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PCR
Polymerase chain reaction. This is an in vitro technique for the selective amplification of defined nucleic acid regions in a DNA mixture by copying the complementary strands of a target DNA molecule simultaneously for a series of cycles until the desired amount is obtained. The principle: Primers are synthesised which have nucleotide sequences complementary to the DNA that flanks the target region. Then, the DNA is heated to separate the complementary DNA strands and cooled to allow the primers to bind to the flanking sequences. Heat-stable DNA (Taq) is added and the reaction allowed to proceed for a series of replication cyles. Twenty cycles will render an approximately millionfold amplification of the soruce DNA.
Penetrance
Defined as the ratio of individuals who have a disease-causing allele and manifest the disease versus those who have this disease causing allele but do not manifest the associated disease. Diseases are usually associated with either high penetrance or low penetrance alleles.
Peptide
Two or more amino acids are joined by a so-called peptide-bond.
Phenotype
Observable properties of an organism produced by the genotype in conjunction with the environment.
Photolithography
Selective masking generates light patterns that direct chemical transformations to specific areas of photosensitive surfaces. Photolithography usually requires expensive equipment and particular knowledge; the technology is protected by patents
Physical map
Map of the linear order of genes on a chromosome with units that indicate their distances determined by methods other than genetic recombination. These include e.g. nucleotide sequencing, electon micrographs of heteroduplex DNAs, etc.)
Piezoelectric
Becoming electrically polarised when subject to the mechanical stress; quartz for example produces an electric charge when squeezed.
Plasmid
Small circular DNA replicating independently of the chromosome in bacteria and unicellular eukaryotes (e.g. yeasts). Plasmids usually carry genes for e.g. antibiotic resistance, colicin production and are widely used in genetic engineering as vectors. Vectors are vehicles into which foreign genes are inserted for subsequent cloning or expression in bacterial cells.
Point mutation
In molecular genetics, a point mutation is caused by the substitution of one nucleotide for another. In classical genetics, point mutation refers to any mutation that is not associated with a cytologically detectable chromosomal aberration or one that has no effect on crossing over.
Polygeny
Most diseases are polygenic, i.e. caused by the interplay of several genes.
Polymorphism
In classical population genetics, polymorphism refers to the coexistance of different alleles in a population/species at significant frequencies (the most frequent allele should be less than 95%%). However, the term polymorphism is frequently used simply to indicate that genetic variation occurs within a certain part of the genome. In extreme, polymorphism can refer to just one nucleotide, e.g., SNP.
Primer walking
Sequencing method in which the sequence data of the sequenced section are exploited to synthesize the following primer. Starting from the first priming site this continues until the DNA template has been sequenced completely.
Principal component analysis
Visual and numerical analysis of collinearity among variables. Allows for the mapping of high-dimensional data to a lower dimension for visualisation, analysis and modelling.
Probe
In general, probe refers to any biochemical/nucleic acid/oligo etc. labelled with radioactive isotopes or tagged in other ways for identification. A probe is used to identify or isolate a gene, its product, or a protein. In microarrays, the probe is immobilised in regular arrangement on the substrate.
Protein arrays
Arrays consisting of proteins themselves or of probes used for capturing proteins.
Pseudogene
Stretch of DNA related in sequence to a funtional gene; it is however inactive as a result of the changes it has accumulated during evolution.
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