The study of modern genetics depends on an understanding of the physical and chemical characteristics of DNA. Some of the most fundamental properties of DNA emerge from the characteristics of its four basic building blocks, called nucleotides. Knowing the composition of nucleotides and the differences between the four nucleotides that make up DNA is central to understanding DNA’s role in living systems.
DNA is a nucleotide polymer, or polynucleotide. Each nucleotide contains three components:
The sugar carbon atoms are numbered 1 to 5, with 1 being the point of attachment of the nitrogenous base, and 5 the point of attachment of the phosphate group. DNA polymers are built from individual nucleotides by linking the phosphate of one nucleotide to the #3 carbon of the neighboring nucleotide. The repeating pattern of phosphate, sugar, then phosphate again is commonly referred to as the backbone of the molecule.
The sugar in DNA is deoxyribose. Deoxyribose differs from ribose (found in RNA) in that the #2 carbon lacks a hydroxyl group (hence the prefix “Deoxy”). This missing hydroxyl group plays a role in the three-dimensional structure and chemical stability of DNA polymers.
Nucleotides in DNA contain four different nitrogenous bases: Thymine, Cytosine, Adenine, or Guanine. There are two groups of bases:
Pyrimidines: Cytosine and Thymine each have a single six-member ring.
Purines: Guanine and Adenine each have a double ring made up of a five-atom ring attached by one side to a six-atom ring.
The order of nucleotides along DNA polymers encode the genetic information carried by DNA. DNA polymers can be tens of millions of nucleotides long. At these lengths, the four letter nucleotide alphabet can encode nearly unlimited information.
Nucleosides are similar to nucleotides except they do not contain a phosphate group. Without this phosphate group, they are unable to form chains.
The illustration above introduces nucleotide and the terminology used to describe them.