Word: amino
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Neat as it was, this scheme still left unanswered one more question: How could DNA or RNA choose from among 20 amino acids to produce complex proteins by using an informational system that had only four code letters?the four bases?at its disposal? An answer to this intriguing problem was suggested by Physicist George Gamow, who likened the four bases to the different suits in a deck of playing cards...
...dealt one at a time, disregarding the order of the cards within the suits, the player encounters only one of four possibilities on each draw (a heart, diamond, spade or club); clearly, if DNA's code worked this way, there would not be enough choices to encode 20 amino acids. If the cards are dealt in pairs, the number of combinations increases to 16 (since each card may combine with its own kind or one of three other suits). But such a two-unit system also would be inadequate. So Gamow reasoned that DNA's four bases...
...National Institutes of Health, to crack the code itself. That same year Nirenberg had succeeded in building up short, synthetic strands of RNA out of only one type of base. Invariably, this artificial RNA induced the manufacture of chains of proteins consisting of only one type of amino acid, phenylalanine. The conclusion was inescapable: in the genetic code, Nirenberg's triplet had to signify phenylalanine...
Using this clue as their Rosetta stone, Nirenberg and other researchers eventually found one or more three-letter code words, or codons, that could call up every single amino acid?plus other words that acted as punctuation, marking the start or completion of a message ordering the production of a protein. Even more remarkable, they learned that the code was universal: the same four letters, taken three at a time to form a single genetic word, code the same amino acids in all living things. Thus by the mid-1960s, scientists finally understood how DNA passes on genetic information with...
...majority of genetic stigmas have somewhat more subtle symptoms and occur when defective genes fail to order the production of essential enzymes that trigger the body's biochemical reactions. Phenylketonuria (PKU) is caused by the absence of the enzyme necessary for the metabolism of the amino acid phenylalanine; as a result, toxins accumulate in the body and eventually cause convulsions and brain damage. Cystic fibrosis, which causes abnormal secretion by certain glands and respiratory-tract blockage that can lead to death by pneumonia, is the most common inborn error of metabolism; it is believed to be caused by a deficiency...