The UGA codon also codes for selenocysteine. Selenoproteins have UGA codons in the middle of their coding regions. In the absence of selenium, protein synthesis stops prematurely in these genes.
to drastic in terms of their effects on phenotype. This will be discussed in more detail in later chapters.
An interesting observation about the genetic code is that, with limited exceptions, the repertoire of amino acids is limited to 20 in all organisms, regardless of growing environments. Thermophilic and cryophilic organisms adapt to growth at 100oC and freezing temperatures, respectively, not by using structurally different amino acids but by varying the combinations of the naturally occurring amino acids. As will be discussed in the next section, cells have strict control and editing systems to protect the genetic code and avoid incorporation of unnatural amino acids into proteins. Recent studies have shown that it is possible to manipulate the genetic code to incorporate modified amino acids.13,14 The ability to introduce chemically or physically reactive sites into proteins in vivo has significant implications in biotechnology.
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