DNA (deoxyribonucleic acid) is the biological molecule responsible for carrying our genetic code. It consists of chains of nucleosides that tend aggregate as pairs of chains and wrap into a double helical shape. In order to understand why DNA adopts such a characteristic shape, a strong background in general and organic chemistry is required.

A closer look at a short segment of DNA reveals that the nucleotides all have two basic segments. One segment is a five-membered ring with many O-H groups attached. This segment is the sugar, deoxyribose. Nucleotides are connected together by phosphate (PO4) groups between the deoxyribose segments. The other segment in each nucleotide is a base containing some combination of carbon, nitrogen and oxygen. Observations have shown that there are four different bases that appear in DNA, adenine, thymine, cytosine, and guanine. The four different nucleotide bases form complexes with nucleotide bases in another strand of DNA and each base consistenly pairs with only one other base. Adenine consistenly pairs with thymine rather than adenine, cytosine or guanine. Cytosine consistently pairs with guanine rather than cytosine, adenine, or thymine.

Rotation of the GC Base Pair (195K)

Think about what you know about intermolecular attractions as the next movie swaps the base pairs and put them together the wrong way.

Swapping the Bases (450K)

The DNA bases also pair in the same relative orientation every time. Think again about what you know about intermolecular attractions as one base in a base pair is rotated. This is shown from two different perspectives.

Base Rotation View 2 (540K)