Principles of Microbiology 1st Edition PDF Free Download
Genetic determinants have been shown to consist of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Enzymes are synthesized under the direction of nucleic acids, and enzymes, thus synthesized, are responsible for essentially all processes that occur in microorganisms. Experiments with. both bacteria (pneumococci) and viruses (radioisotope tests) have established roles of nucleic acids as determinants in microbial variations. Microorganisms are excellent for genetic studies, primarily because of haploid conditions in many of them and the ability of observers to detect mutations easily and test very large populations in short time periods. Permanent changes in bacteria can be differentiated from induced temporary types by fluctuation and replica plate techniques. Changes in DNA structure are termed mutations, and they occur in genes of chromosomes. Each gene may have a multiplicity of mutable sites. Since codons code for amino acids, a missense mutation occurs when a codon that codes for a new amino acid replaces a codon of a DNA chain, and a nonsense mutation results from replacement with a codon that codes for no amino acid. An altered nucleotide sequence may be restored to its original order by a reverse mutation, or an altered nucleotide sequence may be compensated for through a suppressor mutation. Either the DNA structure or the manner in which RNA is read may be altered.
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