Point deleterious mutation

The realization that one could screen or select beneficial point mutations or recombinations and discard deleterious ones, thus mimicking sexual recombination, spawned the field of directed evolution. 

A breakthrough was achieved by recognizing that the process of natural selection can be harnessed to evolve effective enzymes in artificial circumstances. In directed evolution the processes of natural evolution are accelerated in a test tube in order to select proteins with the desired properties. The realization that one could screen or select beneficial point mutations or recombinations and discard deleterious ones, thus mimicking sexual recombination, spawned the field of directed evolution. 

Specific. Each codon is a signal for a specific amino acid. The majority of codons that code for the same amino acid possess similar sequences. For example, in each of the four serine codons (UCU, UCC, UCA, and UCG) the first and second bases are identical. Consequently, a point mutation in the third base of a serine codon would not be deleterious. 

In this picture of cytochrome evolution, point mutations occur at random along the DNA which codes for the amino acid sequence, and the protein produced from this mutated DNA is tested in the organism for its ability to operate as a functioning cytochrome. If the molecule is impaired, then the mutation is deleterious, and if the molecule is ineffective, the mutation is lethal. We never see these lethal mutations in the sequence record because the unfortunate carriers of them are weeded out. Residues such as histidine-18 and methionine-80 are absolutely essential. 

As explained (section V, A, 2), only DLs induced in postmeiotic stages of male germ cells are free of selective elimination prior to ferilization. Point mutations would be much less likely to have a deleterious effect on the cells in which they arose, and therefore would not be subject to such selection. The failure to recover DL mutation from premeiotic cells would not therefore preclude the production of point mutation in such cells. 

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