Nitrogenous Bases
As we've covered nucleotides are made up of nitrogenous bases, a sugar and a phosphate group. There are two types of nitrogenous bases:
- Pyrimidines [Cytosine (C), Uracil (U) and Thymine (T)]
- You can use the acronym "CUT" to remember which bases are pyrimidines.
- Purines [Guanine (G) and Adenine (A)]
- You can us the acronym "GA" as in Purines are a city in Georgia (GA) to remember which bases are purines.
The Nitrogenous bases in DNA pair up with hydrogen bonds in the following ways:
- Adenine (A) with Thymine (T)
You can use the acronym "AT" as in Appalachian Trail to remember this pair.
- Guanine (G) with Cytosine (C)
You can use the acronym "GC" as in General Contractor to remember this pair.
Note: In RNA, Thymine is replaced by Uracil (U)
- A and U pair up
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Cytosine
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Uracil
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Thymine
|
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Guanine |
Adenine |
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Figure 14. The top of this image depicts the detailed chemical structure of Pyrimidines (Cytosine, Uracil and Thymine). Pyrimidines only have one central hexagonal ring structure. The bottom of the image depcts the detailed chemical structure of Purines (Guanine and Adenine). Purines have a central hexagonal ring structure fused to a pentagonal ring. Due to their shapes, pyrimidine nucleotides always pair with purine ones.
