Nucleotides are made up of a phosphate group, a five-ring sugar and a nitrogenous base. Nucleic acids, such as deoxyribonucleic acid (DNA) and ribonucleic (RNA), contain repeating nucleotides. Nucleotides link together to form nucleic acids by connecting the phosphate group of one nucleotide to the sugar of another.
The phosphate group is made up of one phosphorus atom and several oxygen atoms. This phosphate group, called an inorganic phosphate group, links up with a sugar that contains a ring made of five carbons. In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose, which contains one more oxygen atom than deoxyribose.
The nitrogenous bases are so named because each compound contains a ring containing nitrogen. Five nitrogenous bases are found in human cells: adenine, thymine, uracil, guanine and cytosine. Adenine, thymine, guanine and cytosine exist in DNA, whereas adenine, uracil, guanine and cytosine are in RNA. Adenine and guanine are bigger molecules called purines, which contain two rings linked together. Thymine, cytosine and uracil are pyrimidines, which are smaller than purines and have a single ring.
In DNA, two nucleic acid strands made of repeating nucleotides twist around one another to form a structure called a double helix that looks like a ladder. The two strands are bound to one another by the linkage of two nitrogenous bases. The space between the two sides of the helix is so precise that only a purine can bond with a pyrimidine, meaning that adenine and thymine bind together, whereas only cytosine and guanine can bind together.