DNA and RNA each contain four nitrogenous bases. DNA contains adenine, thymine, cytosine and guanine, whereas RNA contains adenine, uracil, cytosine and guanine. This is required at both SL and HL in IB Biology A1.2 Nucleic acids.
| Nucleic acid | Four bases | Complementary pairs |
|---|---|---|
| DNA | Adenine (A), thymine (T), cytosine (C), guanine (G) | AtoT and CtoG |
| RNA | Adenine (A), uracil (U), cytosine (C), guanine (G) | AtoU and CtoG |
A nitrogenous base is one component of a nucleotide. Each nucleotide also contains a pentose sugar and a phosphate group. The sugar and phosphate form the covalently bonded backbone, while the sequence of bases carries genetic information.
DNA and RNA share three bases: adenine, cytosine and guanine. The key difference is that DNA normally uses thymine, whereas RNA uses uracil instead. Therefore, five different bases occur across DNA and RNA altogether, but each nucleic acid contains only four.
In double-stranded DNA, complementary base pairing links the two antiparallel strands through hydrogen bonds. Adenine pairs with thymine, while cytosine pairs with guanine. During transcription, an RNA strand is synthesized from a DNA template, so adenine on the DNA template pairs with uracil in RNA.
For example, the DNA template sequence TACG produces the complementary RNA sequence AUGC. By contrast, its complementary DNA sequence would be ATGC.
A common misconception is that RNA contains all four DNA bases plus uracil. This is incorrect: uracil replaces thymine, so RNA still has four bases.
Exam technique: For a question asking you to distinguish DNA from RNA, state both sides explicitly: “DNA contains thymine but not uracil, whereas RNA contains uracil but not thymine.” If constructing a complementary RNA sequence, use U rather than T and check every base individually.
