Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA: Structure of Chromosomes and Genes
Genes are regions of DNA that encode polypeptide chains and RNA

Let us now turn to the most functionally significant parts of DNA molecules—the genes. In the classical biological sense, a Gene was defined as a segment of a chromosome responsible for a specific phenotypic trait of an Organism, such as eye color. (The word “phenotype” means “appearance”.) Later on, a molecular Definition of the gene emerged, first proposed by George Beadle and Edward Tatum in 1940. They exposed spores of the fungus Neurospora crassa to X-rays and other agents that damage DNA and thereby induce Mutations. Some of the mutants they obtained lacked a specific enzyme, disrupting their METABOLISM (Sec. 13.14). This observation led the authors to conclude that a gene is a piece of genetic material that determines, or codes for, a single enzyme (the “one gene - one enzyme” hypothesis). Subsequently, this definition became more generalized as “one gene - one protein”, since some genes encode Proteins that are not Enzymes. Today, however, an even more precise biochemical definition of a gene can be given.

Recall that many proteins consist of multiple polypeptide chains (Sec. 8.8). In some such proteins, all polypeptide chains are identical, in which case they may all be encoded by the same gene. However, other proteins contain two or more polypeptide chains of different types that vary in their Amino Acid Sequence. The Hemoglobin molecule, for instance, consists of Two Types of polypeptide chains—$\alpha$ and $\beta$—which differ in length and amino acid sequence. Today we know that the $\alpha$- and $\beta$-chains are encoded by two different genes. Therefore, the relationship between a gene and a protein is more accurately expressed by the definition: “one gene - one polypeptide”.

However, not all genes are ultimately expressed to produce polypeptide chains. Some genes encode various types of tRNA, while other genes are responsible for the synthesis of various rRNAs. Genes that encode Polypeptides and RNAs are called structural genes because they determine The Structure of a particular final gene product—an enzyme or a stable RNA molecule. DNA also contains other regions or sequences that perform an exclusively regulatory function. Some of these regulatory regions act as signals indicating the start and end of structural genes; others are involved in initiating or terminating the Transcription of structural genes. Thus, alongside structural genes, the chromosome also contains regulatory sequences.