Question

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?

Short answer

Answer: Three single-base substitutions will result in an amino acid substitution at position 210 for both the GGA and GGU codons specifying glycine.

Step-by-step solution

Understand the genetic code

To solve this problem, we need to understand how the genetic code is translated. The genetic code consists of 64 possible codons, which represent 20 amino acids and 3 stop codons. The codon is a group of three nucleotides (bases), and there are four types of nucleotide bases: adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA or thymine (T) in DNA. In this problem, we are given the wild-type codon for glycine as GGA and GGU.

Analyzing single-base substitutions for GGA codon

We will look at all the possible single-base substitutions for the GGA codon and determine which ones will result in an amino acid substitution: 1. GAA: Glu (Glutamic acid) 2. GCA: Ala (Alanine) 3. GUA: Val (Valine) 4. GGA: Gly (Glycine) - no change 5. GGG: Gly (Glycine) - no change Out of the 5 possible single-base substitutions, two of them result in glycine, meaning 5 - 2 = 3 single-base substitutions will result in an amino acid substitution when the wild-type codon is GGA.

Analyzing single-base substitutions for GGU codon

We will now look at all the possible single-base substitutions for the GGU codon and determine which ones will result in an amino acid substitution: 1. GCU: Ala (Alanine) 2. GAU: Asp (Aspartic acid) 3. GGU: Gly (Glycine) - no change 4. GGG: Gly (Glycine) - no change 5. GUU: Val (Valine) Out of the 5 possible single-base substitutions, two of them result in glycine, so 5 - 2 = 3 single-base substitutions will result in an amino acid substitution when the wild-type codon is GGU.

Comparing the results

Upon analyzing the possible single-base substitutions for both GGA and GGU codons, we found that 3 single-base substitutions will result in an amino acid substitution at position 210 for both codons specifying glycine.

Key concepts

Decoding the Genetic Code

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.

Understanding Single-Base Substitutions

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.

Exploring Amino Acid Substitution Consequences

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.