Question

One type of DNA mutation that can occur as a result of environmental toxin exposure is a deletion, in which one or more nucleotides are deleted during the replication of the cell's genome. However, in mutations in which three base pairs are deleted, the mutated gene often codes for a relatively normal protein. This is likely due to A. the fact that most DNA mutations have no effect on cellular function. B. codons' ability to expand in size to fill the gaps created by base-pair deletions. C. retention of the original reading frame. D. occurrence of a back-mutation.

Short answer

C. Retention of the original reading frame.

Step-by-step solution

Understanding DNA Mutations

Review what a DNA deletion mutation is: a DNA deletion mutation involves the loss of one or more nucleotides from the DNA sequence. This can affect the gene's functionality.

Observing the impact of three-base pair deletions

Note that if three base pairs are deleted, one entire codon is removed. Since codons are groups of three nucleotides that code for a specific amino acid, removing one codon means one fewer amino acid is in the sequence.

Considering the reading frame

Recognize that the reading frame—the way nucleotides are divided into codons—remains unchanged because removing a whole codon (three base pairs) doesn’t shift the sequence of remaining codons.

Evaluating the answer choices

Analyze the choices:A. This isn't necessarily true; some mutations have significant effects.B. Codons do not expand; this is incorrect.C. This accurately describes why a protein can still be relatively normal: the reading frame is retained.D. Back-mutations are mutations that reverse the effects of previous mutations, which is not the situation described here.

Conclusion

Based on the steps above, the correct choice is C. Retention of the original reading frame explains why the protein can still be relatively normal despite the deletion of three base pairs.

Key concepts

Gene Functionality

DNA sequences contain the instructions our cells use to produce proteins. These sequences are composed of genes, which determine the traits and functions of an organism. Each gene is a blueprint for a specific protein. When a mutation, like a nucleotide deletion, occurs in a gene, it can change the protein it codes for. However, not all mutations drastically alter gene functionality. The impact depends on the type and location of the mutation.
A deletion mutation can lead to the loss of genetic information, which may impair the normal function of the resulting protein. If the gene is crucial for a specific function, its alteration can affect the whole organism. Yet, some mutations, especially those involving three nucleotides (a whole codon), might have minimal impact on the protein's functionality. This is because the reading frame remains unchanged, ensuring the rest of the gene is read correctly.

Codons

In DNA, codons are sequences of three nucleotides that correspond to specific amino acids or stop signals during protein synthesis. There are 64 different codons, coding for the 20 amino acids used in proteins and three stop signals that end protein synthesis.
Each codon instructs the cell's machinery to add a particular amino acid or stop constructing the protein. If a mutation causes a codon to be deleted or altered, it can change the structure and function of the resulting protein. However, the deletion of a whole codon (three nucleotides) does not shift the reading frame. This means that while one amino acid may be missing, the rest of the protein is synthesized correctly.
The accuracy of codons in directing amino acid placement is critical for proper protein function. The redundancy in the genetic code, where multiple codons can code for the same amino acid, also helps minimize the impact of some mutations.

Reading Frame

The reading frame is the way nucleotides in DNA are grouped into codons for translation into proteins. The sequence starts from a 'start' codon and is read in sets of three nucleotides, continuing until a 'stop' codon is reached. Maintaining the correct reading frame is essential for the resulting protein to be functional.
A nucleotide deletion can disrupt the reading frame, leading to a frameshift mutation. This shifts the way nucleotides are read, potentially changing every codon downstream of the mutation and resulting in a completely different (and usually nonfunctional) protein. However, if three nucleotides—or one entire codon—are deleted, the reading frame remains unchanged. The rest of the sequence is read correctly, which helps in producing a protein that is still mostly functional.
This retention of the original reading frame is why some three-nucleotide deletions do not result in severe alterations.

Nucleotide Deletion

A nucleotide deletion involves the loss of nucleotides from a DNA sequence. The consequences of a deletion depend on the number of nucleotides removed and their position within the gene.
When nucleotides are deleted, it can lead to a loss of genetic information, possibly impairing gene functionality. If the deletion involves one or two nucleotides, it can cause a frameshift mutation, altering the downstream codons and leading to a dysfunctional protein. However, a deletion of three nucleotides (a whole codon) does not shift the reading frame. While this removes one amino acid from the resulting protein, it does not affect the rest of the sequence.
Nucleotide deletions can occur naturally or due to environmental factors, such as toxins. Understanding how different types of deletions impact gene functionality is essential for studying genetic diseases and developing treatments.