Pyrimidines
- Thymine (T)
- Cytosine (C).
Purines
- Adenine (A)
- Guanine (G)
- We leave out uracil (U) from this tutorial because it is an RNA nitrogenous base.
BASE PAIRING
Adenine-Thymine
- Adenine and thymine pair with each other via TWO hydrogen bonds as follows:
- The hydrogen atom on N3 of THYMINE bonds with N1 of ADENINE.
- An amino group hydrogen on C6 of ADENINE bonds with the carbonyl oxygen on C4 of THYMINE.
Cytosine-Guanine
- Cytosine and guanine pair with each other via hydrogen bonds
- An amino group hydrogen on C2 of GUANINE bonds with the carbonyl oxygen on C2 of CYTOSINE.
- The hydrogen atom bound to N1 of GUANINE bonds with N3 of CYTOSINE.
- An amino group hydrogen on C4 of CYTOSINE bonds with the carbonyl oxygen on C6 of GUANINE.
DNA Base Pairing
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Sugar-Phosphate Backbone
DNA has a fixed backbone that comprises units of:
- Sugar (a deoxyribose)
- Monophosphates
These give each DNA strand directionality: 5' to 3' vs 3' to 5'.
The Nitrogenous Bases
Denote the 4 nitrogen bases.
- The purines: Adenine & Guanine
To remember the purines, think: Larger & think about ATP/GTP (adenine, guanine).
The pyrimidines: Thymine & Cytosine (Uracil is in RNA, only*).
To remember the pyrimidines, think: Smaller & think about the chemistry of UMP to dTMP (thymine) & CTP (cytosine).
Base Pairing
- Adenine pairs with thymine (via two hydrogen bonds).
- Guanine pairs with cytosine (via three hydrogen bonds).