List the salient features of the genetic code. Explain how a frame-shift mutation affects a protein.

1. 1 · Define the code. The relation between the base sequence of mRNA and the amino acid sequence of a protein; read in codons of three bases.
2. 2 · Explain why it is a triplet. 4 bases taken 3 at a time give 4³ = 64 codons, enough for 20 amino acids (Gamow).
3. 3 · List the features.
   • 61 sense codons and 3 stop codons (UAA, UAG, UGA)
   • Unambiguous and specific
   • Degenerate
   • Read contiguously, no punctuation
   • Nearly universal
   • AUG codes for methionine and starts translation
4. 4 · Explain a frame-shift. Inserting or deleting one or two bases shifts the reading frame, so every later codon and amino acid changes.
5. 5 · Contrast with three bases. Adding or removing three bases changes one codon's worth only; the frame after it stays the same.
6. 6 · Link to real life. A universal code lets bacteria read the human insulin gene and make insulin.
7. 7 · Close in one line. So the code is read three bases at a time from a fixed start, and one extra base ruins the message.

Scoring tip: A short 'three-letter word' sentence example of a frame-shift earns marks quickly and shows understanding.