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How can insertions and deletions cause frameshift mutations?

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A frameshift mutation occurs when the number of nucleotides inserted into or deleted from a DNA sequence is not divisible by three.12

In a protein-coding gene, the genetic code is read in triplets called codons, which define the reading frame for protein synthesis. When a nucleotide pair is added or subtracted, the reading frame is shifted by that position, causing every subsequent codon downstream to be altered.32

This shift results in a protein where the sequence before the mutation remains normal, but the remainder of the protein consists of a tail of functionally meaningless amino acids. Consequently, these mutations often lead to a drastic loss of protein function or the complete absence of the protein.32