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proteins that increase the rate of gene transcription n is to say when the gene is expressed. Prokaryotic genes are usually controlled by operon systems, relatively simple systems that tie expression directly to metabolic activity in the cell. Eukaryotic genes are controlled by more complex regulatory systems that respond to hormones, growth factors, internal conditions, and many other influences.

To ensure that each gene is expressed when, and only when, it is needed, each eukaryotic gene has several control regions, termed the promoter and enhancer regions. These do not code for amino acids but are critical for proper gene expression. Mutations in these regions often change the rate at which a gene is expressed, or the factors in the cell or the environment to which it responds.

The promoter region is a sequence of 20 to 200 nucleotides "upstream" of the coding region to which the RNA polymerase enzyme binds, permitting it to begin transcribing the DNA. Promoters differ in size and sequence in prokaryotic and eukaryotic genes. Promoters attract RNA polymerase by transcription factors first binding a variety of other proteins, called transcription factors. In some eukaryotic genes, promoter sites also occur within the coding region, allowing alternative transcripts with fewer exons.

Enhancers, also called activation sites, are located either nearby or far away from the promoter. Because DNA is looped and coiled, however, these sites are actually physically close to the gene's promoter even when distant on the DNA strand. Enhancers are gene-specific, and attract a variety of transcription factors. All of these work together to increase the rate of transcription by increasing the likelihood of RNA polymerase binding. Controlling the availability of these proteins is an important factor in regulating expression of the gene. see also Chromosome, Eukaryotic; Chromosome, Prokaryotic; Crick, Francis; DNA; Evolution of Genes; Gene Expression: Overview of Control; Gene Families; Genetic Code; Mendel, Gregor; Morgan, Thomas Hunt; Muscular Dystrophy; Mutation; Nature of the Gene, History; Nucleotide; Operon; Proteins; RNA Polymerases; RNA Processing; Transcription; Transcription Factors; Watson, James.

Elof Carlson

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Science, 2002.

Carlson, Elof. The Gene: A Critical History. Philadelphia, PA: Saunders Publishing, 1966.

Muller, H. J. "The Development of the Gene Theory." In Genetics in the Twentieth Century, L. C. Dunn, ed. New York: Macmillan, 1951.

Olby, Robert. The Path to the Double Helix. Seattle, WA: University of Washington Press, 1974.

phenotype observable characteristics of an organism

Gene and Environment

Questions of "nature versus nurture" have been asked of most human traits: Is it our genes, inherited from our parents, that make us the way we are, or is it the environment in which we live? A phenotype is a trait that can be observed and described in a population. Although some phenotypes may be

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totally controlled by genetic or environmental factors, most are influenced by a complex combination of the two. Genes and environmental factors may work independently, or they may interact with one another to cause the phenotype.

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