The most unique and interesting aspect of DNA is its three
dimensional helical structure, the double helix. The double
helix consists of two strands of complementary DNA running
antiparallel, that is, 5' to 3' on one strand, and 3'
to 5' on the other strand.
The primary structure of an example sequence of double-stranded
DNA is given below. Notice how every guanine
is matched with a cytosine
and every adenine is matched
with a thymine.
The same sequence of double-stranded DNA is shown below with
the backbone (sugars and phosphates) in purple,
the bases color-coded as above, and hydrogen bonds in cyan.