An RNA strand made of nothing but uracil cracked the genetic code's first word
In 1961, Marshall Nirenberg and Heinrich Matthaei added a synthetic RNA built entirely from the base uracil (poly-U) to a cell-free extract capable of making protein. It churned out a chain of pure phenylalanine, revealing that the triplet UUU codes for that amino acid — the first codon of the genetic code ever deciphered. The breakthrough kicked off a race that cracked all 64 codons within a few years.