Why is screening cells necessary after inserting foreign DNA?

Correct answer: Because not all cells take up foreign DNA

The direct answer is A: screening is necessary because not all cells take up foreign DNA. During transformation, DNA is mixed with or introduced into many cells, but uptake is not guaranteed. Also, a cell may take up a vector without receiving the desired insert, or the insert may be arranged incorrectly. Therefore, selection first helps identify cells with a suitable selectable marker, and screening checks whether the desired recombinant DNA is actually present. Option A is correct. Option B is wrong because cells are not always identical in their DNA uptake or recombinant status. Option C is biologically impossible in this context: DNA does not become water. Option D is not the general reason for screening; an enzyme may produce a colour signal in some tests, but colour change is only a possible detection method, not the fundamental reason. Memory cue: transformation is not guaranteed, so screen to find the true recombinant cells.