Final answer:
In the experiments to crack the genetic code, a cell-free translation system produced polypeptides when synthetic mRNA and amino acids were added, confirmed by Nirenberg and Matthaei's seminal work.
Step-by-step explanation:
In experiments designed to "crack" the genetic code, a cell-free translation system was used to produce polypeptides when synthetic mRNA and specific amino acids were added. Marshall Nirenberg and Heinrich Matthaei were pioneers who utilized this system to reveal that certain sequences of mRNA, or codons, corresponded to specific amino acids. In one of their early experiments, adding mRNA consisting exclusively of uracil (U) led to the production of polypeptides composed entirely of the amino acid phenylalanine. This major breakthrough supported the theory that proteins are synthesized through a process of cellular translation directed by codons, which are nucleotide triplets in mRNA.
Further developments came when Nirenberg and others used specific tRNAs charged with individual amino acids. They successfully identified the codons by observing the synthesis of aminoacyl-tRNA and later polypeptides in the translation system. The profound significance of these experiments rests on their demonstration that each amino acid is encoded by specific nucleotide triplets, and the entirety of the genetic code relies on sets of three nucleotides to dictate protein synthesis
The discovery that the genetic code is universally composed of these triplets, which are called codons, can be largely attributed to the work of scientists like Nirenberg, Matthaei, and later Francis Crick and Sydney Brenner who further fortified the concept of the triplet code. These founding experiments not only elucidated the nature of the genetic code but also laid the groundwork for our understanding of how genetic information is translated into living matter.