All of the information needed to control and build cells is stored in these molecules.
There are two main types of nucleic acid, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both of these molecules are polymers. They are composed of monomer subunits like the carbohydrates and proteins described previously. The monomers used to build nucleic acids are called nucleotides. The nucleotides are often referred to by the single letter abbreviations A, C, G, T and U. Like all of the monomers described so far, the monomers used to build DNA are similar to each other but are not exactly alike. One of the differences between DNA and RNA is the subset of nucleotides used to build the polymers. DNA contains A, C, G and T while RNA contains A, C, G and U.
Deoxyribonucleic Acid (DNA)
DNA is composed of two long strings (polymers) of nucleotides twisted around each other to form the spiral or helical structure shown below. The twisted molecules are arranged in a particular manner, with specific nucleotides always found across from each other. The nucleotide containing adenine (A) always pairs with the nucleotide containing thymine (T). Likewise, guanine (G) always pairs with cytosine (C). If you look closely at the graphics below you can see the nucleotide pairs interacting in the middle of the helix. The polymers that form DNA can be extremely long, reaching millions of nucleotides per each individual DNA molecule. The following graphic depicts a short strand of double-stranded DNA. (1)
DNA is located in the nucleus of cells, a structure that will be described in the next section of the site. All of the nucleated cells in the human body have the same DNA content regardless of their function. The difference is which parts of the DNA are being used in any given cell. For example, the cells that make up the liver contain the same DNA as the cells that make up muscles. The dramatically different activities of these two cell types is dependent on the portions of DNA that are active in the cells. DNA is the storage form of genetic information and acts as a blueprint for cells. As we shall see, changes in the sequence of DNA can lead to alterations in cell behavior. Unregulated growth, as well as many of the other changes seen in cancer, are ultimately the result of mutations, changes in the structure of DNA.