Basepair (bp)


A basepair is a single subunit of double‐stranded nucleic acid, consisting of two nucleotides in opposite strands linked by hydrogen bonds. Only certain combinations of nucleotides can form base pairs, so the sequence of nucleotides in one strand is complementary to the sequence in the other. Base pairing is essential not just for maintaining the double‐helical structure of DNA but also for the replication of nucleic acids, for making an RNA copy of a DNA strand, and for repairing damage to DNA. So, while the sequence of nucleotides within a strand encodes information, it is base pairing between strands that allows nucleic acids to perform most of their functions. Base pairing also underpins many aspects of biotechnology, such as DNA sequencing.

Key Concepts

  • Base pairing is the formation of hydrogen bonds between the bases (subunits) of two strands of DNA or RNA.
  • Only certain combinations of bases (complementary pairs) can form basepairs.
  • When DNA is replicated, or when an RNA copy is made in a process called transcription, base pairing ensures that the new DNA or RNA strand is a mirror image, or complement, of the old strand.
  • When damaged DNA is repaired, base pairing is used to reconstruct a damaged strand using the undamaged strand as a template.
  • Base pairing is also exploited in many aspects of biotechnology, such as the polymerase chain reaction (PCR) and DNA sequencing.

Keywords: DNA; nucleic acids; double helix; RNA; nucleotide; basepair

Figure 1. (a) Structure of double‐stranded DNA. The backbones of the two intertwined strands, consisting of alternating deoxyribose and phosphate groups, are shown schematically as helices. The bases form pairs between the backbones, linked by hydrogen bonds (represented as white ellipses), with approximately 10 pairs per turn of the double helix. (b) The four basepairs. Adenine and guanine are both purines, each consisting of a linked five‐ and six‐atom ring of carbon and nitrogen. Thymine and cytosine are pyrimidines, consisting of a six‐atom ring alone. The points at which each base attaches to the backbone are indicated by arrows: the G:C and A:T basepairs have nearly identical attachment points, and this symmetry is preserved when they are flipped to give C:G and A:T pairs. It is this feature that allows any sequence of bases to be accommodated within a regular double helix.


Watson J and Crick FHC (1953) Molecular structure of nucleic acid: a structure for deoxyribose nucleic acid. Nature 171: 737–738.

Further Reading

Watson J (2001) The Double Helix: A Personal Account of the Discovery of the Structure of DNA. New York: Touchstone Books.

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Dear, Paul H(Jan 2018) Basepair (bp). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005929.pub2]