lysodeikticus (a bac t erf um)

lysodeikticus (a bac t erf um)

very important. It held that successive groups of nucleotides along a DNA chain code for successive amino acids along a given polypeptide chain. That colinearity does in fact exist was shown by elegant mutational analysis on bacterial proteins, carried out in the early 1960s by Charles Yanofsky and Sydney Brenner. Equally important were the genetic analyses by Brenner and Crick, which in 1961 first established that groups of three nucleotides are used to specify individual amino acids.

But which specific groups of three bases [codons) determine which specific amino acids could only be learned by biochemical analysis. The major breakthrough came when Marshall Nirenbeig and Heinrich Matthaei, then working together, observed in 1961, that the addition of the synthetic polynucleotide poly U (UUUUU . . .) to a cell-free system capable of making proteins leads to the synthesis of polypeptide chains containing only the amino acid phenylalanine. The nucleotide groups UUU thus must specify phenylalanine. Use of increasingly more complex, defined polynucleotides as synthetic messenger RNAs rapidly led to the identification of more and more codons. Particularly important in completing the code was the use of polynucleotides like AGUAGU, put together by organic chemist Har Gobind Khorana. These further defined polynucleotides were critical to test more specific sets of codons. Completion of the code in 19B6 revealed that 61 out of the 64 possible permuted groups corresponded to amino acids, with most amino acids being encoded by more than one nucleotide triplet (Table 2-3).

figure 2-18 Demonstration that RNA is synthesized in the nucleus and moves to the cytoplasm, (a) Autoradiograph of a cet! (Telrobymena) exposed to radioactive cytidine for IS minutes. Superimposed on a photograph of a thm section of the cell is a photograph of an exposed st/ver emulsson. Each dark spot represents the path of an electron emitted From a 5H (tritium) atom that has been incorporated into RNA Almost afl the newly made RNA is bund within the nudeus, (b) Autoradiograph of a similar ceil exposed to radioactive cytidme for 12 minutes end then allowed to grow (or 8B minutes in the presence of nonradioactive cytidine. Practically all the label incorporated into RMA pn tlte first 12 minutes has left the nudeus and moved into the cytoplasm. (Source; Courtesy of DM Prescrit, University of Colorado Medical School; reproduced from 1964 Progr. NudeicAadRes. Ill: 35, With permission.)

TABLE 2-3 The Genetic Code second position

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