Why is a protein tag added in an expression construct?

Correct answer: To make detection and purification easier

The direct answer is A: a protein tag makes detection and purification easier. A tag is a short added peptide or protein sequence attached to the recombinant protein, usually through the expression construct. Reasoning step by step: the construct encodes the target protein together with the tag; the host produces the fusion protein; an antibody or binding material can recognise the tag; the tagged protein can then be detected, separated or purified more conveniently. A His-tag is a common example because it can bind suitable nickel-based purification resin under appropriate conditions. The tag may sometimes affect folding or activity, so its position and later removal may need consideration. Option A is correct. Option B is wrong because a tag is not designed to stop DNA uptake. Option C is wrong because its normal purpose is not to kill the host. Option D is wrong because a protein tag does not manufacture an antibiotic; antibiotic resistance markers are separate genetic elements. Memory cue: tag = label, so it helps find and clean the desired protein.