Microbiology

Microbiology Study Guide

©2018 of 132 In the case of frameshift mutation, the sequence of amino acids in proteins alters completely by the addition or deletion of the pair of nucleotides from the molecule of DNA. Spontaneous mutation represents mutation that takes place naturally, without inducing any intervention in the laboratory. On the contrary, induced mutation is possible through an intervention in the laboratory. Selection of Mutants Detecting the mutant is done through selecting or invoking a test for the alterations in the phenotype. The tests are generally undertaken with the use of bacteria due to their capability to reproduce and grow in very large numbers. Another property of bacteria that makes them a good medium for mutant tests is that every bacterial cell has only one copy of each gene, so the outcome is not biased. The selection of mutants can be direct or indirect. In the case of direct selection, the un-mutated parent cells are eliminated and the mutated cells are accepted and detected. In contrast, indirect selection takes place by looking for mutations in other kinds of genes by choosing the cells that are incapable of performing specific operations. Mutagens Mutagens are the physical and chemical agents that make mutations possible. Nitrous acid is a common mutagen that helps interrupt the genetic code by converting adenine to hypoxanthine. Another chemical mutagen is a base analog, which is similar to the nitrogenous base and becomes a part of the DNA molecule by error. This molecule of DNA is not effective in the synthesis of the proteins. X-rays, gamma rays, and ultraviolet light comprise the class of physical mutagens. The covalent bonds in the DNA molecules are broken into fragments by the x-rays and gamma rays. In contrast, dimers are formed by combining the adjacent bases of thymine with the help of ultraviolet light. This leads to an interruption of the genetic code. Photoreactivation is the technique used to repair the damages, with the help of the enzymes of bacteria, caused by the radiations. The mutation rate is computed by finding the probability of the mutation happening at the division of the cell. It has been found that the mutation rate in bacteria is one in a billion—that is, out of every one billion cells, one is mutant. Recombination The DNA of the recipient can be altered by different bacterial recombinations; they are transformation, transduction, and conjugation. Transformations Discovered in 1928 by Frederick Griffith, transformation was successful when the live bacteria pneumococci were combined with fragments of dead pathogenic pneumococci; the harmless bacteria took the genes of the dead fragments and became pathogenic. This proved the capability of bacteria Achieve Page 32