**Question:** Frame shift mutation is caused by

A. insertion of a nucleotide
B. deletion of a nucleotide
C. substitution of a nucleotide
D. duplication of a nucleotide

**Core Concept:** A frame shift mutation is a type of genetic variation in which an extra or missing nucleotide causes the reading frame of the mRNA sequence to shift, leading to a change in the amino acid sequence of the resulting protein. This can result in a loss of function or a gain of function, depending on the specific mutation.

**Why the Correct Answer is Right:** A frame shift mutation occurs due to the insertion or deletion of a single nucleotide. When a nucleotide is inserted or deleted, the reading frame of the mRNA sequence is disrupted, causing the protein sequence to shift. This results in a change in the amino acid sequence, which can lead to a loss of protein function or a novel protein function, referred to as a gain of function.

**Why Each Wrong Option is Incorrect:**

A. Insertion of a nucleotide does not always cause a frame shift mutation. The size of the insertion must be 1 nucleotide longer than the original sequence to disrupt the reading frame. If it is one nucleotide shorter or identical, it will not cause a frame shift mutation.

B. Deletion of a nucleotide does not always cause a frame shift mutation. The size of the deletion must be 1 nucleotide shorter than the original sequence to disrupt the reading frame. If it is one nucleotide longer or identical, it will not cause a frame shift mutation.

C. Substitution of a nucleotide does not cause a frame shift mutation. A substitution changes the amino acid sequence directly, but it does not alter the reading frame.

D. Duplication of a nucleotide does not cause a frame shift mutation. The size of the duplication must be 1 nucleotide longer than the original sequence to disrupt the reading frame. If it is one nucleotide shorter or identical, it will not cause a frame shift mutation.

**Clinical Pearl:** Understanding frame shift mutations is crucial for genetic counseling and disease diagnosis. These mutations can lead to severe genetic disorders, such as cystic fibrosis, where a single nucleotide deletion causes the famous "ΔF508" mutation.

**Correct Answer:** D. duplication of a nucleotide

Frame shift mutations are caused by insertions or deletions of nucleotides, not duplications or substitutions. Duplications of a nucleotide do not disrupt the reading frame, while insertions and deletions can cause a change in the reading frame, leading to a change in the amino acid sequence and potentially a loss of function or gain of function.