What was Marshall Nirenberg and Heinrich Matthaei’s contribution to our current understanding of the genetic code?

Marshall Nirenberg and Heinrich Matthaei used a cell-free system to show that a sequence of uracil bases (UUU) directs the production of the amino acid phenylalanine. This helped identify how three-base codons in RNA specify amino acids. Their contribution was the discovery of codons, not the DNA model, Photo 51, or DNA extraction.

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