Question

Which of the following mutations would you expect to have the greatest effect on a living cell? (a) a mutation that changes a single base in a region of noncoding DNA; (b) a mutation that changes the third letter of one of the three-base "words" in a particular gene; (c) a mutation that deletes one base in the middle of a gene.

Short answer

The mutation in (c) that deletes one base in the middle of a gene would have the greatest effect.

Step-by-step solution

Understanding the Types of Mutations

To solve this problem, we must first understand what each described mutation means. A mutation in noncoding DNA might not affect the organism because noncoding regions don't usually translate into proteins. A change in the third base of a codon might not affect the protein primarily due to the redundancy in the genetic code, but a deletion within a gene can disrupt the reading frame, potentially altering all downstream amino acids.

Evaluating the Effectiveness of Each Mutation

(a) A mutation in noncoding DNA is likely to have little to no effect unless it affects regulatory elements. (b) A mutation in the third base of a codon might still produce the same amino acid because of the genetic code's redundancy, in which multiple codons can code for the same amino acid. (c) A deletion of one base in the middle of a gene causes a frameshift mutation, changing the reading frame of the mRNA.

Conclusion and Impact Analysis

Frameshift mutations (like the one described in part (c)) are usually the most severe because they alter the entire protein sequence downstream from the mutation. This can result in a completely nonfunctional protein, leading to more substantial effects on the cell than the other types of mutations.

Key concepts

Noncoding DNA mutations

Noncoding DNA refers to regions of DNA that do not code for proteins. It's often referred to as "junk DNA," but this doesn't mean it's unimportant. These regions can have crucial roles, such as regulating gene expression. When mutations occur in these noncoding areas, they generally have little or no direct impact on protein production.
However, if these mutations affect regions that regulate important processes, like enhancers or promoters, they can significantly influence how genes are expressed.

• Most noncoding DNA mutations are harmless.
• Some might affect regulatory functions and impact gene expression.
• Effects depend on the mutation's location relative to key regulatory elements.

Understanding the context of noncoding mutations helps us appreciate their subtle roles in genetic regulation.

Genetic code redundancy

The genetic code is redundant, meaning multiple codons can encode the same amino acid. This redundancy provides a safety net against mutations. When a mutation changes the third base of a codon, often the resulting codon still translates to the same amino acid due to this redundancy.
This is known as a "silent mutation" because it has no effect on the final protein product.

• Redundancy helps reduce the effects of mutations.
• Silent mutations do not alter protein structure.
• This feature makes many single base changes neutral with no phenotypic effect.

This characteristic of the genetic code protects organisms from potentially harmful mutations by maintaining protein function even when DNA changes.

Frameshift mutations

Frameshift mutations involve the insertion or deletion of a nucleotide within a gene, disrupting the reading frame of the sequence. The reading frame is a way of dividing the sequence of nucleotides into a set of consecutive, non-overlapping triplets or "codons."
When a nucleotide is added or removed, it shifts the entire sequence down or up, altering every subsequent codon. This often results in a completely different and nonfunctional protein.

• Frameshift mutations can cause dramatic changes in protein function.
• Impact all codons downstream from the mutation site.
• Can lead to incomplete or faulty proteins, resulting in significant cellular dysfunction.

Frameshift mutations are one of the most severe types of mutations because they often lead to major disruptions in protein production and function, which can have profound effects on cellular activity and overall organism health.