The ABCs of DNA — Part 2

Diagram of Deoxyribose sugar with attachment points to Phosphate backbone and bases indicated.

DNA Structure

Nuclear DNA molecules consist of a string of ring-shaped sugar molecules attached to a phosphate backbone on one side forming the sides of the twisted-ladder-like structure and one of the four DNA bases on the other. As the name deoxyribo suggests, the ring-shaped 5-carbon sugar on nuclear DNA has lost an oxygen molecule. There are four bases* that comprise the DNA code: adenine (A), guanine (G), cytosine (C), and thymine (T). The arrangement of these four bases along the ester-bonded, sugar-phosphate backbone determines what protein is coded for, when it is manufactured and how much is made.

The four bases (ATGC) are of one of two types: a purine or a pyramidine. Purines are dicyclic aromatic compounds meaning they have two rings. and pyramidines are monocyclic aromatic compounds having one ring as illustrated below:

The structure of DNA Bases: Adenine, Guanie, Cytosine and Thymine

Another important way to divide the four bases is by the number of hydrogen bonds it can form. Adenine and thymine form two hydrogen bonds each while cytosine and guanine each form three hydrogen bonds. Because of the different number of available hydrogen bond points, cytosine and guanine form a base pair rung on the DNA ladder while adenine and thymine form another rung.
While the ladder analogy is useful for visualizing the DNA double helix, as you can see, the bases are nearly-flat, circular molecules more like coins than rod shaped ladder rungs. Because of this, each base of the polymer that makes up one-half of the double helix ladder, is stacked between the bases above and below it. To be more accurate, the twisted ladder of the double helix should be more appropriately visualized as a twisted stepladder. Envision the flat steps running parallel to the ground, the sugar-phosphate backbone running up and down perpendicular to the ground and then twist the whole stepladder clockwise. The attraction between each base and the bases above and below it on one side of the twisted stepladder is what keeps the polymer of the DNA molecule intact and in position so that it can be easily read.

*There is a fifth base called uracil, a pyrimidine, that only occurs in RNA and replaces thymine but it is not important for the discussion of nuclear DNA.

Chemical Structures Credit: Rutgers University, Dept. of Molecular Biology and Biochemistry