Word: amino
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Long before the three new Nobel laureates began their experiments, scientists had learned that the message of heredity is carried by large molecules of deoxyribonucleic acid (DNA) in the chromosomes. Researchers had deduced that somehow DNA directs the cells to assemble amino acids into the proteins that form the basic structural material of all living beings and impart their characteristics. Then, in 1953, James Watson (author of The Double Helix] and Francis Crick put together more of the puzzle; they discovered that DNA consists of twin helices that are held together by regularly spaced links similar to the stairs...
...chemical compounds called nucleotides. There are only four different kinds of nucleotides in DNA, but the order in which they appear along the length of the helix varies considerably, suggesting that they are arranged in a coded sequence. To be able to call up one of the 20 different amino acids using only four nucleotide "letters," scientists decided, each genetic code "word" has to be three letters long. But how to break the code...
...Nirenberg, then an obscure young scientist at the National Institutes of Health, provided the biological Rosetta stone. After synthesizing a single helix with half-stairs that were the equivalent of only one of DNA's nucleotides-adenine (A)-he added it to a solution containing all 20 amino acids. Only one protein was produced in the solution. It consisted entirely of a chain of amino-acid molecules called phenylalanine. Thus, Nirenberg concluded, a three-letter code word made up of adenine nucleotides (AAA) was nature's instruction to the cell to use phenylalanine in building a protein...
Punched-Tape Message. Nirenberg refined his technique and began to match other three-letter combinations of nucleotides with particular amino acids. The task was also taken up independently by Khorana at the University of Wisconsin. Other scientists pitched in, and by 1965 the genetic code had been largely deciphered. Khorana was also able to determine that each of the three-letter words is always read separately and does not share any of its letters with another word. The words are read off continuously along a strand of DNA, much as a punched-tape message is read by a teletype machine...
...dopamine, a natural body chemical essential to normal nerve activity in the midbrain. So, researchers reasoned, why not give the patients extra dopamine? The trouble is that dopamine cannot cross the natural barrier be tween the bloodstream and the brain to reach the deprived cells. But dopa, an amino acid that comes in three forms including L-dopa, crosses the barrier by a process not yet fully understood. It is broken down in the brain to yield the desired dopamine...