The DNA Double Helix and Base Pairing

Watson and Crick's 1953 model established four structural features that remain central to molecular biology and forensic science. First, DNA is a double-stranded helix: two polynucleotide chains coil around a central axis to form a right-handed spiral. Second, the chains are antiparallel: one runs 5' to 3' and the other runs 3' to 5', where 5' and 3' refer to the carbon positions on the deoxyribose sugar to which the phosphate is attached. Third, the bases face inward and the sugar-phosphate backbones face outward: the hydrophobic bases are stacked in the interior, stabilised by base-stacking interactions as well as hydrogen bonds, while the charged backbones are exposed to water. Fourth, base pairing is complementary and specific: A pairs only with T, and G pairs only with C.

The helix makes one complete turn every ten base pairs, which corresponds to a rise of approximately 3.4 nanometres. The diameter of the double helix is approximately 2 nanometres. These dimensions are uniform regardless of the sequence, because A-T and G-C base pairs are the same width. The two strands do not wrap symmetrically, which creates two grooves of different sizes along the outside of the helix: the major groove (wider, about 2.2 nm) and the minor groove (narrower, about 1.2 nm). Proteins that interact with DNA, including the polymerases used in PCR, typically recognise sequence-specific features within the major groove.

Base-stacking interactions, the hydrophobic interactions between adjacent base pairs along the helix axis, contribute as much to DNA stability as hydrogen bonding does, possibly more. A DNA strand with all hydrogen bonds disrupted (fully denatured) but with bases still stacked can re-anneal rapidly. The stacking interactions are why GC-rich sequences are harder to denature: more hydrogen bonds per pair combined with efficient base stacking make G-C pairs collectively more stable.

Each nucleotide in a DNA strand has a deoxyribose sugar at its centre. The carbons of this sugar are numbered 1' through 5'. The base is attached to the 1' carbon; the next nucleotide in the chain is connected by a phosphodiester bond between the 3' carbon of the current nucleotide's sugar and the 5' carbon of the next nucleotide's sugar. This creates directionality: at one end of a strand, the 5' carbon has a free phosphate group (the 5' end); at the other end, the 3' carbon has a free hydroxyl group (the 3' end). By convention, DNA sequences are always written 5' to 3' from left to right.

In the double helix, the complementary strand runs in the opposite direction. If the template strand reads 5'-ATGCGT-3', the complementary strand reads 3'-TACGCA-5', which by the 5' to 3' convention is written 5'-ACGCAT-3'. This antiparallel arrangement is not merely a chemical curiosity: DNA polymerase can only synthesise new DNA in the 5' to 3' direction, which is why PCR primers are designed to have their 3' ends pointing toward the region to be amplified.

Hydrogen bonds form between specific donor and acceptor groups on opposing bases. In an A-T base pair, adenine's amino group at position 6 donates a hydrogen bond to thymine's carbonyl oxygen at position 4, and thymine's N3 nitrogen accepts a hydrogen bond from adenine's N1. This gives A-T two hydrogen bonds per pair. In a G-C base pair, guanine's N1-H donates to cytosine's N3, cytosine's amino group at position 4 donates to guanine's carbonyl at position 6, and guanine's carbonyl at position 2 accepts from cytosine's amino group at position 2. This gives G-C three hydrogen bonds per pair.

Each individual hydrogen bond has an energy of around 2 to 5 kilojoules per mole, which is weak by chemical bond standards. But a human genome contains approximately 3.2 billion base pairs, each contributing base-stacking and hydrogen-bonding interactions. Collectively, these weak interactions produce a molecule that is highly stable across a wide range of temperatures, pH values, and ionic concentrations, far more stable than RNA or proteins under equivalent conditions.

The specificity of the base pairs is the reason PCR amplification works as a forensic tool. Two short DNA sequences (primers) are designed to be complementary to the two flanking regions of an STR locus. When the reaction is cooled after denaturation (the annealing step), the primers bind only to their correct complementary sequences because A-T and G-C are the only geometrically and energetically favoured pairings. Mismatches, where a primer base faces a non-complementary template base, introduce distortions that prevent stable hybridisation at the annealing temperature.

Biological evidence at a crime scene is exposed to physical, chemical, and biological agents that break down macromolecules. Proteins denature within hours to days in ambient conditions and are rapidly digested by environmental proteases. RNA has a half-life of minutes in the presence of ubiquitous ribonucleases. DNA is dramatically more stable, and four structural features explain why.

• No 2'-hydroxyl group. RNA carries a hydroxyl group at the 2' carbon of each ribose sugar; in the presence of metal ions or alkaline conditions, this group attacks the adjacent phosphodiester bond in a rapid intramolecular reaction that cleaves the backbone. DNA lacks this group entirely, removing the main hydrolytic vulnerability that degrades RNA.
• Complementary strand redundancy. The double-stranded structure means that if one strand is nicked, oxidised, or partially degraded, the complementary strand preserves the sequence information and can serve as a repair or amplification template. Single-stranded molecules lack this backup.
• Stacked hydrophobic interior. The bases are buried inside the helix, away from water and oxidising agents. The hydrophobic core resists enzymatic attack from most nucleases, which need access to the bases to cleave efficiently.
• Protein packaging in cells. In living cells, nuclear DNA is tightly wound around histone proteins and further organised into chromatin. Even after cell death, residual protein binding and the compact chromatin structure provide physical protection, especially in dense tissues like bone and teeth, where mineral encases the DNA.

Bone and teeth are the most stable biological substrates for DNA recovery because the mineral matrix (hydroxyapatite) physically protects the DNA inside the dentinal tubules and osteon channels from mechanical disruption, UV radiation, and microbial attack. Skeletal remains from ancient and historical contexts have yielded amplifiable DNA thousands of years old. Blood and semen dried on absorbent fabric can retain amplifiable DNA for decades if kept dry and away from UV light. Touch DNA deposited on hard surfaces is at much greater risk, because the cell material is thin, exposed, and vulnerable to UV, humidity, and contact transfer.

DNA degradation in biological evidence follows several overlapping pathways. Hydrolysis, the cleavage of chemical bonds by water, attacks both the phosphodiester backbone (producing strand breaks) and the glycosidic bonds attaching bases to the sugar (producing abasic sites, also called apurinic or apyrimidinic sites). Oxidative damage from reactive oxygen species, produced during cellular decay and from environmental exposure, causes base modifications, particularly 8-oxoguanine, which is one of the most common forms of oxidative DNA damage and causes miscoding during PCR amplification. Microbial and fungal nucleases degrade DNA when microorganisms colonise decomposing biological material.

The practical consequence of degradation is DNA fragmentation: the long, intact chromosomal molecules are broken into progressively shorter fragments over time. STR profiling requires fragments typically 100 to 400 base pairs long for full-profile amplification. As degradation advances, larger alleles fail to amplify before smaller ones (a phenomenon called allelic dropout, or partial dropout when only some alleles are lost). When average fragment length falls below 100 to 150 base pairs, even miniSTR systems and SNP-based methods are required. Ancient DNA and skeletal remains from mass fatality incidents commonly present in this state.

Cold and dry conditions dramatically slow all degradation pathways. This is why evidence stored at minus 20 degrees Celsius in a dry environment can retain amplifiable DNA almost indefinitely, and why frozen or desiccated ancient specimens yield DNA when comparable specimens from warm, humid burial sites do not. Evidence collection and storage protocols, including the use of paper bags rather than plastic (which traps moisture), drying stains before sealing, and refrigerating or freezing samples rapidly, are direct applications of understanding DNA's chemical vulnerabilities.

Every core forensic DNA typing method depends directly on the base-pairing rules of the double helix. PCR uses oligonucleotide primers, short single-stranded sequences typically 15 to 30 nucleotides long, that anneal by complementary base pairing to specific flanking sequences on either side of a target locus. After annealing, a thermostable DNA polymerase extends from the 3' end of each primer, synthesising a new strand complementary to the template and replicating the target region. Repeated cycles of denaturation, annealing, and extension produce exponential amplification.

STR profiling, the primary identification method used by forensic laboratories in the United States (CODIS system, 20 core loci), the United Kingdom (National DNA Database), the European Union (ENFSI-recommended loci), India (under the DNA Technology (Use and Application) Regulation Act 2019, which established the National DNA Data Bank), and other jurisdictions, targets short tandem repeats: sequences of 2 to 6 base pairs that repeat in tandem at a locus. The number of repeats at a locus varies among individuals, creating alleles that differ in length. PCR amplifies these loci using fluorescently labelled primers; capillary electrophoresis separates the products by size; and the allele sizes at each locus are compared between a crime scene sample and a reference sample to generate a match probability.

Next-generation sequencing methods, increasingly used for forensic purposes including massively parallel sequencing-based STR typing, mitochondrial DNA sequencing for degraded samples, and single nucleotide polymorphism (SNP) panels for ancestry and phenotyping, also depend on base complementarity at every step: library preparation, primer annealing, template hybridisation, and base-by-base sequencing. The double helix is not just a structural feature of DNA; it is the mechanism by which forensic typing methods read genetic information.

For deeper coverage of how these methods are applied to specific evidence categories, see Forensic Serology and Forensic Biotechnology. For the population genetics and statistics that convert a DNA match into a match probability, see Chromosomes, Genes, and the Human Genome.