Problem 7 - Campbell Biology

Question

Which of the following mutations would be most likely to have a harmful effect on an organism? \begin{equation}\begin{array}{l}{\text { (A) a deletion of three nucleotides near the middle of a gene }} \\ {\text { (B) a single nucleotide deletion in the middle of an intron }} \\ {\text { (C) a single nucleotide deletion near the end of the coding }} \\ {\text { sequence }} \\ {\text { (D) a single nucleotide insertion downstream of, and close to, }} \\ {\text { the start of the coding sequence }}\end{array}\end{equation}

Step-by-Step Solution

Answer

Option D is the most harmful.

Step 1: Understand the Problem

Determine which mutation would most likely harm an organism by negatively affecting gene expression or protein function.

Step 2: Option A - Deletion of Three Nucleotides in the Middle of a Gene

A deletion of three nucleotides will remove a single amino acid from the polypeptide chain, but it will not cause a frameshift. This is less likely to be harmful since the rest of the protein sequence remains unchanged.

Step 3: Option B - Single Nucleotide Deletion in the Middle of an Intron

Introns are non-coding regions, so a single nucleotide deletion here will likely have a minimal effect on the gene’s function. Intronic mutations usually do not affect the protein unless they disrupt splicing.

Step 4: Option C - Single Nucleotide Deletion Near the End of the Coding Sequence

This will result in a frameshift mutation, altering the reading frame of the gene. However, being near the end of the coding sequence might result in fewer amino acids being altered, possibly reducing the harmful effects.

Step 5: Option D - Single Nucleotide Insertion Downstream of the Start of the Coding Sequence

A single nucleotide insertion near the start of the coding sequence will cause a frameshift mutation, changing all downstream amino acids. This is likely to produce a nonfunctional protein and is typically the most harmful mutation.

Step 6: Determine the Most Harmful Mutation

Based on the analysis, the single nucleotide insertion downstream of the start of the coding sequence (Option D) is most likely to be harmful due to the extensive impact on the protein structure.

Key Concepts

Frameshift Mutation

Frameshift mutations occur when there is an insertion or deletion of nucleotides that are not in multiples of three. This disrupts the triplet codon reading frame of DNA during translation. As a result, every amino acid after the point of insertion or deletion is altered, which typically leads to significant changes in the resulting protein. This can make the protein non-functional and can have severe consequences for the organism.

Imagine a sentence where removing a letter causes all subsequent letters to shift, drastically changing the meaning. Similarly, frameshift mutations alter the entire downstream sequence, usually leading to non-functional proteins.

Gene Expression

Gene expression is the process by which the information encoded in a gene is used to direct the production of a functional protein or RNA. This involves transcription of DNA to mRNA and translation of mRNA to protein. Gene expression can be tightly regulated and can be influenced by mutations.

For example, mutations in the promoter region of a gene can increase or decrease transcription levels, affecting how much protein is produced. Mutations within the gene can change the protein structure, impacting its function and the overall phenotype of the organism. Proper gene expression is crucial for normal cellular function.

Protein Function

Proteins are essential molecules that perform a wide range of functions within cells, including structural roles, enzymatic activity, and cellular signaling. Mutations can affect protein function in various ways:

• Frameshift mutations can render a protein non-functional by altering its amino acid sequence.
• Missense mutations substitute one amino acid for another, potentially impacting protein folding or function.
• Nonsense mutations introduce premature stop codons, truncating the protein.

These changes can be detrimental, causing diseases or developmental issues.

Nucleotide Insertion

Nucleotide insertion involves the addition of one or more nucleotides into the DNA sequence. If the insertion is not a multiple of three, it results in a frameshift mutation, drastically altering the downstream reading frame.

Such an insertion near the start of the coding sequence is particularly harmful because it affects the entire protein sequence, potentially generating non-functional proteins. On the other hand, insertions within non-coding regions (introns) or multiples of three within coding sequences may have lesser effects on the resulting protein.

Nucleotide Deletion

Nucleotide deletion removes one or more nucleotides from the DNA sequence. Similar to insertions, deletions not in multiples of three cause frameshift mutations, altering the downstream reading frame and affecting protein function.

For instance, deleting a single nucleotide at the beginning of a coding sequence can drastically affect protein structure, leading to potential loss of function. However, deletions in non-coding regions or in multiples of three within coding sequences may have milder effects.

Coding Sequence

The coding sequence is the portion of a gene's DNA or RNA that codes for protein. It consists of exons, the regions that are expressed as protein. Changes in this sequence directly affect the amino acid sequence and, consequently, the protein's structure and function.

Mutations within the coding sequence can have various outcomes, from minor alterations that do not significantly impact protein function to severe changes that produce completely non-functional proteins. The position and type of mutation play crucial roles in determining the overall impact.

Intron

Introns are non-coding regions of a gene that are transcribed into mRNA but are spliced out before translation. Although introns do not code for protein, they play regulatory roles. Mutations within introns can affect gene expression, splicing, and mRNA stability.

Such mutations might alter the splicing machinery, potentially leading to the inclusion of intronic sequences in the mature mRNA, or skipping of exons, both of which can produce aberrant proteins. Therefore, while a mutation in an intron is often less detrimental, it can still have significant effects if splicing is disrupted.

Splicing

Splicing is the process by which introns are removed from pre-mRNA and exons are joined together to form mature mRNA, which can be translated into protein. This process is crucial for generating functional proteins.

Mutations that disrupt normal splicing can have severe consequences. If an exon is skipped or an intron is included, the resulting mRNA can be abnormal, potentially producing non-functional or harmful proteins. Precise splicing is integral for maintaining the correct sequence and functionality of proteins.