GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND HEREDITY OF TRAITS

19.1. PROTEIN SYNTHESIS AND THE GENETIC CODE

19.1.4. The genetic code

Each Gene is represented by a specific region of a DNA molecule. The specific information contained within a gene is determined by the sequence

of nitrogenous bases in the DNA chain. The "alphabet" used to record this DNA information consists of four "letters"—bases: A for adenine, G for guanine, T for thymine, and C for cytosine. In mRNA, thymine is replaced by uracil—U.

The Specificity of enzyme Proteins, whose synthesis is controlled by genes, is determined by the Amino Acid Sequence in polypeptide chains. A specific code serves to translate The nucleotide sequence into The amino acid sequence. Each amino acid is specified by a group of three adjacent NUCLEOTIDES, known as a triplet (or codon). Why do exactly three nucleotides encode a single amino acid? DNA contains four types of bases, whereas proteins consist of 20 different Amino Acids. If the code were single-letter, it would only be possible to encode four amino acids instead of 20. A two-letter code is also insufficient, as it can only encode 4² - 16 amino acids. However, a three-letter code (4³ - 64) can easily encode all 20 amino acids with plenty of room to spare. The triplet nature of METABOLISM/28.html">The Genetic Code was proven in F. Crick's laboratories. The genetic code is degenerate, meaning that a single amino acid residue can be encoded by multiple different codons. Moreover, this degeneracy is uneven: Arginine, for example, is encoded by six codons, whereas Methionine is encoded by only one. In 1966, through the concerted efforts of scientists worldwide, the decoding of the genetic code was completed (Table 19.1). It was established that 61 codons correspond to the 20 amino acids. Certain codons perform specific regulatory Functions, acting as "start" or "stop" signals for the polypeptide chain. The genetic code is universal across All living organisms: a bacterium, a tiger, and a carnation all use the exact same codons for the same amino acid residues.