DNA, Protein Synthesis & Gene Technology

Base-pairing (adenine with thymine, cytosine with guanine) constrains the DOUBLE-stranded molecule and the complementary strand, but a single strand on its own has no pairing constraint: its four base percentages only have to add to 100.

Trigger: any question giving base counts or percentages and asking about the complementary strand or the whole double helix.

Instances:
(i) within one strand: the four percentages sum to 100, so a missing base is 100 minus the other three, nothing more;
(ii) complementary strand: it is the mirror of the given strand, adenine opposite thymine and cytosine opposite guanine, so each base percentage swaps with its partner;
(iii) whole double helix: totals satisfy $A=T$ and $C=G$, so guanine and cytosine each take half of whatever adenine and thymine leave;
(iv) counts, not just percentages: for $n$ base pairs there are $2n$ bases, so turn a percentage into a count first, then split the non-adenine, non-thymine remainder equally between cytosine and guanine.