Genetic code point mutations

Mutations are often classified according to the effect they have on the structure of the gene s protein product. This change in protein structure can be predicted using the genetic code table in conjunction with the base sequence of DNA or mRNA. A variety of such mutations is listed in Table 1-4-1. Point mutations and frameshifts are illustrated in more detail in Figure 1-4-2. 

Table 1-4-3. Important Points About the Genetic Code, Mutations, and Dranstation...

It is indispensable for any formal genetic analysis to be able to isolate mutants defective in genes coding for important cellular functions. In addition to the spontaneous mutations mentioned above, as caused by ISH elements, several protocols of induced mutagenesis were introduced. The most efficient method so far is the use of the alkylating agent ethyl methane sulfonate to cause point mutations... 

FRAMESHIFT MUTATION A gene mutation that occurs by the addition or deletion of one or a few base pairs and causes a shift in the reading frame of the genetic code, thereby altering the message encoded by all DNA base pairs that are read after the point of the mutation. 

Mutagens affect DNA in different ways. Some mutagens, such as nitrogen mustard, bind to a base and cause it to make a different amino acid. These mutagens cause point mutations, because they change the genetic code at one point, so changing a protein s amino acid sequence. 

Point mutation— A change in the genetic code at one point, which changes a protein s amino acid sequence. 

Figure 14.5. The standard genetic code minimizes the effects of point mutation better than the vast majority of alternative plausible genetic codes that could assign 20 amino acids to 64 codons in synonymous codon blocks (adapted from Freeland and Hurst 1998 [94]).

Some mutations are silent mutations that is, they cause no change in the protein. Often, however, a mutation has a negative effect on the health of the organism. The effect of a mutation depends on how it alters the genetic code for a protein. Consider the two codons for glutamic acid GAA and GAG. A point mutation that alters the third nucleotide of GAA to GAG will still result in the incorporation of glutamic acid at the correct position in the protein. Similarly, a GAG to GAA mutation will also be silent. 

Proteins are composed of chains of amino acids. In the genetic code of deoxyribonucleic acid (DNA), a codon or three-base sequence codes for the placement of each amino acid for example, the codon UUU places phenylalanine at that location in the protein and replacement of the third base with adenine results in the placement of leucine instead of phenylalanine. If a portion of the original code read. . . UUUACG. . . , deleting one of the uridine bases would cause that portion of the code to read. . . UUACG. .. the sequence UUA would then specify leucine. A point mutation changing one base might result in the formation of a different protein. 

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