BRAINCAKEComplementary Base Pairing
The two strands of a DNA double helix are held together by hydrogen bonds between the bases. The bases pair in a fixed way, called complementary base pairing:
• adenine (A) pairs with thymine (T), joined by two hydrogen bonds
• cytosine (C) pairs with guanine (G), joined by three hydrogen bonds
Each pair is always one purine (A or G, two rings) with one pyrimidine (T or C, one ring). This keeps the distance between the two backbones the same all along the molecule, so the helix has a constant width.
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Key terms in this lesson
- purine
- A nitrogenous base with a double-ring structure; adenine and guanine are the purine bases in DNA and RNA.
- pyrimidine
- A nitrogenous base with a single-ring structure; cytosine, thymine and uracil are pyrimidine bases.
More in DNA, Genes and Protein Synthesis
- Nucleotides
- ATP
- Nucleic Acids
- Phosphodiester Bonds
- Evidence for Semi-Conservative DNA Replication
- DNA Replication
- The Genetic Code
- Types of RNA
All 20 lessons in DNA, Genes and Protein Synthesis · All Edexcel AS-level Biology topics