DNA

DNA is deoxyribonucleic acid, the template for genetic instructions. DNA undergoes replication into complementary strands of DNA for reproduction, and transcription into RNA for the production of proteins. The nucleotide bases, adenine, thymine, cytosine, and guanine are coupled inside the double helix, while the ribose backbone curves around the edge.

Above - nucleobases form complementary pairs through hydrogen bonding. Adenine and thymine are coupled (broken lines), and cytosine couples with guanine. Below left - stick model with hydrogen bonds as broken lines. (click to enlarge image)

More at:
http://www.dnaftb.org/dnaftb/
http://www.dnalc.org/home.html
http://www.pbs.org/wgbh/aso/tryit/dna/