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APPENDIX A Glossary The following have been taken directly or modified from definitions in A Dictionary of Genetics, 3rd edition, by Robert C. King and William D. Stansfield, Oxford University Press, New York, 1985, with permission from the publisher. Allele One of a series of possible alternative forms of a given gene, differing in DNA sequence and affecting the functioning of a single product (RNA and/or protein). cDNA Complementary DNA produced from a RNA template by the action of RNA-dependent DNA polymerase (reverse transcrip- tase). Chromosome (1) In prokaryotes, the circular DNA molecule con- taining the entire set of genetic instructions essential for life of the cell. (2) In the eukaryotic nucleus, one of the threadlike structures consisting of chromatin (DNA plus associated protein) and carrying genetic information arranged in linear sequence. Clone (1) A group of genetically identical cells or organisms all descended from a single common ancestral cell or organism by mitosis in eukaryotes or by binary fission in prokaryotes. (2) Genetically engineered replicas of DNA sequences. Codon The nucleotide triplet in messenger RNA that specifies the amino acid to be inserted in a specific position in the forming polypeptide during translation. Cosmid Vectors designed for cloning large fragments of eukaryotic DNA. Crossing over The exchange of genetic material between homologous chromosomes. 105

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06 APPENDIX A Cytogenetics The science that combines the methods and findings of cytology and genetics. Electrophoresis The movement of the charged molecules in solution in an electrical field. The solution is generally held in a porous support medium, such as a gel made of agarose or polyacrylamide. Electrophoresis is generally used to separate molecules from a mixture on the basis of `differences in net electrical charge and also by size or geometry of the molecules, in a manner that depends on the characteristics of the gel matrix. Exon A portion of split gene that is included in the transcript of a gene and that survives splicing of the RNA in the cell nucleus to become part of a messenger RNA or a structural RNA in the cell cytoplasm. Gene A hereditary unit that, in the classical sense, occupies a specific position Focus) within the genome or chromosome; a unit that has one or more specific effects upon the phenotype of the organism; a unit that can mutate to various allelic forms; a unit that codes for a single protein or functional RNA molecule. Intron in split genes a segment that is transcribed into nuclear RNA, but is subsequently removed from within the transcript by RNA splicing and rapidly degraded. Most genes in the nuclei of eukaryotes contain introns. Linkage map A chromosome map showing the relative positions of the known genes on the chromosomes of a given species, as determinecl by the inheritance of linked traits. Oligonucleotide A polymer made up of a few (between 2 and 20) nucleotides. Open reading frame Regions in a DNA molecule where successive nucleotide triplets can potentially be read as codons specifying amino acids and where the sequence of these triplets is not interrupted by stop codons. Polymorphism The existence of two or more genetically different classes in the same interbreeding population (Ah-positive and Rh- negative humans, for example). Recombination The occurrence of progeny with combinations of genes other than those that occurred in the parents as a result of independent assortment or crossing over. Restriction fragment length polymorphisms Variations occurring within a species in the length of DNA fragments generated by a specific endonuclease. Such variations are generated by mutations that create or abolish recognition sites for these enzymes. For example, restriction endonuclease mapping of human structural genes for beta hemoglobin chains has shown that parents with the sickle cell mutation produce abnormal restriction fragments. ...

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APPENDIX A 107 Reverse transcription DNA synthesis from an RNA template, me- diated by reverse transcriptase. Somatic cell Any cell of the eukaryotic body other than those destined to become sex cells. In diploid organisms, most somatic cells contain the IN number of chromosomes. Stop codon A ribonucleotide triplet signaling the termination of the translation of a protein chain. ..