When the amino acid sequences of insulin isolated from different organisms were determined, differences were noted. For example, alanine was substituted for threonine, serine for glycine, and valine for isoleucine at corresponding positions in the protein. List the single-base changes that could occur in codons of the genetic code to produce these amino acid changes.

In the triplet binding technique, radioactivity remains on the filter when the amino acid corresponding to the codon is labeled. Explain the rationale for this technique.

In studies of the amino acid sequence of wild-type and mutant forms of tryptophan synthetase in E. coli, the following changes have been observed:

Determine a set of triplet codes in which only a single-nucleotide change produces each amino acid change.

In studies using repeating copolymers, AC . . . incorporates threonine and histidine, and CAACAA . . . incorporates glutamine, asparagine, and threonine. What triplet code can definitely be assigned to threonine?

Refer to Table 13.1. Can you hypothesize why a synthetic RNA composed of a mixture of poly U poly A would not stimulate incorporation of ¹⁴C-phenylalanine into protein?

In a coding experiment using repeating copolymers, the following data were obtained:

AGG is known to code for arginine. Taking into account the wobble hypothesis, assign each of the four codons produced in the experiment to its correct amino acid.

Why doesn't polynucleotide phosphorylase (Ochoa's enzyme) synthesize RNA in vivo?