The Genetic Code

Cracking the Genetic Code

The Central Dogma: DNA makes RNA makes protein

After 1953: DNA somehow "codes" for proteins
Proteins are made on ribosomes , which contain
long-lived (stable) & short-lived (labile) RNA

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 amino acids require a minimum 3-letter triplet code
                 4 x 4 x 4 = 64, is there redundancy? (hmmm ....)

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 physically 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 1961)

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

    UUU UUU UUU      poly-phenylalanine, therefore    UUU codes phe
    AAA AAA AAA    poly-lysine                                     AAA codes lys
    CCC CCC CCC    poly-proline                                   CCC codes pro
    GGG GGG GGG      poly-glycine                                  GGG codes gly

Experiments with RNA di-, tri-, & tetra-nucleotide polymers (Khorana 1965)

(1) UG + UG + UG + UG UGU-GUG-UGU-GUG
cysteine + valine + cysteine + valine + . . . . etc

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

GUG & valine occur & produced in both experiments
therefore, GUG makes valine

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

        Since we know GUG makes valine,
           and we know GGG makes glycine,
                             if GGU makes gly as well (hmmm...),
        Therefore, UGG makes trypotophan
                & maybe GGN makes glycine: code is "ambiguous" in 3rd position

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

[First example of "Big Science" Biology: NIH organized major effort through Nirenberg]