There is much talk about the information in DNA. This famous double-helix macro molecule carries a wealth of information encoded in each strand of the helix. Like the letters in this sentence, the DNA information is a sequence of letters. Instead of 26 letters, the DNA language consists of four letters. And instead of ink on paper, the DNA letters are represented by four different nucleotide molecules. The four nucleotides used in DNA are adenine, thymine, guanine and cytosine (abbreviated a, t, g and c, respectively). So the information encoded on a segment of a DNA strand might look something like this: ctgctagcat. These nucleotides are chemically bound together and they form one strand in the double helix. The design inherent in this information is certainly interesting, but the design of the DNA structure is equally fascinating.
It is well known that DNA consists of two strands. What is less well known is that the two strands are only delicately connected to each other. It is sort of a nano version of maglev technology, where the DNA strands are held in position, hovering next to each other, by a series of weak attractions known as hydrogen bonds. These bonds form when a hydrogen's lone proton, in one nucleotide, interacts with electrons from the complementary nucleotide in the other strand. The key is that, something like a lock and key arrangement, hydrogen bonds require a certain degree of geometrical alignment.
The geometrical alignment of complementary nucleotides in opposing DNA strands is precise. Adenine and guanine are the larger nucleotides whereas thymine and cytosine are the smaller nucleotides. Not surprisingly, a complementary nucleotide pair consist of a big and a small nucleotide. Adenine is not found paired with guanine (two large nucleotides) nor is thymine found paired with cytosine (two small nucleotides). This way the DNA double helix has a constant width. Furthermore, adenine is only found paired with thymine (barring error), and guanine is only found paired with cytosine. Each pair (at and gc) has hydrogen bonds that precisely align, as can be seen here (scroll down a bit). Notice that the gc pair has three hydrogen bonds whereas the at pair has only two hydrogen bonds.
These pairing rules mean that the DNA strands are complementary. If you know the sequence of one strand, then you can infer the sequence of the complementary strand. In our example above, if one segment reads, ctgctagcat, then the complementary segment would read: gacgatcgta. And since hydrogen bonds are fairly weak, it is not too difficult to pry the strands apart. This is important because protein machines need to access the DNA sequence. These machines may do this in order to copy down a segment of DNA information for subsequent use in the cell, or they may do this to make a duplicate copy of the entire genome before the cell divides.
Of course the details of these incredible structures and molecular activities fill textbooks. The design of the information is not the only fascinating design aspect of DNA.
