Cytosine: Structure & Definition

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  • 0:04 What Is Cytosine?
  • 0:25 Structure
  • 1:07 Function in Nucleic Acids
  • 2:03 Other Functions
  • 2:30 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
Cytosine is one of the bases that spell out genes in your DNA. It is so versatile that it's been called the 'wild card' of nucleic acids. Learn about the structure and functions of the letter 'C' in DNA's alphabet!

What Is Cytosine?

Cytosine is an important part of DNA and RNA, where it is one of the nitrogenous bases coding the genetic information these molecules carry. Cytosine can even be modified into different bases to carry epigenetic information. Cytosine has other roles in the cell, too, as the energy carrier and cofactor CTP.


As a nitrogenous base, cytosine is full of nitrogen atoms (it has three). It also has one ring of carbon, which makes it a pyrimidine. A purine, on the other hand, has two rings of carbon. There are two pyrimidines, cytosine and thymine, and two purines, adenine and guanine, in DNA. RNA also has two pyrimidines, cytosine and uracil, and two purines, adenine and guanine.

In DNA, adenine and thymine are present in the same percentages and always pair with each other. That leaves cytosine to pair with its double-ringed buddy, guanine. Cytosine also pairs with guanine in RNA.

Function in Nucleic Acids

Cytosine can be part of a nucleotide, a molecule that includes a nitrogenous base along with a sugar and one or more phosphates. When nucleotides join together, they can form the nucleic acids DNA and RNA.

When cytosine is on one strand of a nucleic acid, the other strand will contain a guanine to match. These two are friends because they fit together perfectly with three hydrogen bonds. Cytosine can be easily converted into other bases, and so it's been called the wild card base. In fact, sometimes it accidentally loses a few atoms and becomes uracil, one of the bases in RNA. But don't worry, our cells can find and fix this error.

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