The Genetic Code

Cracking the Genetic Code

The Central Dogma: DNA makes RNA makes protein

>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:
                rRNA is the "workshop", mRNA is the "blueprint"

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, so 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 in "spiked" monomer recations.

        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. Which codons can be inferred?
2. Will these experiments have implications for the degeneracy of the code? Explain.