Problem 6 - Campbell Biology Concepts & Connections

Question

A geneticist found that a particular mutation had no effect on the polypeptide encoded by a gene. This mutation probably involved a. deletion of one nucleotide. b. alteration of the start codon. c. insertion of one nucleotide. d. substitution of one nucleotide.

Step-by-Step Solution

Answer

d. substitution of one nucleotide.

Step 1 - Define the Problem

Identify what type of mutation had no effect on the polypeptide encoded by the gene.

Step 2 - Understand Mutation Types

Review the impact of different mutations: a) Deletion of one nucleotide: This causes a frameshift mutation, altering the entire reading frame and potentially affecting the polypeptide.b) Alteration of the start codon: This could prevent the polypeptide from being synthesized at all.c) Insertion of one nucleotide: Similar to deletion, this also causes a frameshift mutation, altering the reading frame.d) Substitution of one nucleotide: This might lead to a silent mutation, which does not affect the polypeptide due to the redundancy of the genetic code.

Step 3 - Analyze the Effects

Determine which mutation type could have no effect on the polypeptide: - Deletion and insertion of a nucleotide would both shift the reading frame, causing significant changes.- Alteration of the start codon would prevent translation initiation.- Substitution of one nucleotide can result in a silent mutation where the codon change does not alter the amino acid.

Step 4 - Conclude

Since the mutation had no effect on the polypeptide, it is most likely a substitution of one nucleotide, as this can result in a silent mutation.

Key Concepts

Substitution Mutation

Substitution mutation occurs when one nucleotide in DNA is replaced with another. This type of mutation can be silent, missense, or nonsense. It is considered point mutation because it affects only one point in the DNA sequence. These mutations can lead to various consequences:
- **Silent mutation**: Here, the change does not affect the resulting protein.
- **Missense mutation**: This results in a different amino acid in the protein, potentially altering its function.
- **Nonsense mutation**: This creates a stop codon, leading to a truncated, often nonfunctional protein.
In the context of our exercise, the substitution mutation could be silent if it does not change the function of the polypeptide.

Silent Mutation

A silent mutation is a type of substitution mutation that does not change the amino acid sequence of the resulting protein. This is due to the redundancy of the genetic code. Even though the DNA sequence is altered, the same amino acid is produced, keeping the protein functional. For example:
- If the codon GAA (which codes for glutamic acid) is changed to GAG, it still codes for glutamic acid because of genetic code redundancy.
Therefore, silent mutations do not impact the protein, aligning with the scenario described in our exercise, where a mutation had no effect.

Frameshift Mutation

A frameshift mutation involves the insertion or deletion of nucleotides that alters the reading frame of the gene. Since genes are read in three-nucleotide codons, adding or removing nucleotides shifts this frame, potentially changing every subsequent amino acid in the protein. This can destroy the protein's functionality. There are two main types:
- **Insertion mutation**: Adds extra nucleotides.
- **Deletion mutation**: Removes nucleotides.
Both are likely to have dramatic effects on the polypeptide, in contrast to silent substitution mutations.

Genetic Code Redundancy

Genetic code redundancy means that multiple codons can code for the same amino acid. This feature is essential for silent mutations. For instance,
- The amino acid leucine can be coded by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG.
Redundancy acts as a buffer against some mutations. If a codon changes (due to a substitution mutation), the new codon might still specify the same amino acid, ensuring that the protein's functionality remains unaffected. This redundancy explains why some mutations, like silent ones, have no observable effect on the polypeptide product.