Why is DNA described as complementary?

Answer 1

If you are referring to two random DNA strands, then they may or may not be complimentary as they contain different nucleotides and code for different genes. However, if you are talking about DNA strands that comprise a DNA double-helix, then yes they must be complimentary because the nucleotiside adenosine (A) always binds with thymine (T) and guanine (G) always binds with cytosine (C); this is the only way that a double-helix can be formed.

Answer 2

Each strand of the DNA molecule "complements" the other. For example, in the following sequence: 5' - ATCTAGC - 3' the complement is:

3' - TAGATCG - 5'

* Note the antiparallel arrangement of the strands (5' --> 3' vs 3' --> 5')

C forms hydrogen bonds with G and A forms hydrogen bonds with T.