A frameshift mutation occurs when bases are added or deleted from the DNA sequence, leading to a shift in the reading frame of the genetic code.
The original DNA segment is: AGC TTA TCG.
Now, let's analyze each of the given responses:
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AGC TCG: This exhibits a deletion of one base (the second "T"), resulting in a shift in the reading frame.
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AGC TTA TAG: This is not a frameshift mutation, as it has simply replaced "TCG" with "TAG" (substitution), but the length is the same.
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TTA TCG: This has deleted the "AGC" at the start, which is still a deletion but does not represent a frameshift mutation starting from the first base.
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AGC TTT CG: This introduces a base pair change and a deletion, but it retains the overall length, which could mask a frameshift.
The best option that clearly indicates a frameshift mutation through deletion is:
AGC TCG.