Building blocks of DNA
Each DNA strand is assembled from repeating nucleotide units, making DNA a polynucleotide.
Every DNA nucleotide contains three components:
- a phosphate ion
- 2-deoxyribose, a pentose sugar
- one nitrogen-containing base
The four bases found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T).
The base is connected to the 1′ carbon of 2-deoxyribose through a ring nitrogen, while the phosphate is linked through the oxygen-containing 5′ group.
The detailed structures of the phosphate ion, 2-deoxyribose and the bases are provided in the Chemistry Data Booklet, so the important skill is recognising their correct connections.
| Bases | Type | Connection to the sugar |
|---|---|---|
| Adenine and guanine | Purines | Through a ring N in the five-membered part of the base |
| Cytosine and thymine | Pyrimidines | Through a ring N in the six-membered ring |
Exam Tip: A complete DNA nucleotide contains phosphate, 2-deoxyribose and one base. When joining structures from the Data Booklet, check both the attachment point on the sugar and the ring nitrogen used by the base.