A substitution mutation occurs when one base in a DNA sequence is replaced with another base. Let's analyze the provided sequences:
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Original Sequence:
- Strand 1: C G T T A C T G C A A T
- Strand 2: G C A A T G A C G T T A
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Proposed Mutated Sequences:
First option (no mutation):
- Strand 1: C G T T A C T G C A A T
- Strand 2: G C A A T G A C G T T A
(This is identical to the original sequence; no mutation)
Second option (substitution mutation):
- Strand 1: C G T A C T G C A A T
- Strand 2: G C A T G A C G T T A
(In strand 1, T is replaced with A; this indicates a substitution mutation)
Third option (substitution mutation):
- Strand 1: C G T G A C T G C A A T
- Strand 2: G C A C T G A C G T T A
(In strand 1, T is replaced with G and in strand 2 A is replaced with C; this indicates a substitution mutation)
Fourth option (substitution mutation):
- Strand 1: C G T C T A C T G C A A T
- Strand 2: G C A G A T G A C G T T A
(In strand 1, T is replaced with C and in strand 2 A is replaced with G; this indicates a substitution mutation)
Thus, options 2, 3, and 4 all show substitution mutations, but based on your question format, you want to identify which shows a substitution mutation.
The second option is the clearest example, as it shows a single point substitution:
C G T A C T G C A A T
G C A T G A C G T T A
This is where T has been replaced by A in the first strand.
Therefore, the sequence showing a substitution mutation is the second option.