72. Your friend runs a BLASTp on his unknown sequence and finds a match with E-value of 3 with
a target. What does this mean?
A. There are 𝑒3 sequences in the database that match the unknown sequence.
B. There are 3 mismatches between the unknown and target sequences.
C. The random chance of unknown sequence matching the target is 1 in 3.
D. There are 3 hits of similar score that can be found by random chance.

Introduction:

When running a BLASTp (Basic Local Alignment Search Tool for proteins) analysis on a sequence, one of the key metrics provided is the E-value. This value helps assess the statistical significance of the sequence match between the query and the target in a database. Understanding the meaning of the E-value is crucial for interpreting the results accurately.

Explanation of the Answer:

In your friend’s case, an E-value of 3 indicates the following:

1. A. There are e3e^3e3 sequences in the database that match the unknown sequence.

   • Incorrect. The E-value does not represent e3e^3e3 sequences in the database. It represents the probability of a match occurring by random chance. This answer misrepresents the statistical interpretation of the E-value.

2. B. There are 3 mismatches between the unknown and target sequences.

   • Incorrect. The E-value is not directly related to the number of mismatches between sequences. It is a measure of the statistical significance of the match, not the number of mismatches or similarities.

3. C. The random chance of the unknown sequence matching the target is 1 in 3.

   • Incorrect. This is a misinterpretation. The E-value indicates the number of times you might expect to see a match with a similar score due to random chance. An E-value of 3 means that 3 matches of similar quality might occur by random chance, not that there is a 1 in 3 probability of a match.

4. D. There are 3 hits of similar score that can be found by random chance.

   • Correct. The E-value of 3 means that, on average, you would expect to find 3 matches with a similar or better score purely by random chance in the database. A lower E-value indicates a more statistically significant match, while a higher E-value suggests the match could easily have occurred by random chance.

Conclusion:

The E-value of 3 in your friend’s BLASTp results indicates that there are, on average, 3 hits with a similar or better score that could be found by random chance. This means the match might not be highly significant, as it could occur due to chance rather than true biological similarity.

The correct answer is: D. There are 3 hits of similar score that can be found by random chance.