On what principle does a nutritional marker work in yeast?

Correct answer: Restoring a nutrient requirement

The direct answer is A: a nutritional marker restores a nutrient requirement in an auxotrophic yeast. An auxotroph lacks a functional pathway needed to make a particular nutrient, so it cannot grow on medium lacking that nutrient. Step by step: begin with yeast that cannot synthesise nutrient X; place a functional marker gene on the vector; introduce the vector into the yeast; the marker restores the missing biochemical function; on selective medium without X, transformed cells grow while many non-transformed cells do not. Thus growth itself can indicate successful transformation. Option A is correct. Option B is wrong because DNA burning is neither the principle nor the purpose of a nutritional marker. Option C is wrong because agar melting is a preparation or physical process, not genetic selection. Option D is wrong because the host does not become dry. The marker may be used for selection, but growth also depends on suitable medium and other conditions. Memory cue: auxotroph needs a nutrient; a matching marker gives back the ability to make it.