The building block of nucleic acids is called a nucleotide. A nucleotide is composed of three types of chemicals: organic bases, a phosphate group and a sugar with five carbon atoms.
The nucleotides for DNA and RNA have three of the same organic bases: adenine, guanine and cytosine. The fourth base is one of the key differences between the two. DNA has a base called thymine, while RNA has a base called uracil. Each nucleic acid also has a different sugar. DNA contains deoxyribose, which is why it is called deoxyribonucleic acid. The sugar found in RNA is called ribose, according to the Royal Society of Chemistry.