Word: rna
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Scientists suspected that DNA had a helper, a single-stranded chemical first cousin called ribonucleic acid (RNA). Most of the cell's RNA is found in ribosomes. These are globular bodies in the material outside the cell's nucleus that seem to be highly active centers of protein synthesis. But if this ribosomal RNA played a role in protein making, how did it obtain and execute the instructions from the master molecule DNA inside the nucleus...
...after wrestling with the question, Francis Crick postulated (and Harvard Biochemists Paul Zamecnik and Mahlon Hoagland confirmed) a second form of RNA, which was later found to carry specific amino acids floating in the cytoplasm to the ribosomes; this substance became known as transfer RNA. Then in the early 1960s, biologists discovered a third kind of RNA?shortly after its existence had been theorized by Jacques Monod and François Jacob of France's Pasteur Institute. Called messenger RNA, it provided the missing piece in the molecular puzzle. It was formed on an uncoiled strip of DNA in the nucleus...
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...
...CRICK'S team at Cambridge proved Gamow's ingenious "triplet" theory. They demonstrated that RNA formed from only one or two base units could not effect the manufacture of proteins. But when they added a third base unit, protein formation began immediately. It remained, however, for an unknown young biochemist named Marshall Nirenberg, at the 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...
That process, shown in the accompanying color chart, was summarized by Crick in a series of rules that became known as the Central Dogma. Most scientists interpreted the key rule of that dogma to be that genetic information flowed in one direction: from DNA to RNA to protein. To the surprise of many molecular biologists, however, it has recently been shown that part of the process can sometimes be reversed. This finding, in the opinion of molecular biologists like Columbia's Sol Spiegelman, may offer an important clue to the workings of cancer cells (see box, page...