There are two possible codons: AUA and UAU; only two amino acids can be incorporated into a polymer, and the only polymer produced is poly(Tyr-Ile).

The codon for methionine is AUG, so an RNA that produces poly(Met) must be a repeating sequence of AUG. There are three reading frames for (AUG)_n. The (AUG-AUG-AUG)_n frame produces poly(Met); UGA is a termination codon, so no polypeptide forms; and (GAU-GAU-GAU)_n produces poly(Asp).

Start and stop codons are in red:

Four possible RNA sequences can encode Met-Asn-Trp-Tyr (the variations in codons are in red):

Addition of a Leu residue, which has 6 possible codons, increases the number of possible RNA sequences to 24: any of the 6 codons—UUA, UUG, CUU, CUC, CUA, or CUG—can be added to the end of each of the four sequences above.

Met-Tyr-Gln. These are the first three amino acids, beginning at the second AUG (initiation) codon: AUG-UAU-CAG. The first reading frame, AUG-UGU-UGA, ends in a stop codon after only two amino acids.

5’-AUG-GGU-CGU-GAG-UCA-UCG-UUA-AUU-GUA-GCU-GGA-GGG-GAG-GAA-UGA-3’

Met-Gly-Arg-Glu-Ser-Ser-Leu-Ile-Val-Ala-Gly-Gly-Glu-Glu-(stop)

Ten tRNAs are needed for these 14 amino acids (two to encode the three Gly; one to encode both Ser; one to encode the three Glu; and one for each of the other residues).

S-17

5’-AUG-GGU-CGU-GAG-UCA-UCG-UUA-AUU-GUA-GCU-GGA-GGG-GAG-GAA-UGA-3’

Met-Gly-Arg-Glu-Ser-Ser-Leu-Ile-Val-Ala-Gly-Gly-Glu-Glu-Trp

The peptide is 15 amino acids long, instead of 14; 10 tRNAs are needed, one for each different amino acid.

5′-ACC-AUA-UUG-CUC-UCU-UCG-3′

Thr - Ile  - Leu- Leu - Ser  - Ser

Pro - Leu - Met - Phe - Pro - Pro

Ala - Val  - Val  - Ile - Thr- Thr

Ser - Thr  - Ser - Val  - Ala  - Ala

Ile - Lys - Trp - Pro - Phe - Leu

Asn - Arg - Phe  - His - Tyr  - Trp

          Met-(stop) Arg-Cys-(stop)

Met-Pro-Ala-Glu-Val. A tRNA^Tyr with an anticodon mutation (e.g., 5′-AUA to 5′-UUA) will suppress the first stop codon, resulting in Met-Pro-Ala-Glu-Val-Tyr-Ser-Glu-Ala.

(a) Poly(Arg-Glu). (b) Poly(Val-Cys). (c) Poly(Glu), poly(Thr), and poly(Asn). (d) Poly(Lys), poly(Glu), and poly(Arg). (e) Poly(Leu-Leu-Thr-Tyr).

Leu: U₂C and UC₂ (observed proportion 22.2; calculated proportion 20). Phe: UUU and U₂C (obs. 100; calc. 100). Pro: UC₂ and CCC (obs. 5.1; calc. 4). Ser: U₂C and UC₂ (obs. 23.6; calc. 20).

The anticodon contains an inosine: 5′-ICC.

Incorporation of a Sec residue occurs only at a UGA codon that has an adjacent sequence called a SECIS element; it also requires a specialized tRNA (SelC, or tRNA^Sec); an enzyme, RelA, that catalyzes conversion of the Ser that is initially used to charge tRNA^Sec to Sec, to form Sec-tRNA^Sec; and a specialized elongation factor called SelB that recognizes the SECIS element.

The maximum number for a four amino acid change is 11 nucleotides between the insertion (X) and the deletion:

CAT-XCA-TCA-TCA-TCA(omit T)-CAT

The minimum number is 7 nucleotides:

CAT-CAX-TCA-TCA-T(omit C)AT-CAT

A-CGU-CGA-GUA-GCA-GUA-UCG-AUU-GAG-CUC-UUA-GAU-AAG-AUC-GC

The other reading frames would encode stop codons (red) in the middle of a protein:

Only one set of answers is shown.

(a) Three different codons are present in the three reading frames of this oligonucleotide, and thus a maximum of three different aminoacyl-tRNAs might bind. (b) With limited knowledge of the genetic code, possible explanations included the presence of a four-base code rather than a triplet code, or a relaxed specificity due to reaction conditions. (c) AAG was assigned to Lys. (d) AAA had already been assigned to Lys in the earliest code-cracking experiments that used homopolymeric poly(A). (e) GAA was assigned to Glu. (f) Increased Mg²⁺ concentrations and lower temperatures relaxed the specificity of the binding. (g) AAG = Lys; GAA = Glu; AGA = Arg. (h) GAC and GAU encode Asp. The Asp-tRNA^Asp may have elicited a positive signal in some experiments, because coding specificity is weakest in the third (wobble) position of the codon.