Learning Objectives

• Define the five core features of the genetic code.
• Explain the Wobble Hypothesis and its role in tRNA-codon pairing.
• Identify the exceptions to the redundancy and universality rules.
• Distinguish between unambiguous and degenerate properties.

1. Core Properties of the Code

The genetic code is a set of rules used by living cells to translate information encoded within genetic material into proteins.

2. The Wobble Hypothesis

The Wobble Hypothesis explains how cells can function with fewer than 61 tRNAs for 61 sense codons.

• The first two nucleotides of the codon require strict base pairing with the anticodon.
• The third position of the codon can “wobble” or differ, allowing a single tRNA to recognize multiple codons.

Activity

3. Important Exceptions

Success on the USMLE often requires knowing where the “rules” break:

• Non-Degenerate Exceptions: Methionine (AUG) and Tryptophan (UGG) are the only amino acids encoded by just one codon each.
• Non-Universal Exceptions: In animals, Mitochondria have their own genetic code, where certain codons specify different amino acids compared to nuclear DNA.
• Overlapping Exceptions: Some Viruses use overlapping reading frames to maximize the information density of their small genomes.

Activity

4. Clinical Mnemonic: Reading the Code

Activity