Cracking the Genetic Code 


The Central Dogma: DNA makes RNA makes protein
Central Dogma



>1953: DNA somehow "codes" for proteins
    Proteins are made on ribosomes
                 and ribosomes contain ribosomal RNA (rRNA)
    "one gene, one ribosome, one enzyme" hypothesis:
        genes make rRNA for enzyme-specific ribosomes (not)

Deciphering the code
       Can't be 1- or 2-letter code:
            1 letter 4 amino acids only; 2 letters 4 x 4  =  16 only
           20 primary amino acids require a minimum 3-letter triplet code

        Can't overlap: neighbors aren't constrained
                                 point mutations affect only single amino acids

        Can't be direct DNA protein:
            not chemically compatible (H-bonds vs. covalent bonds)
            in eukaryotes, DNA & protein are separated

       Adaptor Hypothesis (Crick 1957)
                some molecule must serve as intermediary

       messenger RNA  hypothesis (Jacob & Monod 1961)
           rRNA is too stable; "other RNA" is labile (transient)
                "other RNA" acts as messenger: mRNA is the "blueprint"
                rRNA is the "workshop"

Experiments with RNA homopolymers (Nirenberg & Matthaei 1961)

    cell-free in vitro protein synthesis:
       polynucleotide phosphorylase polymerizes rNDP monomers into RNA
                 rNDP  +  rNDP  + ... +  rNDP  <=>  3'-rNM-rNM-...-rNMP-5' + nPi

(1) UUUUUUUUU      poly-phenylalanine, therefore    UUU codes phe
    AAAAAAAAA      poly-lysine                                     AAA codes lys
  CCCCCCCCC      poly-proline                                    CCC codes pro
  GGGGGGGGG    poly-glycine                                   GGG codes gly

Experiments with RNA di-, tri-, & tetra-nucleotide polymers (Nirenberg et al., 1965; Khorana 1965)

(2)UG + UG + UG + UG UGU-GUG-UGU-GUG  cysteine + valine

(3)UGG + UGG + UGG + UGG UGG-UGG-UGG or
                              GGU-GGU-GGU or
                                GUG-
GUG-GUG
        mixture of poly-trypotophan, poly-glycine, & poly-valine

      Since only GUG & valine occur in both experiments
            therefore, GUG makes valine

(4) UGGG UGGG UGGG UGGG   UGG - GUG - GGU - GGG
                        trp, val, gly in 1:1:2 ratio

        Since we know GUG makes valine,
                            and GGG makes glycine,
                        then if GGU makes the other half of the gly ratio as well (GGN? Hmmm...)
        Therefore, UGG makes trypotophan

     and so on for all 64 triplets

The complete code was deciphered by 1965 (Nobel prize 1968)
    The code is a logical relationship between DNA, RNA, & protein
    The code precedes the biochemical understanding of transcription & translation

HOMEWORK: Consider the complete set of experiments possible with mono- & di-nucleotide RNA primers:
                                 1. How many codons (& which) can be inferred?
                                 2. Will these experiments have implications for the degeneracy of the code? Explain.


All text material 2014 by Steven M. Carr 


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