Index >> Cellular Components >> Breaking the Genetic Code

Breaking the Genetic Code - Once it was clear that m-RNA on ribosomes guides the amino acid insertion, it was of interest to understand the sequence of nucleotides that code for each amino acid. This aspect of breaking the genetic code was first reported by Marshall Nirenberg and J.H.

Mathaei in 1961 who found that ill extracts of E. coli, the natural m-RNA could be replaced by synthetic m-RNA of known sequences. By doing this, the sequence of nucleotides that code for amino acids was revealed. The first synthetic polynucleotide used was polyuridylic acid (poly U) which was found to guide the polymerization of phenylalanine into polyphenylalanine. These experiments were extended by Khorana and others using synthetic messengers of known sequences and the sequence of nucleotides that direct the insertion of every known amino acid was established.

The discovery of t-RNA and the reaction by which an amino acid becomes attached to it came from M.B. Hoagland. He found that each amino acid first reacts with A TP, catalysed by a specific activating enzyme , to give an activated amino acid. This activated amino acid does not separate from the enzyme but immediately reacts with a molecule of t-RNA that is specific for that particular amino acid. This reaction is also catalysed by the same enzyme (aminoacyl-t-RNA synthetase) to give an aminoacyl-t-RNA The sequence and the structure of t-RNAs is now well established. All have a clover leaf structure and the anticodon for a particular amino acid is always found in the central loop. The other loops contain sequences common to all t-RNAs suggesting common function in binding the t-RNA ribosome-m-RNA complex. Interestingly, once the amino acid is linked to the 3 OH end of adenine, t-RNA recognizes its correct triplet on the m-RNA.