Difference Between Adenine and Guanine

Adenine vs Guanine

Nuclear acids are nucleotide polymers, which contain four different nucleotide bases; adenine, guanine, cytosine, and thymine (uracil in RNA). These four bases can be put into two major categories namely purines and pyrimidines. Adenine and guanine are the purines while cytosine, thymine, and uracil are the pyrimidines. To keep the same length of DNA, the base pairs must always be made up of one pyrimidine and one purine. The purines are composed of a two- ring system made from a pyrimidine type six- membered ring fused with a five- membered imidazole ring.


Adenine is a purine found in all DNA, RNA and ATP. It is made up of a six-membered ring attached to a five-membered ring. The structure of adenine, basically, differs from guanine by the presence of an additional point of unsaturation between C-6 and N-1 positions of its six-membered ring. Adenine always gets paired with thymine in DNA, and uracil in RNA by means of two hydrogen bonds. In addition to DNA and RNA, adenine is also found in adenosine triphosphate (ATP), which is considered as the energy currency of organisms. In ATP, adenine is attached to a five carbon sugar.


Guanine is the purine which pairs with cytosine in DNA and RNA.  Like adenine, guanine is also composed of six-membered ring, attached to the five-membered ring. However, guanine has amine or ketone groups attached to C-2 or C-6 positions in its six- membered ring. The guanine nucleoside is known as guanosine. Guanine can be found as two forms; the major keto form and rare enol form. It binds the cytosine by means of three hydrogen bonds.

What is the difference between Adenine and Guanine?

• Adenine always binds thymine, while guanine always binds cytosine.

• Three hydrogen bonds are formed between guanine and cytosine, whereas two hydrogen bonds are formed between adenine and thymine.

• Adenine gets paired with different bases in DNA and RNA (thymine and uracil), but guanine always binds a single base called cytosine in both DNA and RNA.

• Unlike in guanine, adenine has an additional point of unsaturation between C-6 and N-1 in its six-membered ring.

• Guanine has amine or ketone group attached to C-2 or C-6 positions while adenine has only amine group attached to C-6 position.

• Nucleside of adenine is called adenosine while that of guanine is called guanosine.

• Unlike guanine, adenine is important to construct ATP.

• The chemical formula of adenine is C5H5N5, whereas that of guanine is C5H5N5O.