The two strands in DNA are not identical but are complementary. Explain.

Explanation

DNA has double helix structure in which the two strands are held together by H-bonds between purine base of one strand and pyrimidine base of the other and vice-versa. Thymine (T) pairs with adenine (A) through two H-bonds and cytosine (C) pairs with guanine (G) through there H-bonds.
So, opposite of each adenine (A) on one strand there must be a thymine (T) on the other strand and opposite of guanine (G), there must be cytosine (C). In other words, the two strands of DNA are complementary and not identical.