Base Pairs: Definition & Types

Nitrogenous bases pair with each other using hydrogen bonds. Learn how base pairs keep DNA together and how they help RNA take on useful shapes. Afterwards, you can test your knowledge with a quiz.
Base pairs keep the double strand of DNA together. If we think of DNA as a twisted ladder, each rung is a pair of nitrogenous bases, such as adenine and thymine or guanine and cytosine. Remember that there are four bases in DNA, and their sequence spells out the information DNA carries. You can think of genes as the words or sentences they spell. The pairs are stuck together, not with real covalent bonds, but with a weak attraction we call a hydrogen bond.
Types of Base Pairs
Each nitrogenous base has a partner. In DNA, adenine and thymine pair up, and so do guanine and cytosine.
Adenine-Thymine base pair. Dotted lines show hydrogen bonds. R is where the base attaches to the backbone of DNA.
Adenine-thymine base pair
In the related molecule RNA, thymine is replaced by its close relative uracil, so the pairs are adenine-uracil and guanine-cytosine.
Guanine-Cytosine base pair. Dotted lines show hydrogen bonds. R is where the base attaches to the backbone of DNA.
guanine-cytosine base pair
Structure of Base Pairs
The bases’ ability to pair up comes from their nitrogen atoms (they are called nitrogenous bases, after all!) Each nitrogen (shown above as a letter N) has a pair of electrons that makes it slightly negative, so it can be attracted to the slightly positive hydrogen atoms on its partner (shown as a letter H). We call this a hydrogen bond, and it’s drawn as a dashed line in the illustrations above.
Each base has a place where it attaches to the sugar-phosphate backbone of DNA or RNA. In our ladder analogy, the two backbones are the two vertical parts of the ladder. The hydrogen bonds occur between the base on one strand and its partner on the other strand.
As you see in the illustration, guanine-cytosine pairs are connected by three hydrogen bonds, and adenine-thymine pairs are connected by two. Adenine-uracil bonds, not shown, are also connected by two.
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