Nitrogenous Base: Definition & Pairs

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  • 0:05 What Are Nitrogenous Bases?
  • 1:31 Linking DNA & RNA With…
  • 4:07 Lesson Summary
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Lesson Transcript
Adrienne Brundage
Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

A nitrogenous base is simply a nitrogen-containing molecule that has the same chemical properties as a base. They are particularly important since they make up the building blocks of DNA and RNA: adenine, guanine, cytosine, thymine and uracil.

What Are Nitrogenous Bases?

A nitrogenous base is simply a molecule that contains nitrogen and has the chemical properties of a base. Let's break it down a bit to understand what the definition really means. Nitrogenous bases fall into the class of chemical compounds known as organic compounds or those which contain carbon. What makes a nitrogenous base is that it not only contains carbon, but it also contains the element nitrogen. Elemental nitrogen is normally found as a gas in the Earth's atmosphere. It is generally odorless, tasteless and colorless and reacts very easily with the other elements. This reactivity makes it an important part of compounds necessary for life.

Nitrogenous bases are also, as the name implies, bases. A base is a substance that can donate pairs of electrons to other elements or molecules and form a new molecule in the process. It is the opposite of an acid, which is a substance that can accept pairs of electrons from elements or molecules. In the case of nitrogenous bases, nitrogen bonds with carbon, hydrogen and oxygen atoms. These different elements can arrange themselves in rings, and we name the compound based on its ring formation. Bases that form single rings, like this molecule, are called pyrimidines.

Bases that form double rings, like this molecule, are called purines. Purines are much larger than pyrimidines because of this double ring.

Linking DNA & RNA with Base Pairs

Purines and pyrimidines serve as a basis for the building blocks of DNA and RNA. DNA, or deoxyribonucleic acid, is the molecule that encodes the instructions for life. RNA, or ribonucleic acid, is the molecule that is responsible for the coding, decoding, regulation and expression of genes. RNA is single-stranded whereas DNA is double-stranded.

These instructions are encoded in the order of nitrogenous bases all along the DNA and RNA molecules. These bases are formed starting with either the single-ring pyrimidine or the double-ring purine. Then, some extra nitrogen, hydrogen or oxygen molecules are added on to the basic ring to make the nitrogenous bases: adenine, guanine, cytosine, thymine (DNA only) or uracil (RNA only). When bound to the phosphate backbone of DNA and RNA, the nitrogenous bases are called nucleotides. Cytosine and thymine are both pyrimidine bases.

Cytosine starts out as the single ring pyrimidine, and an extra oxygen, nitrogen and three hydrogen atoms are attached to the ring. Cytosine is very unstable, and can change form if left on its own for too long.

Thymine also starts out as the single-ring pyrimidine, but has two extra oxygen atoms as well as two hydrogen atoms and a methyl group attached to the main ring.

Uracil is also a pyrimidine base, but it is only found in RNA, not DNA. It is very similar to thymine expect it does not have the methyl group, so it acts differently from thymine. Adenine and guanine are both purine bases.

Adenine starts out as the double-ring purine, and an extra nitrogen and two hydrogen atoms are attached.

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Additional Activities

Nitrogenous Base Model

In this lesson, students will be building a model of each of the five nitrogenous bases using multicolored gum drops and toothpicks to represent the bonds. You will need four different colors of gum drops for this activity, and at least one box of toothpicks. You will need at least 36 gum drops of one color for carbon, 25 of another color for hydrogen, 15 for nitrogen, and 5 for oxygen. If gum drops aren't available, you can also use four different colors of modeling clay for this activity and role the colors into small balls.


In this sweet lab, you'll be creating a candy model of each of the four nitrogenous bases, adenine, thymine, uracil, guanine and cytosine. Follow the steps below to create your model from gum drops and toothpicks.

  1. Research the molecular structure of each of the five bases and draw them on paper first.
  2. Next, identify the four different types of atoms you'll need to represent and choose a gum drop color for each. Write down your key on your paper as well.
  3. Next, choose one base to start with and connect the atoms using toothpicks. Don't forget to use two toothpicks for double bonds and one toothpick for single bonds.
  4. Repeat step 3 until you have built all five bases.


  1. Which bases are purines versus pyrimidines?
  2. Which bases are physically larger, purines or pyrimidines? Why?
  3. Why do purines always pair with pyrimidines in nucleic acids? What would happen if two purines paired together or two pyrimidines? Why would this be a problem for the DNA?

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