MCAT Content / Nucleic Acid Structure And Function / Nucleotides And Nucleosides

Nucleotides and Nucleosides

Topic: Nucleic Acid Structure And Function

Nucleotides are important molecules that consist of a nucleoside and a phosphate group.

DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a polynucleotide: DNA or RNA. Each nucleotide is made up of three components:

  1. a nitrogenous base
  2. a pentose (five-carbon) sugar
  3. a phosphate group

Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups.

The nitrogenous bases are organic molecules and are so named because they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential of binding an extra hydrogen, and thus, decreasing the hydrogen ion concentration in its environment, making it more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T).

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 on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose. The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).

The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, which forms a 5′3′ phosphodiester linkage.

nucleoside is comprised of a pentose sugar attached to a nitrogenous base. Nucleotides form when one or more phosphate groups are attached to a nucleoside.


Key Points

• The two main types of nucleic acids are DNA and RNA.

• Both DNA and RNA are made from nucleotides, each containing a five-carbon sugar backbone, a phosphate group, and a nitrogen base.

• DNA provides the code for the cell ‘s activities, while RNA converts that code into proteins to carry out cellular functions.

• The sequence of nitrogen bases (A, T, C, G) in DNA is what forms an organism’s traits.

• The nitrogen bases A and T (or U in RNA) always go together and C and G always go together, forming the 5′-3′ phosphodiester linkage found in the nucleic acid molecules.

• A nucleoside is comprised of a pentose sugar attached to a nitrogenous base. Nucleotides form when one or more phosphate groups are attached to a nucleoside.

Practice Questions

Khan Academy

Molecular targets of antiretroviral therapies


MCAT Official Prep (AAMC)


Key Terms

Nucleotide: the monomer comprising DNA or RNA molecules; consists of a nitrogenous heterocyclic base that can be a purine or pyrimidine, a five-carbon pentose sugar, and a phosphate group

Nitrogenous bases: Organic molecules, which are part of the nucleotides in DNA, showing base-like chemical properties.

Pentose sugar: A monosaccharide with a five-carbon ring.

Phosphate group: A molecule containing one atom of phosphorus covalently bound to four oxygen residues.

Nucleoside: A molecule comprised of a pentose sugar attached to a nitrogenous base.



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