Gene Theory
Gene Theory
Gene Concept
The term “gene” (Gr. genos, descent) was coined by W. Johannsen in 1909 to refer to the hereditary factors of Mendel. Initially, they were regarded as indivisible units of the chromosomes on which they were located. Later studies with multiple mutant alleles demonstrated that alleles are in fact divisible by recombination; that is, portions of a gene are separable. Furthermore, parts of many genes in eukaryotes are separated by sections of DNA that do not specify a part of the finished product (introns).
As the chief unit of genetic information, genes encode products essential for specifying the basic architecture of every cell, nature and life of the cell, specific protein syntheses, enzyme formation, self-reproduction of the cell, and, directly or indirectly, the entire metabolic function of the cell. Because of their ability to mutate, to be assorted and shuffled in different combinations, genes have become the basis for our modern interpretation of evolution. Genes are units of molecular information that can maintain their identities for many generations, can be selfduplicated in each generation, and can control processes by allowing their specificities to be copied.
One Gene–One Enzyme Hypothesis
Since genes act to produce different phenotypes, we may infer that their action follows the scheme: gene → gene product → phenotypic expression. Furthermore, we may suspect that the gene product is usually a protein, because proteins act as enzymes, antibodies, hormones, and structural elements throughout the body.
The first clear, well-documented study to link genes and enzymes was carried out on the common bread mold Neurospora by Beadle and Tatum in the early 1940s. This organism was ideally suited to a study of gene function for several reasons: these molds are much simpler to handle than fruit flies, they grow readily in well-defined chemical media, and they are haploid organisms that are consequently unencumbered with dominance relationships. Furthermore, mutations were readily induced by irradiation with ultraviolet light. Ultraviolet-light-induced mutants, grown and tested in specific nutrient media, had single-gene mutations that were inherited in accord with Mendelian principles of segregation. Each mutant strain was defective in one enzyme, which prevented that strain from synthesizing one or more complex molecules. Putting it another way, the ability to synthesize a particular molecule was controlled by a single gene.
From these experiments Beadle and Tatum set forth an important and exciting formulation: one gene produces one enzyme. For this work they were awarded the Nobel Prize in 1958. The new hypothesis was soon validated by the research of others who studied other biosynthetic pathways. Hundreds of inherited disorders, including dozens of human hereditary diseases, are caused by single mutant genes that result in the loss of a specific enzyme. We now know that a particular protein may be made of several chains of amino acids (polypeptides), each of which may be specified by a different gene, and not all proteins specified by genes are enzymes (for example, structural proteins, antibodies, transport proteins, and hormones). Furthermore, genes directing the synthesis of various kinds of RNA were not included in Beadle and Tatum’s formulation. Therefore a gene now may be defined more inclusively as a nucleic acid sequence (usually DNA) that encodes a functional polypeptide or RNA sequence.
Gene Concept
The term “gene” (Gr. genos, descent) was coined by W. Johannsen in 1909 to refer to the hereditary factors of Mendel. Initially, they were regarded as indivisible units of the chromosomes on which they were located. Later studies with multiple mutant alleles demonstrated that alleles are in fact divisible by recombination; that is, portions of a gene are separable. Furthermore, parts of many genes in eukaryotes are separated by sections of DNA that do not specify a part of the finished product (introns).
As the chief unit of genetic information, genes encode products essential for specifying the basic architecture of every cell, nature and life of the cell, specific protein syntheses, enzyme formation, self-reproduction of the cell, and, directly or indirectly, the entire metabolic function of the cell. Because of their ability to mutate, to be assorted and shuffled in different combinations, genes have become the basis for our modern interpretation of evolution. Genes are units of molecular information that can maintain their identities for many generations, can be selfduplicated in each generation, and can control processes by allowing their specificities to be copied.
One Gene–One Enzyme Hypothesis
Since genes act to produce different phenotypes, we may infer that their action follows the scheme: gene → gene product → phenotypic expression. Furthermore, we may suspect that the gene product is usually a protein, because proteins act as enzymes, antibodies, hormones, and structural elements throughout the body.
The first clear, well-documented study to link genes and enzymes was carried out on the common bread mold Neurospora by Beadle and Tatum in the early 1940s. This organism was ideally suited to a study of gene function for several reasons: these molds are much simpler to handle than fruit flies, they grow readily in well-defined chemical media, and they are haploid organisms that are consequently unencumbered with dominance relationships. Furthermore, mutations were readily induced by irradiation with ultraviolet light. Ultraviolet-light-induced mutants, grown and tested in specific nutrient media, had single-gene mutations that were inherited in accord with Mendelian principles of segregation. Each mutant strain was defective in one enzyme, which prevented that strain from synthesizing one or more complex molecules. Putting it another way, the ability to synthesize a particular molecule was controlled by a single gene.
From these experiments Beadle and Tatum set forth an important and exciting formulation: one gene produces one enzyme. For this work they were awarded the Nobel Prize in 1958. The new hypothesis was soon validated by the research of others who studied other biosynthetic pathways. Hundreds of inherited disorders, including dozens of human hereditary diseases, are caused by single mutant genes that result in the loss of a specific enzyme. We now know that a particular protein may be made of several chains of amino acids (polypeptides), each of which may be specified by a different gene, and not all proteins specified by genes are enzymes (for example, structural proteins, antibodies, transport proteins, and hormones). Furthermore, genes directing the synthesis of various kinds of RNA were not included in Beadle and Tatum’s formulation. Therefore a gene now may be defined more inclusively as a nucleic acid sequence (usually DNA) that encodes a functional polypeptide or RNA sequence.
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