the wobble hypothesis

There are sixty-one codons that name amino acids, so you might expect a cell to need sixty-one different transfer RNAs to read them all. In fact it gets by with far fewer — about thirty to forty. How can one tRNA read more than one codon? The answer is a bit of deliberate sloppiness at the third letter, which Francis Crick named the wobble.

A tRNA recognizes a codon by base-pairing its three-letter anticodon against the codon. The first two pairings (codon positions 1 and 2) are read strictly, by the usual rules A-U and G-C. But the third pairing is held loosely; the geometry there lets the anticodon's first base 'wobble' and pair with more than one partner. For instance, a G in that wobble spot can pair with either C or U, and a special modified base called inosine (I) is so flexible it can pair with U, C, or A. So a single tRNA whose anticodon begins with I can read three different codons that share their first two letters. This is exactly why the genetic code's redundancy is concentrated in the third position — and why it costs the cell so much less to read the whole code.

Wobble is a beautiful piece of economy, but it is controlled, not chaotic. The looseness is permitted only at the third codon position; the first two are read tightly so that the amino acid identity is never lost. This is also why mutations in the third codon base are so often silent: the wobble tolerance means several third-base spellings deliver the very same amino acid. Wobble shows that fidelity and flexibility can coexist — the cell relaxes its grip exactly where a slip does no harm.