Genome Project - Primer on molecular genetics - 1992, страница 8

An electric current is passed through amedium containing the mixture, and each kind of molecule travels through the medium ata different rate, depending on its electrical charge and size. Separation is based on thesedifferences. Agarose and acrylamide gels are the media commonly used for electrophoresis of proteins and nucleic acids.34Endonuclease: An enzyme that cleaves its nucleic acid substrate at internal sites in thenucleotide sequence.Enzyme: A protein that acts as a catalyst, speeding the rate at which a biochemicalreaction proceeds but not altering the direction or nature of the reaction.EST: Expressed sequence tag. See sequence tagged site.Eukaryote: Cell or organism with membrane-bound, structurally discrete nucleus andother well-developed subcellular compartments.

Eukaryotes include all organisms exceptviruses, bacteria, and blue-green algae. Compare prokaryote. See chromosomes.Evolutionarily conserved: See conserved sequence.Exogenous DNA: DNA originating outside an organism.Exons: The protein-coding DNA sequences of a gene. Compare introns.Exonuclease: An enzyme that cleaves nucleotides sequentially from free ends of a linearnucleic acid substrate.Expressed gene: See gene expression.FISH (fluorescence in situ hybridization): A physical mapping approach that usesfluorescein tags to detect hybridization of probes with metaphase chromosomes and withthe less-condensed somatic interphase chromatin.Flow cytometry: Analysis of biological material by detection of the light-absorbing orfluorescing properties of cells or subcellular fractions (i.e., chromosomes) passing in anarrow stream through a laser beam.

An absorbance or fluorescence profile of the sampleis produced. Automated sorting devices, used to fractionate samples, sort successivedroplets of the analyzed stream into different fractions depending on the fluorescenceemitted by each droplet.Flow karyotyping: Use of flow cytometry to analyze and/or separate chromosomes onthe basis of their DNA content.Gamete: Mature male or female reproductive cell (sperm or ovum) with a haploid set ofchromosomes (23 for humans).Gene: The fundamental physical and functional unit of heredity. A gene is an orderedsequence of nucleotides located in a particular position on a particular chromosome thatencodes a specific functional product (i.e., a protein or RNA molecule).

See gene expression.Gene expression: The process by which a gene’s coded information is converted into thestructures present and operating in the cell. Expressed genes include those that aretranscribed into mRNA and then translated into protein and those that are transcribed intoRNA but not translated into protein (e.g., transfer and ribosomal RNAs).35GlossaryGene families: Groups of closely related genes that make similar products.Gene library: See genomic library.Gene mapping: Determination of the relative positions of genes on a DNA molecule(chromosome or plasmid) and of the distance, in linkage units or physical units, betweenthem.Gene product: The biochemical material, either RNA or protein, resulting from expressionof a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease-causing alleles.Genetic code: The sequence of nucleotides, coded in triplets ( codons) along the mRNA,that determines the sequence of amino acids in protein synthesis.

The DNA sequence ofa gene can be used to predict the mRNA sequence, and the genetic code can in turn beused to predict the amino acid sequence.Genetic engineering technologies: See recombinant DNA technologies.Genetic map: See linkage map.Genetic material: See genome.Genetics: The study of the patterns of inheritance of specific traits.Genome: All the genetic material in the chromosomes of a particular organism; its size isgenerally given as its total number of base pairs.Genome projects: Research and technology development efforts aimed at mapping andsequencing some or all of the genome of human beings and other organisms.Genomic library: A collection of clones made from a set of randomly generated overlapping DNA fragments representing the entire genome of an organism. Compare library,arrayed library.Guanine (G): A nitrogenous base, one member of the base pair G-C (guanine andcytosine).Haploid: A single set of chromosomes (half the full set of genetic material), present in theegg and sperm cells of animals and in the egg and pollen cells of plants.

Human beingshave 23 chromosomes in their reproductive cells. Compare diploid.Heterozygosity: The presence of different alleles at one or more loci on homologouschromosomes.Homeobox: A short stretch of nucleotides whose base sequence is virtually identical inall the genes that contain it. It has been found in many organisms from fruit flies to humanbeings. In the fruit fly, a homeobox appears to determine when particular groups of genesare expressed during development.36Homologies: Similarities in DNA or protein sequences between individuals of the samespecies or among different species.Homologous chromosomes: A pair of chromosomes containing the same linear genesequences, each derived from one parent.Human gene therapy: Insertion of normal DNA directly into cells to correct a geneticdefect.Human Genome Initiative: Collective name for several projects begun in 1986 by DOEto (1) create an ordered set of DNA segments from known chromosomal locations, (2)develop new computational methods for analyzing genetic map and DNA sequence data,and (3) develop new techniques and instruments for detecting and analyzing DNA.

ThisDOE initiative is now known as the Human Genome Program. The national effort, led byDOE and NIH, is known as the Human Genome Project.Hybridization: The process of joining two complementary strands of DNA or one each ofDNA and RNA to form a double-stranded molecule.Informatics: The study of the application of computer and statistical techniques to themanagement of information. In genome projects, informatics includes the development ofmethods to search databases quickly, to analyze DNA sequence information, and topredict protein sequence and structure from DNA sequence data.In situ hybridization: Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.Interphase: The period in the cell cycle when DNA is replicated in the nucleus; followedby mitosis.Introns: The DNA base sequences interrupting the protein-coding sequences of a gene;these sequences are transcribed into RNA but are cut out of the message before it istranslated into protein.

Compare exons .In vitro: Outside a living organism.Karyotype: A photomicrograph of an individual’s chromosomes arranged in a standardformat showing the number, size, and shape of each chromosome type; used in lowresolution physical mapping to correlate gross chromosomal abnormalities with thecharacteristics of specific diseases.kb: See kilobase.Kilobase (kb): Unit of length for DNA fragments equal to 1000 nucleotides.Library: An unordered collection of clones (i.e., cloned DNA from a particular organism),whose relationship to each other can be established by physical mapping. Comparegenomic library, arrayed library.37GlossaryLinkage: The proximity of two or more markers (e.g., genes, RFLP markers) on a chromosome; the closer together the markers are, the lower the probability that they will beseparated during DNA repair or replication processes (binary fission in prokaryotes,mitosis or meiosis in eukaryotes), and hence the greater the probability that they will beinherited together.Linkage map: A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together.

Distance is measured incentimorgans (cM).Localize: Determination of the original position ( locus) of a gene or other marker on achromosome.Locus (pl. loci): The position on a chromosome of a gene or other chromosome marker;also, the DNA at that position. The use of locus is sometimes restricted to mean regionsof DNA that are expressed. See gene expression.Macrorestriction map: Map depicting the order of and distance between sites at whichrestriction enzymes cleave chromosomes.Mapping: See gene mapping, linkage map, physical map.Marker: An identifiable physical location on a chromosome (e.g., restriction enzymecutting site, gene) whose inheritance can be monitored.

Markers can be expressedregions of DNA (genes) or some segment of DNA with no known coding function butwhose pattern of inheritance can be determined. See RFLP, restriction fragment lengthpolymorphism.Mb: See megabase.Megabase (Mb): Unit of length for DNA fragments equal to 1 million nucleotides androughly equal to 1 cM.Meiosis: The process of two consecutive cell divisions in the diploid progenitors of sexcells. Meiosis results in four rather than two daughter cells, each with a haploid set ofchromosomes.Messenger RNA (mRNA): RNA that serves as a template for protein synthesis. Seegenetic code.Metaphase: A stage in mitosis or meiosis during which the chromosomes are alignedalong the equatorial plane of the cell.Mitosis: The process of nuclear division in cells that produces daughter cells that aregenetically identical to each other and to the parent cell.mRNA: See messenger RNA.Multifactorial or multigenic disorders: See polygenic disorders.38Multiplexing: A sequencing approach that uses several pooled samples simultaneously,greatly increasing sequencing speed.Mutation: Any heritable change in DNA sequence.