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Transcript
THE GENETIC CODE and how we got to the decipherment In the early sixties, molecular biologists were able to "decrypt" the genetic code. There was a problem that left them perplexed: how can someone write 20 "words" with an alphabet of only 4"letters"? That is: is : how can the four nitrogenous bases (A U C G) specify 20 different amino acids? acids The only possibility was a triplet code, based on the codons codons,, each one has three nitrogenous bases. Having only four letters (A U C G) clearly, a codon of a single letter could encode only four amino acids, not 20; and a codon of two letters would have coded 4x 4 = 16, still too little. But a codon of three letters could encode 4x4x4 = 64 codons codons,, more than enough for 20 amino acids. acids But how did they arrive to the deciphering of the genetic code? The first step in the decoding of the genetic code was made in 1961, ten years after the "discovery" of the structure of DNA by Watson and Crick. The scientists who carried out the first experiments to decipher the genetic code were the biochemical Niremberg Marshall, winner of the Nobel Prize for medicine, and his German colleague Heinrich Matthaei Matthaei.. They realized that they could use a simple artificial polynucleotide as a messenger, rather than a natural messenger RNA, much more complex, in order to identify the polypeptide encoded by this artificial messenger RNA. The two researchers prepared an artificial messenger RNA, previously discovered by biochemist Severo Ochoa, containing only a nitrogenous base, uracil uracil,, repeated over and over again. This RNA molecule was called poly poly-- U . Adding a poly - U in a test tube containing the ingredients necessary for protein synthesis ( ribosomes ribosomes,, enzymes, amino acids, ATP), could compose a polypeptide chain composed entirely of a single type of amino acid: phenylalaninel . Thus, a poly u encoded the phenylalanine; consequently, UUU was the codon (the "code word") to specify the phenylalanine. With a similar method were decrypted all possible codons for the 20 amino acids. Similar experiments were carried out later, using artificial RNA in which two or three nucleotides were repeated over and over according to a certain sequence. Of the 64 codons encoded, 61 were amino acids and the remaining 3 were signs of stopping. A s c a n b e s e e n f rom t h e t a b l e b e l ow, t h e re a re g e n e ra l ly d i f fe re n t c o d o n s for t h e s a m e a m i n o a c i d : t h e g e n e t i c c od e i s re d u n d a n t . Fu r t h e r m ore , for a l m os t a l l orga n i s ms i s a lways t h e s a m e : i t i s s a i d, t h e re fore , t h a t t h e g e n e t i c c od e i s a n universal code