A glycine residue is in position 210 of the tryptophan synthetase enzyme of wild-type \(E\). coli. If the codon specifying glycine is GGA, how many single- base substitutions will result in an amino acid substitution at position \(210 ?\) What are they? How many will result if the wild-type codon is GGU?

The genetic code is a foundational concept in molecular genetics, acting as the biological cipher that translates genetic information from the language of nucleic acids into the language of proteins. Imagine it as a dictionary where each 'word' or codon consists of three nucleotide bases—adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA (or thymine [T] in DNA). There are 64 possible codons, each corresponding to specific amino acids or signaling the end of a protein chain (stop codons).

In our problem, the focus was on glycine specified by codons GGA and GGU. The exercise asked how many single-base changes in those codons would alter the amino acid at position 210 of the tryptophan synthetase enzyme. Through this exercise, students get an engaging demonstration of how single letters in the genetic 'text' can have significant effects on the resulting protein's structure and function.

A single-base substitution is a type of genetic mutation where one base pair is improperly swapped for another. In the context of codons, this means changing one of the three bases, which can sometimes result in encoding a different amino acid—this is termed a 'missense' mutation. To assess the impact, each base in the given codon is systematically substituted with the remaining three nucleotide options, and the resulting codon's new assignment is compared to the original.

Our exercise demonstrated this process step by step, analyzing the specific genotype of an E. coli enzyme. Generally, single-base substitutions could result in no change (silent mutation), a different amino acid (missense mutation), or a stop signal (nonsense mutation) that prematurely terminates protein synthesis. The critical understanding here is that not all substitutions are consequential—a concept central to understanding genetic mutations and their potential impacts on organisms.

When a single-base substitution alters the encoded amino acid, we term it an amino acid substitution. These can have mild to drastic effects on a protein's form and function. It's like changing one ingredient in a recipe; sometimes, the dish is unaffected, other times, it's inedible. In protein structures, such a substitution could simply alter a non-essential part or could disrupt the active site of an enzyme, as potentially investigated in our glycine residue example.

The solution indicated that out of five possible mutations for each codon, three would change the amino acid at position 210, leading students to comprehend that genetic stability and variability are finely balanced. By actively doing these exercises, students can recognize that the continuity of life hinges on the integrity and flexibility of genetic information—a thrilling revelation that adds depth to the study of biology.