Andrew did a mutant screen with his favorite bacterium that he used for marine phage hunting. He identified three base-substitution mutations on the bacterial recA gene: Mutant A: A base substitution occurred, which changed T to C, and consequentially abolished transcription of the recA gene.
Mutant B: A base substitution, which changed codon 2 from ACC to ACT, which did not change encoded amino acid.
Mutant C: A base substitution occurred, which changed codon 3 from TCA to a stop codon TGA.

Based on the above info, which of the mutants would still have functional RecA protein?

a. Mutant B
b. Mutant A
c. Mutant C

a. Mutant B

Explanation:

This question is describing a kind of mutation called SUBSTITUTION MUTATION, in which one or more nucleotide base is replaced by another in a sequence. In this case, the base-substitution mutations occcurs in the bacterial recA gene sequence.

However, base substitutions can be grouped into three based on the different results it yields namely: missense mutation, nonsense mutation, silent mutation.

- In Mutant A: A base substitution occurred, which changed T to C, and consequentially abolished transcription of the recA gene. This is an example of missense mutation because it changes the sequence.

- In Mutant B: A base substitution, which changed codon 2 from ACC to ACT, which did not change encoded amino acid. This is an example of silent mutation because it does not alter the sequence of the gene.

- In Mutant C: A base substitution occurred, which changed codon 3 from TCA to a stop codon TGA. This is an example of nonsense mutation because it leads to a STOP CODON.

According to the question, MUTANT B would still have functional RecA protein because it doesn't change the encoded amino acid sequence, which will still lead to the production of the RecA protein.

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