What is the key feature of dna that allows it to be copied?

Complementary base pairing

Explanation:

Complementary base pairing: Complementary base pairing is the phenomenon where in DNA the base guanine always pairs with cytosine with three hydrogen bonds and adenine always binds to thymine with two hydrogen bonds.

While in DNA replications, each nucleotide pairs with its partner in a specific manner, T with A and G with C. Both the strands of DNA are complementary with each other in case of nucleotides.

So in new molecule of DNA, one strand in new and other is parental, because DNA is replicated in semi-conservative way.