Nucleotides are composed of a pentose sugar molecule, a nitrogenous base, and a phosphate group. These make up one nucleotide monomer which, when combined into a polymer, can create nucleic acids, DNA or RNA.
Each nucleotide is made up of three components:
- a nitrogenous base
- a pentose (five-carbon) sugar
- a phosphate group
When multiple nucleotide monomers combine, they create polymers called nucleic acids, such as DNA and RNA.
A molecule with just a nitrogenous base and a sugar is known as a nucleoside. Once at least one phosphate is covalently attached, it is known as a nucleotide
Nitrogenous Bases
The nitrogenous bases are organic molecules that contain carbon and nitrogen. They are bases because they contain an amino group that forms hydrogen bonds within the double-stranded DNA polymer to bases on the adjacent strand (shown in the figure below). There are two families of bases, Purines, and Pyrimidines. Adenine (A) and guanine (G) are classified as purines, and have a primary structure consisting of two carbon-nitrogen rings. Cytosine (C), thymine (T), and uracil (U) are classified as pyrimidines which have a single carbon-nitrogen ring as their primary structure. Because of the sizes and functional groups of the bases, base pairing is highly specific in DNA: A only pairs with T, and G only pairs with C, as shown below. RNA contains A U C and G.
Uracil is identical to thymine, without the methyl group.
Five-Carbon Sugar
The pentose sugar in DNA is deoxyribose, and in RNA it is ribose. The difference between the sugars is the presence of the hydroxyl group (-OH), on the second carbon of the ribose, and hydrogen on the second carbon of the deoxyribose.
Phosphate Group
The phosphate residue forms a phosphodiester bond between the ribose sugars attached to each base. The phosphodiester linkage forms the backbone of the double helix structure of DNA and the single strand structure of RNA.
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Key Points
• Nucleotides are comprised of a ribose sugar, a nitrogenous base, and a phosphate group.
• There are two families of nitrogenous bases, the pyrimidines (C, T, and U) and the purines (A and G).
• Bases A T G and C are found in DNA and A U C and G found in RNA.
• The ribose sugar and phosphate form a phosphodiester bond, which links several nucleotide monomers together in a polymer to form a nucleic acid such as RNA (single strand) or DNA (double strand with specifically paired bases).
Key Terms
nucleotide: the monomers which make up DNA or RNA polymer molecules. These consist of a nitrogenous heterocyclic base that can be a purine or pyrimidine, a five-carbon pentose sugar, and a phosphate group
monomer: a relatively small molecule which can be covalently bonded to other monomers to form a polymer
polymer: many monomers joined together to form long chains
nucleic acid: polymers comprised of many nucleotides bonded together, found in cells in two forms, RNA and DNA
DNA: deoxyribonucleic acid, the genetic material found within the cell nucleus
RNA: ribonucleic acid, codes for proteins at ribosomes
purines: a nitrogenous base consisting of two carbon-nitrogen rings
pyrimidines: a nitrogenous base consisting of a single carbon-nitrogen ring
phosphodiester bond: the linkage that occurs when two hydroxyl groups in a phosphate molecule react with hydroxyl groups on ribose sugars to form two ester bonds
nucleoside: a nitrogenous base attached to a ribose sugar without a phosphate molecule
