DNA Structure and Base Pairing
Summary
Deoxyribonucleic acid (DNA) is a directional polymer whose common duplex form contains two complementary, antiparallel strands. Canonical Watson–Crick base pairs match adenine with thymine and guanine with cytosine. The order of bases determines not only the chemical groups exposed in the DNA grooves but also local conformational and mechanical properties of the duplex. These properties provide physical information that DNA-binding proteins can recognize.
Scope
Covered
- DNA strand direction, complementarity, and reverse complements.
- Canonical base pairing and the major and minor grooves of duplex DNA.
- Sequence-dependent base stacking, shape, and deformability.
Not covered
- DNA replication, transcription, chromatin organization, or gene regulation.
- Noncanonical DNA structures in detail.
- Any particular protein's sequence preference.
Key concepts and notation
| Term or symbol | Definition | Notes |
|---|---|---|
| Nucleotide | A nitrogenous base, deoxyribose sugar, and phosphate group | DNA commonly uses A, C, G, and T |
| (5') and (3') | Ends defined by carbon positions in deoxyribose | A sequence is conventionally written (5'\rightarrow3') |
| Complement | Base substitution A↔T and C↔G | Defined position by position |
| Reverse complement | Complement followed by reversal | Written in the same (5'\rightarrow3') convention as the original strand |
| Base pair | Two bases paired across a duplex | Canonical pairs are A·T and G·C |
| Base-pair step | Two consecutive base pairs | Its geometry depends on both pairs and neighboring sequence |
Core knowledge
Directional, antiparallel strands
Phosphodiester bonds connect nucleotides into a strand with chemically different (5') and (3') ends. In the common double-helical form, the two strands run in opposite directions. If one strand is (5')-A G T C-(3'), its paired strand is (3')-T C A G-(5'), which is written (5')-G A C T-(3') when reported in the conventional direction. This operation is the reverse complement, and applying it twice returns the original sequence [1,2].
For ordinary double-stranded DNA, a sequence and its reverse complement describe opposite orientations of the same base-pair tract. Orientation can still matter when neighboring DNA, asymmetric protein complexes, chemical labels, or the experimental construct distinguish the two directions.
Canonical pairing and helix stabilization
Watson–Crick pairing matches A with T and G with C while maintaining a regular duplex geometry [1]. Hydrogen bonding contributes pairing specificity, whereas base stacking, solvent, ions, and the sugar-phosphate backbone all contribute to duplex stability. Counting hydrogen bonds alone is therefore not a complete description of sequence-dependent stability.
Grooves expose sequence information
The geometry of the double helix produces a major groove and a minor groove. The edges of base pairs expose patterns of hydrogen-bond donors, acceptors, nonpolar groups, and electrostatic potential. The major groove distinguishes the four Watson–Crick base-pair orientations more directly than the minor groove, while minor-groove width and electrostatic potential can carry sequence-dependent structural information [3,4].
Sequence-dependent structure
DNA is not a perfectly uniform cylinder. Base stacking and backbone constraints make parameters such as roll, twist, slide, propeller twist, minor-groove width, and bending propensity depend on sequence and context [3-5]. A nucleotide can therefore influence the physical presentation of neighboring bases. Dinucleotide and longer contexts are often needed to describe these effects; isolated single-base identities do not determine all local geometry.
Conditions, limitations, and uncertainty
- B-DNA is the prevalent reference form under many physiological conditions, but DNA can adopt other conformations and noncanonical base pairs.
- Salt concentration, temperature, pH, chemical modification, mismatches, and supercoiling can change duplex stability and shape.
- Reverse-complement equivalence is a property of an unoriented duplex tract, not a guarantee that every biological or experimental system treats both orientations identically.
- General shape tendencies do not uniquely determine a protein's binding preference; protein structure and assay conditions also matter.
Related knowledge resources
transcription_factor_dna_binding: how proteins read base chemistry and DNA shape.binding_sites_motifs_and_sequence_context: representations of recurring sequence preferences.
References
- Watson JD, Crick FHC. Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid. Nature. 1953;171:737–738. https://doi.org/10.1038/171737a0. [Primary research]
- Dickerson RE. The DNA helix and how it is read. Scientific American. 1983;249:94–111. https://doi.org/10.1038/scientificamerican1283-94. [Review]
- Rohs R, Jin X, West SM, Joshi R, Honig B, Mann RS. Origins of specificity in protein-DNA recognition. Annual Review of Biochemistry. 2010;79:233–269. https://doi.org/10.1146/annurev-biochem-060408-091030. [Review]
- Slattery M, Zhou T, Yang L, Dantas Machado AC, Gordân R, Rohs R. Absence of a simple code: how transcription factors read the genome. Trends in Biochemical Sciences. 2014;39:381–399. https://doi.org/10.1016/j.tibs.2014.07.002. [Review]
- Hunter CA. Sequence-dependent DNA structure: the role of base stacking interactions. Journal of Molecular Biology. 1993;230:1025–1054. https://doi.org/10.1006/jmbi.1993.1217. [Primary research]