Nitrogenous Base: Definition & Pairs

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
Instructor: Adrienne Brundage
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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