Marshall Nirenberg and Heinrich Matthaei significantly advanced our understanding of the genetic code by conducting experiments that deciphered how sequences of nucleotides in RNA correspond to specific amino acids in proteins. In 1961, they used synthetic RNA composed of repeating uracil nucleotides to determine that the codon UUU codes for the amino acid phenylalanine. This groundbreaking work was pivotal in establishing the relationship between DNA, RNA, and protein synthesis, ultimately leading to the elucidation of the entire genetic code. Their research laid the foundation for modern molecular Biology and genetics.