Problem 24 - Biochemistry: Concepts and Connections

Question

A nonsense mutation is a substitution mutation that creates a chain- terminating codon in the mRNA corresponding to the mutant gene. Identify three substitution mutations that could change a tryptophan codon to a nonsense triplet.

Step-by-Step Solution

Answer

Mutations UGG to UAA, UGG to UAG, and UGG to UGA create stop codons.

Step 1: Understand the Codons Involved

The codon for tryptophan (Trp) in mRNA is UGG. We need to find mutations that will convert this codon to a stop codon. The commonly used stop codons are UAA, UAG, and UGA.

Step 2: Identify Possible Mutations for Each Stop Codon

From UGG to UAA, change the second base G to A and the third base G to A. For UAG, change the third base G to A. For UGA, only the third base G needs to be changed to A.

Step 3: List All Possible Substitution Mutations

For UGG to UAA: Change G at position 2 to A, and G at position 3 to A resulting in UAA. For UGG to UAG: Change only G at position 3 to A resulting in UAG. For UGG to UGA: Change G at position 3 to A resulting in UGA.

Key Concepts

Substitution Mutation

A substitution mutation involves one nucleotide being replaced by another in a DNA or RNA sequence. This can lead to various outcomes:

• Silent mutations, which do not change the protein.
• Missense mutations, which result in a different amino acid being incorporated.
• Nonsense mutations, which introduce a stop codon, halting protein synthesis prematurely.

In the context of nonsense mutations, a single nucleotide change can convert a codon that normally codes for an amino acid into a stop codon. This stops the translation process before a protein is fully synthesized, potentially leading to nonfunctional proteins. Understanding this process is crucial for comprehending genetic diseases and developing gene therapies.

Stop Codon

Stop codons signal the end of a protein-coding sequence during mRNA translation. Standard stop codons include UAA, UAG, and UGA. These do not code for any amino acids and instruct the cellular machinery to terminate protein synthesis.

Stop codons are vital for proper gene expression. Without them, proteins could become excessively long or incomplete, impacting their function and potentially leading to cellular dysfunction. In genetic studies, identifying stop codons is essential for assessing mutations and their effects on protein synthesis.

Tryptophan Codon

The tryptophan codon is represented by UGG in mRNA sequences. Tryptophan is an essential amino acid, meaning it must be obtained through diet as the body cannot synthesize it. Its codon, UGG, is unique as it only codes for tryptophan, unlike most amino acids which have multiple codons.

• Because UGG is singular, any alteration can easily result in a change in protein composition.
• Substitution mutations that alter the UGG codon can potentially transform it into a stop codon, thus shutting down translation early.

Understanding the role and coding of tryptophan is important in genetic coding and studies, especially in relation to protein function and regulation.

mRNA Translation

mRNA translation is the process of converting mRNA sequences into proteins. It occurs in ribosomes, the cellular machinery responsible for synthesizing proteins. This process involves three main stages:

• Initiation: The ribosome assembles around the mRNA.
• Elongation: tRNA molecules bring amino acids to the ribosome, where they are added to the growing polypeptide chain following the mRNA sequence.
• Termination: The process ends when the ribosome encounters a stop codon, releasing the completed protein.

This finely tuned sequence of events ensures that cells produce the correct proteins needed for their functions. Disruptions such as nonsense mutations in mRNA translation can cause synthesis to stop abruptly, possibly leading to incomplete or faulty proteins.