The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins. The deciphering of the genetic code was a major milestone in molecular biology and genetics, and it involved understanding how the sequence of nucleotides in DNA or RNA is translated into the sequence of amino acids in a protein. One of the key steps in breaking the genetic code was the deciphering of a poly-U mRNA, which played a crucial role in understanding how codons (three-nucleotide sequences) correspond to specific amino acids.

Discovery of the Genetic Code

The deciphering of the genetic code began with the discovery that DNA carries genetic information and that this information is transcribed into messenger RNA (mRNA), which is then translated into proteins. In 1961, Marshall Nirenberg and J. Heinrich Matthaei conducted an experiment using synthetic RNA made up entirely of uracil (poly-U mRNA). This experiment was pivotal in deciphering the genetic code.

Steps in Breaking the Genetic Code: Deciphering Poly-U mRNA

  • Synthesis of Poly-U mRNA: The first step involved synthesizing a strand of mRNA composed entirely of uracil (poly-U). This was achieved by chemically synthesizing a long chain of uracil nucleotides.
  • In Vitro Translation: Nirenberg and Matthaei then used an in vitro translation system, which allowed them to translate the synthetic poly-U mRNA into a polypeptide chain without the need for living cells. This system contained ribosomes, transfer RNA (tRNA), amino acids, and other components necessary for translation.
  • Identification of Polypeptides: By using radioactive amino acids, Nirenberg and Matthaei were able to identify the polypeptides that were synthesized when the poly-U mRNA was translated in the in vitro system.
  • Deciphering the Codon UUU: The key breakthrough came when they found that poly-U mRNA produced a polypeptide chain consisting only of the amino acid phenylalanine. This led them to conclude that the codon UUU (uracil-uracil-uracil) corresponds to phenylalanine.
  • Implications for Genetic Code: The identification of UUU as a codon for phenylalanine was groundbreaking as it provided the first direct evidence linking a specific codon to an amino acid. This laid the foundation for further deciphering of the genetic code.

Significance of Deciphering Poly-U mRNA

The deciphering of poly-U mRNA had profound implications for understanding how codons are translated into amino acids. It provided crucial evidence that specific sequences of nucleotides in mRNA correspond to specific amino acids, thus unraveling the basic principles of the genetic code.


The deciphering of poly-U mRNA was a critical step in breaking the genetic code and understanding how information is translated from nucleic acids to proteins. This foundational research laid the groundwork for subsequent discoveries that led to a comprehensive understanding of the genetic code.

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