genetic
Nucleic Acids The nucleic acids differ in their type of pentose sugar. Deoxyribonucleic acid DNA is nucleotide that stores genetic information. DNA contains deoxyribose so-called because it has one less atom of oxygen than ribose plus one phosphate group and one nitrogen-containing base. The choices of base for DNA are adenine, cytosine, guanine, and thymine. Ribonucleic acid RNA is a ribose-containing nucleotide that helps manifest the genetic code as protein. RNA contains ribose, one phosphate group, and one nitrogen-containing base, but the choices of base for RNA are adenine, cytosine, guanine, and uracil. The nitrogen-containing bases adenine and guanine are classified as purines. A purine is a nitrogen-containing molecule with a double ring structure, which accommodates several nitrogen atoms. The bases cytosine, thymine found in DNA only and uracil found in RNA only are pyramidines. A pyramidine is a nitrogen-containing base with a single ring structure Bonds formed by dehydration synthesis between the pentose sugar of one nucleic acid monomer and the phosphate group of another form a backbone, from which the components nitrogen-containing bases protrude. In DNA, two such backbones attach at their protruding bases via hydrogen bonds. These twist to form a shape known as a double helix Figure 2.29 . The sequence of nitrogen-containing bases within a strand of DNA form the genes that act as a molecular code instructing cells in the assembly of amino acids into proteins. Humans have almost 22,000 genes in their DNA, locked up in the 46 chromosomes inside the nucleus of each cell except red blood cells which lose their nuclei during development . These genes carry the genetic code to build ones body, and are unique for each individual except identical twins.