Nucleic acids are large biomolecules, and include both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA carries your cells' genetic information. RNA uses this genetic information, and helps the cells make proteins. Both types of nucleic acids consist of building blocks called nucleotides, though there are some differences in the nucleotides that make up the two types of nucleic acids.
Nucleic Acid Structure
DNA and RNA both have chemical "backbones" made up of alternating sugar and phosphate molecules; a phosphate is a compound with the formula PO4. The sugar in DNA is called deoxyribose, while the sugar in RNA is called ribose and has an extra oxygen molecule. Attached to the molecule's backbone are nitrogen-containing bases. RNA comes in different shapes, but typically consists of a single backbone with bases attached, whereas DNA looks more like a twisting ladder of two parallel backbones, with bases forming the "rungs"--the so-called double helix structure.
The building block unit of a nucleic acid is called a nucleotide. A nucleotide consists of a single sugar, a phosphate, and a nitrogen-containing base. There are four different bases each in DNA and RNA. Both DNA and RNA contain the bases adenine, guanine, and cytosine. As its fourth base, RNA uses uracil, whereas DNA uses thymine as its fourth base.
Because of the major similarities between DNA and RNA, they're made of the same basic elements. The sugars and nitrogen-containing bases contain predominantly carbon and hydrogen. There are also oxygen atoms in the sugars. The phosphates, part of the backbone of both DNA and RNA, consist of phosphorus and oxygen. The bases, in addition to carbon and hydrogen, contain oxygen and nitrogen.
Reasons for Differences
The major reason for the differences in structure between RNA and DNA have to do with molecular stability. Deoxyribose makes DNA much more stable than RNA, which is important, because DNA encodes an organism's genetic information for the life of the organism. RNA is a transient molecule that each cell makes and degrades on a regular basis. The single-stranded nature of RNA allows it to perform its function, which is to transmit information quickly.