Marshall Nirenberg and Heinrich Matthaei used poly-U mRNA in a cell-free system to make a polyphenylalanine protein chain. This showed that UUU must be the code that specifies the amino acid phenylalanine.
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Paul Zamecnik first developed the cell-free extract system, which Marshall Nirenberg adapted to decipher the genetic code. Paul Zamecnik and Mahlon Hoagland also isolated activated tRNA, the "adaptor" that shuttled amino acids to ribsomes for incorporati
Marshall Nirenberg, Har Gobind Khorana, and Robert Holley shared the 1968 Nobel Prize for Physiology and Medicine. Nirenberg and Khorana cracked the genetic code. Holley sequenced and deduced the structure of the first tRNA molecule.