RNA and DNA are two types of nucleic acids. The differences between these two molecules involve their chemical composition and, sometimes, their structure.
RNA stands for ribonucleic acid, and DNA stands for deoxyribonucleic acid. As the names imply, one difference between these two molecules is an oxygen atom within their component structures. RNA contains the sugar ribose, DNA contains the sugar deoxyribose. Ribose and deoxyribose are five carbon sugar molecules (monosaccharides) that make up the backbone of the nucleic acid molecule by alternating with phosphate groups. Ribose has a chemical formula of C5H10O5, whereas the chemical formula of deoxyribose is C5H10O4. Deoxyribose lacks a hydroxyl group at the 2′ position on the sugar’s five carbon (pentose) ring.
The sugar phosphate backbone of each nucleic acid binds to nitrogen bases to form a strand. The sugar and base unit is a nucleoside (with the phosphate group replacing the 5′ hydroxyl it is a nucleotide). There are four types of RNA bases – adenine (A), cytosine (C), guanine (G), and uracil (U). DNA also has four types of bases, including adenine (A), cytosine (C), and guanine (G), but instead of uracil it contains thymine (T). The difference in T and U between the two nucleic acids is an exact replacement: in DNA thymine is the counterpart to adenine, and in RNA uracil is the counterpart to adenine. In both molecules, C and G are counterparts. Chemically, uracil is an unmethylated form of thymine.
Another difference between RNA and DNA is the common structure in which they are found. RNA is most often a short single-stranded molecule. On the other hand, DNA is most often double-stranded. DNA forms the well known double helix conformation discovered by Watson and Crick in the 1950s. However, even within DNA there are differences in structure – the molecule can form different twists and unzip itself into a temporary single strand state for biological processes. In the same manner RNA can exist in a double-stranded form, it has the ability to bind via hydrogen bonds between its bases, but is usually the result of folding upon itself. Unlike DNA’s helical structure consisting of two complementary nucleic acid strands, RNA contains repetitive, complementary sequences that allow the molecule to fold upon itself and form highly complex structures from a single nucleic acid strand. An example of this is the transfer RNA (tRNA) molecule that forms a classic cloverleaf structure to transport amino acids for protein synthesis. RNAs also bind to one another to form larger structures in order to perform biological functions, such as the ribosome, another RNA structure involved in protein synthesis.
A less demarcated difference between RNA and DNA is their functions. DNA contains the genetic blueprint of the human organism and many other organisms. RNA is the signaling molecule that carries that information to the steps of protein synthesis, even participating in the structures that carry out translation. On the other hand, some organisms use RNA as their genetic blueprint, bypassing DNA altogether. However, DNA can be reverse transcribed from the RNA genomes in order to become part of cells that utilize DNA. So both nucleic acids are used in this capacity, though RNA is the universally required molecule for known organisms utilizing nucleic acids for gene expression.