The Genetic Code

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 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 1961)

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

    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 (Khorana 1965)

(1)UG + UG + UG + UG => UGU-GUG-UGU-GUG => cysteine + valine

(2)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

(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

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