Genetic code logic: Crick showed that codons consist of three bases and proposed degeneracy of the code. Which experimental strategy did he use to infer triplets and degeneracy?

Introduction / Context:
Understanding how nucleotide sequences encode amino acids was a central question in early molecular genetics. Francis Crick and colleagues used specific genetic experiments to deduce that the genetic code is read in non-overlapping triplets and that multiple codons can specify the same amino acid (degeneracy).

Given Data / Assumptions:

• The question asks which method Crick used to infer triplet codons and degeneracy.
• Historically relevant approaches included mutational analysis in phage systems.
• Frameshift mutations change the reading frame unless compensated.

Concept / Approach:
If codons are read in groups of three, adding or deleting one nucleotide will shift the reading frame and scramble downstream protein sequence. However, adding or deleting three nucleotides may restore the original frame. Observing suppression of the mutant phenotype by compensating insertions or deletions provides strong evidence for a triplet code.

Step-by-Step Solution:
Identify Crick’s key experiment → mutational analysis using proflavin to induce single-base insertions or deletions. Reason: 1 or 2 base changes cause frameshift and loss of function; 3 base changes restore reading frame and function. Triplet inference arises because three-base sets preserve codon boundaries. Degeneracy inference follows from mapping multiple codons to the same amino acid without altering protein function.

Verification / Alternative check:
Complementary experiments by Nirenberg, Khorana and others later directly assigned codons to amino acids, confirming triplet, non-overlapping, and degenerate features.

Why Other Options Are Wrong:
Gel electrophoresis and density gradients can separate molecules but do not reveal codon size; restriction digests and structural techniques didn’t infer triplet logic; X-ray crystallography of ribosomes came much later.

Common Pitfalls:
Confusing triplet inference (genetic) with later biochemical decoding; assuming any DNA technique from that era addresses codon length directly.

Final Answer:
Frameshift mutagenesis.