Bacteria genetic code

The azo reductases in aerobic bacteria were found to be existent when azoreductases from obligate aerobic bacteria were isolated and characterized from strains K22 and KF46 and were shown to be flavin-free after purification, characterization, and comparison 364, 362,363. These intracellular azoreductases showed high specificity to dye structures. Furthermore, Blumel and Stolz cloned and characterized the genetic code of the aerobic azo reductase from Pagmentiphaga... 

Before the triplet nature of codons had been established, Crick and associates used frame-shift mutations in a clever way to demonstrate that the genetic code did consist of triplets of nucleotides.7 55/55a Consider what will happen if two strains of bacteria, each containing a frame-shift mutation (e.g., a -1 deletion), are mated. Genetic recombination can occur to yield mutants containing both of the frame-shift mutations. 

In mammalian cells, some 1% of the total cellular DNA is found in the mitochondria. This DNA is double stranded, circular, and small, with a molecular weight of about 10 million, which is in the same range as that of viral DNAs. Some four to ten molecules of DNA per mitochondrion, along with some ribosomes, are found in the matrix space. DNA replication, transcription, and synthesis of some mitochondrial proteins take place in the matrix space. This protein synthesis very much resembles that of bacteria. The mitochondrial genetic code differs from the "universal" genetic code (Chapter 12) used for nuclearly encoded proteins and bacteria. The reasons for this are unknown. 

The code appears to be universal to the extent that synthetic polyribonucleotides seem to code in the same way in mammals, bacteria, and other species, and components of the protein-synthesizing system of various species can operate with components from certain others. Mitochondria and ciliated protozoa have genetic codes slightly different from the standard. For example, in human mitochondria, UGA codes for tryptophan instead of as a termination signal, AUA codes for methionine rather than isoleucine, and AGA and AGG are termination signals rather than coding for arginine. 

As with the establishment of the genetic code and information on the molecular mechanism of DNA replication (Chap. 16), the present detailed knowledge of the mechanism of DNA transcription to produce RNA rests largely upon studies with bacteria, particularly E. coli. It is convenient to treat transcription in bacteria first. 

The genetic code is universal. All biological systems (or nearly all) use the same code. This principle has important consequences to the developments in rDNA. Thus, the code in messenger RNAs of a human cell can be translated by bacterial protein synthesis machinery into a protein of the same sequence as in the human cell. Production of human proteins in bacteria and other organisms would be immensely difficult, if not impossible, if each organism had a different genetic code. 

Second, microbial chemical transformations are accomplished by means of enzymes, proteins that act as catalysts. Catalysts bind with reactants and hold them in such an orientation that they more readily react. The products of the reaction are then released, leaving the catalyst ready to facilitate another transformation. (It is possible for an enzyme to be destroyed if a chemical mimics the proper substrate sufficiently to bind, but fails to react and subsequently release from the enzyme.) Because each enzyme is produced in response to a section of the genetic code (DNA) in the organism and many enzymes are extremely specific, it is possible that some strains of a species of bacteria may accomplish a certain chemical transformation while other individuals cannot. By using modern techniques of molecular biology, scientists can insert specific biotransformation capabilities into bacteria by means of genetic transfer. This procedure is easiest if the genetic material is associated with plasmids, which are small circular molecules of DNA that can exist independently within a bacterial cell. 

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