Nucleic acid is a complex molecule found in all cells. The two types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is mainly found in the nucleus of a cell. But RNA may be found throughout the cell. Even bacterial cells, which do not have a nucleus, contain DNA and RNA. Certain viruses contain only RNA while some viruses contain only DNA.
DNA plays a vital part in heredity, the passing on of biological characteristics from one generation to the next. It is the hereditary material of an organism. Short sections of DNA called genes determine an organism’s characteristics. Genes are located in chromosomes, the threadlike structures in the nucleus. When a cell divides, its chromosomes and genes are duplicated and passed on to the two resulting daughter cells.
DNA contains phosphate, a sugar called deoxyribose, and compounds called bases. These are arranged in units of phosphate-sugar-base—phosphate-sugar-base, repeated hundreds of thousands of times to form long, coiled chains. This fundamental chemical structure is common to all DNA. However, there are four bases in DNA—adenine, guanine, thymine, and cytosine. The exact proportions of each of these bases and the precise order in which they are arranged, are unique for each kind of living thing. It is this exact order and composition that must be faithfully copied each time a cell divides. Scientists can synthesize (chemically reconstruct) some kinds of DNA molecules that are able to reproduce themselves.
RNA also consists of long chains of repeating phosphate-sugar-base units. However, the sugar in RNA is ribose (rather than deoxyribose as in DNA) and the bases are adenine, guanine, cytosine, and uracil (rather than thymine). RNA is important in the formation of proteins.
Some RNA molecules, called messenger RNA or mRNA, leave the nucleus carrying instructions for making proteins. These mRNA molecules go to ribosomes, the cell structures where proteins are made. Proteins are chains of smaller organic molecules called amino acids. There are 20 kinds of amino acids involved in protein production.
