What Are the Three Parts of a Nucleotide?

The three parts of a nucleotide are the base, the sugar, and the phosphate. Nucleotides are the building blocks of DNA (2′-deoxyribonucleic acid) and RNA (ribonucleic acid). DNA and RNA code genetic information, transport energy throughout cells, and serve as cell signaling molecules. Here is a closer look at the components of a nucleotide, how they are connected, and how they differ between DNA and RNA.

Parts of a Nucleotide

In both DNA and RNA, a nucleotide consists of three parts:

1. Nitrogenous base (base): A nitrogenous base (simply called a “base” in the context of biochemistry) is an organic molecule that contains nitrogen. Chemically, it’s a base because of the pair of electrons on the nitrogen atom. There are two types of nitrogenous bases: purines and pyrimidines. Purines have a double-ring structure, while purines consist of a single ring. Adenine (A) and guanine (G) are purines. Thymine (T), uracil (U), and cytosine (C) are pyrimidines.
2. Pentose sugar: The sugar is called a “pentose sugar” because it contains five (pent-) carbon atoms. The carbon atoms are numbered. The sugar in DNA is 2′-deoxyribose, while the sugar in RNA is ribose. The only difference between the two sugars is that 2′-deoxyribose has one less oxygen atom attached to the second carbon.
3. Phosphate group: A nucleotide has at least one phosphate (PO₄^3-) group. One oxygen atom of the phosphate connects to the 5′ carbon in the sugar. When phosphate groups link together (as in ATP or adenosine triphosphate), the link looks like O-P-O-P-O-P-O. The nucleotide name refers to the number of phosphate groups it contains. For example, adenosine monosphosphate (AMP) has one (mono-) phosphate, while adenosine diphosphate (ADP) has two (di-) phosphates.

Nucleotides in DNA and RNA

DNA and RNA share some similarities, but they contain slightly different sugars and bases in their nucleotides.

The DNA double helix resembles a twisted ladder, where the phosphate groups form the backbone of the ladder and the bases form the rungs.

How the Parts of a Nucleotide Are Connected

The three parts of a nucleotide are connected via covalent bonds.

• The nitrogenous bases bonds to the first or primary carbon atom of the sugar.
• The number 5 carbon of the sugar bonds to the phosphate group. A free nucleotide may have one, two, or three phosphate groups that attach as a chain to the sugar’s 5-carbon. When nucleotides connect to form DNA or RNA, the phosphate of one nucleotide forms a phosphodiester bond with the 3-carbon of the sugar of the next nucleotide. The resulting chain of sugars and phosphates is the backbone of a nucleic acid.
• The purines and pyrimidines form bonds with one another. Adenine forms two bonds with thymine in DNA or two bonds with uracil in RNA. Guanine form three bonds with cytosine in both DNA and RNA.

Difference Between Nucleotides and Nucleosides

Nucleotide names include their nucleoside and the number of phosphate groups they contain.

A nucleoside is essentially a nucleotide minus the phosphate group. A nucleotide has a nitrogenous base, a pentose sugar (ribose or 2′-deoxyribose), and one or more phosphate groups, while a nucleoside consists of the nitrogenous base and pentose sugar. Nucleosides become nucleotides via a process called phosphorylation. Enzymes called kinases perform phosphorylation in cells.

The major nucleosides are adenosine, deoxyadenosine, guanosine, deoxyguanosine, 5-methyluridine, thymidine, uridine, deoxyuridine, cytidine, and deoxycytidine.

References

• Alberts, B.; et al. (2002). Molecular Biology of the Cell (4th ed.). Garland Science. ISBN 0-8153-3218-1.
• McMurry, J. E.; Begley, T. P. (2005). The Organic Chemistry of Biological Pathways. Roberts & Company. ISBN 978-0-9747077-1-6.
• Nelson, David L.; Cox, Michael M. (2005). Principles of Biochemistry (4th ed.). New York: W. H. Freeman. ISBN 0-7167-4339-6.