Klenovv fragment of £. coli DNA polymerase I and two 20-base primers, one 

 complementary to the (+)-strand at one end of the sequence to be amplified and 

 the other complementary to the (-)-strand at the other end. Because the newly 

 synthesized DNA strands can subsequently serve as additional templates for the 

 same primer sequences, successive rounds of primer annealing, strand elongation 

 and dissociation produce rapid and highly specific amplification of the desired 

 sequence. PCR also can be used to detect the existence of the defined sequence in a 

 DNA sample. 



Polymorphism: Difference in DNA sequence among individuals. Genetic variations 

 occurring in more than I percent of a population would be considered useful 

 polymorphisms for genetic linkage analysis. Compare nuitation. 



Primer: Short preexisting polynucleotide chain to which new deoxyribonucleotides can 

 be added by DNA polymerase. 



Probe: Single-stranded DNA or RNA molecules of specific sequence, labeled either 

 radioactively or immunologically, that are used to detect the complementary base 

 sequence by hybridization. 



Prokaryote: Cell or organism lacking membrane-bound, structurally discrete nucleus 

 and subcellular compartments. Bacteria are prokaryotes. Compare eiikaryote. See 

 chromosome. 



Promoter: A site on DNA to which RNA polymerase will bind and initiate 

 transcription. 



Protein: A large molecule composed of one or more chains of amino acids in a 

 specific sequence; the sequence is determined by the sequence of nucleotides in 



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