Complementary base pairing is the specific pairing of nitrogenous bases in nucleic acids: adenine pairs with thymine in DNA, while cytosine pairs with guanine. In RNA, uracil replaces thymine and pairs with adenine. This is required at both IB Biology SL and HL.
The mechanism
Each nitrogenous base has a chemical structure that allows it to form hydrogen bonds only with its complementary base. Complementary bases therefore fit together consistently and specifically.
| Nucleic acid context | Complementary pairs | Biological role |
|---|---|---|
| Double-stranded DNA | A to T and C to G | Holds the two antiparallel DNA strands together |
| DNA replication | A to T and C to G | Allows each original strand to act as a template for a new strand |
| Transcription from DNA to RNA | DNA A to RNA U, DNA T to RNA A, DNA C to RNA G, DNA G to RNA C | Produces an RNA sequence complementary to the DNA template strand |
| Translation | Complementary mRNA codon to tRNA anticodon pairing | Positions the correct amino acid during polypeptide synthesis |
During semi-conservative DNA replication, helicase separates the two DNA strands by breaking the hydrogen bonds between the bases. Each original strand acts as a template. Free nucleotides align by complementary base pairing, and DNA polymerase links them into a new strand. For example, the DNA sequence A–C–G–T determines the complementary sequence T–G–C–A.
Complementarity therefore allows genetic information to be copied accurately and expressed through transcription and translation. A common misconception is that complementary bases are joined by covalent bonds. The bases are joined between strands by hydrogen bonds; covalent bonds form the sugarphosphate backbone within each strand.
Exam technique
For an IB explain question, do more than list AT and CG. Link specific pairing to template-directed nucleotide arrangement and explain how this enables accurate replication or gene expression. In transcription questions, remember that RNA contains uracil, not thymine, and use the DNA template strand given.
