The letters RNA stand for ribonucleic acid, a type of nucleic acid. There are two types of nucleic acid--DNA (deoxyribonucleic acid) and RNA.DNA carries a cell's genetic information, but this information cannot be accessed without RNA. In some organisms, such as in certain viruses, RNA is the carrier of genetic information. It has also been discovered that RNA can sometimes act as a catalyst of chemical reactions.

RNA is almost always a single-stranded molecule constructed as a long chain of nucleotides. Each nucleotide contains a ribose molecule (a type of sugar), a phosphate group, and one of four nitrogen-containing cyclic units. The nitrogen-containing units are the bases: adenine, cytosine, guanine, and uracil. The nucleotides of DNA and RNA differ in two respects: an -OH group of the sugar present in RNA is -H in DNA and one of the bases of DNA is thymine rather than uracil...