Questions: The Genetic Code

Question 1

Which of the following best describes a 'degenerate' genetic code?

• A. The code contains errors and ambiguities that must be corrected by proofreading
• B. Multiple codons can specify the same amino acid
• C. The same codon can specify different amino acids in different organisms
• D. The code can only be read in one direction along the mRNA

Answer: (B)

Degeneracy means redundancy — multiple codons map to the same amino acid (e.g., both UUU and UUC code for phenylalanine). It does NOT mean imprecision; each individual codon maps to exactly one amino acid. Students often confuse the technical meaning of 'degenerate' (redundant) with its colloquial meaning (degraded or flawed).

Question 2

The genetic code is absolutely identical in most living organism on Earth.

• T. True
• F. False

Answer: False

The code is described as 'nearly universal,' not absolutely universal. A handful of exceptions exist — notably in mitochondrial genomes of various organisms, where a few codon assignments differ from the standard table (e.g., UGA codes for tryptophan in some mitochondria rather than serving as a stop codon). This near-universality is still remarkable evidence for common ancestry.

Question 3

A point mutation changes the third base of a codon from C to U. Under what conditions would this mutation have no effect on the protein produced?

Model answer: When both the original codon and the mutated codon are synonymous — they code for the same amino acid. This frequently occurs because the degeneracy of the code often clusters synonymous codons at the third (wobble) position, so third-position substitutions often produce no amino acid change.

The degeneracy of the genetic code is not random; synonymous codons are clustered so that variation at the third codon position (the wobble position) is least likely to change the amino acid. For example, all four codons of the form CCX encode proline, so any substitution at the third position is silent. This architectural feature provides partial protection against point mutations.