Start and Stop Signals: Specific triplets act as punctuation for the cell's protein-making machinery. A 'start' codon (typically coding for methionine) initiates translation, while 'stop' codons signal the end of the polypeptide chain.
Codon-Anticodon Pairing: During translation, mRNA codons pair with complementary sequences on tRNA molecules called anticodons. This ensures that the correct amino acid, carried by the tRNA, is inserted into the growing chain.
Reading Direction: The code is read linearly along the mRNA strand. This sequential reading ensures that the amino acids are joined in the precise order required for the protein's specific function.
| Feature | Codon | Anticodon |
|---|---|---|
| Location | Found on the mRNA molecule | Found on the tRNA molecule |
| Function | Specifies the amino acid to be added | Pairs with mRNA to deliver the amino acid |
| Origin | Transcribed from the DNA template | Part of the tRNA structure |
Explain Degeneracy: When asked about the 'degenerate' nature of the code, always mention that there are 64 possible codons but only 20 amino acids. This mathematical discrepancy is the reason why multiple codons can code for the same amino acid.
Mutation Impact: Understand that because the code is non-overlapping, a single base substitution usually only affects one amino acid. However, an insertion or deletion can shift the entire reading frame, altering every subsequent amino acid.
Universality and Engineering: If a question asks why a human gene can function in a bacterium, the answer is the universality of the genetic code. The bacterial ribosomes 'read' the human mRNA codons exactly the same way human ribosomes would.
