dsDNA Calculators

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Double-stranded DNA (dsDNA) is the predominant form of DNA in cells — two antiparallel polynucleotide strands held together by Watson-Crick base pairing: adenine pairs with thymine (A=T, 2 hydrogen bonds) and guanine pairs with cytosine (G≡C, 3 hydrogen bonds). The two strands run antiparallel (one 5'→3', the other 3'→5'). B-DNA, the predominant form under physiological conditions, forms a right-handed double helix with ~10.5 base pairs per turn and a rise of 0.34 nm per base pair. dsDNA is quantified by UV absorbance at 260 nm (A₂₆₀ = 1.0 ≈ 50 μg/mL) or by fluorescent dyes (PicoGreen, SYBR Gold).

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B-DNA Structure Parameters

Helix sense: right-handed. Base pairs per turn: 10.5 bp. Rise per base pair: 0.34 nm. Pitch (helix repeat): 10.5 × 0.34 = 3.57 nm. Major groove width: ~12 Å (protein binding site). Minor groove width: ~6 Å. Diameter: ~2 nm (20 Å).

Watson-Crick Base Pairing

A=T: 2 hydrogen bonds. G≡C: 3 hydrogen bonds (GC more stable). Complementarity: 5'-ATGCGCAT-3' paired with 3'-TACGCGTA-5'. Each strand template for replication and transcription.

Melting Temperature

Short duplexes: Tm ≈ 2°C × (A+T) + 4°C × (G+C). Longer DNA: Tm = 81.5 + 16.6 × log[Na⁺] + 0.41 × %GC − 675/length. Higher GC → higher Tm. 3 H-bonds per GC vs. 2 per AT.

Quantification

A₂₆₀ = 1.0 → ~50 μg/mL dsDNA. A₂₆₀/A₂₈₀ ≈ 1.8–2.0 for pure dsDNA. PicoGreen fluorometry: more sensitive; specific for dsDNA; preferred for low concentrations (< 5 ng/μL).

Glossary

dsDNA (Double-Stranded DNA)
Two antiparallel complementary strands held by Watson-Crick base pairs (A=T, 2 H-bonds; G≡C, 3 H-bonds); B-form: right-handed helix, 10.5 bp/turn, 0.34 nm rise; A₂₆₀ = 1.0 ≈ 50 μg/mL.
Watson-Crick Base Pairing
A=T (2 hydrogen bonds) and G≡C (3 hydrogen bonds); antiparallel complementary strands; governs DNA replication and transcription fidelity; higher GC content → higher Tm.
Melting Temperature (Tm)
Temperature at which 50% of dsDNA is single-stranded; short oligos: Tm ≈ 2×(A+T) + 4×(G+C); long DNA: 81.5 + 16.6×log[Na⁺] + 0.41×%GC − 675/length.

Frequently Asked Questions

Double-stranded DNA (dsDNA): two complementary antiparallel strands held together by hydrogen bonds (Watson-Crick base pairing). A pairs with T (2 H-bonds); G pairs with C (3 H-bonds). Standard form in genomes of cells and most viruses. Single-stranded DNA (ssDNA): one strand only; less stable; found as intermediates during replication, as primers, and in some viruses (e.g., M13, ΦX174). Properties differ: dsDNA has higher melting temperature; more rigid structure; 50 μg/mL per A₂₆₀ unit. ssDNA: 33 μg/mL per A₂₆₀; more flexible; different secondary structures (hairpins, G-quadruplexes).

B-DNA (most common, physiological conditions): right-handed double helix; 10.5 bp/turn; rise 0.34 nm/bp; major groove wide (protein binding); most transcription factors and restriction enzymes bind B-form DNA. A-DNA (high salt or dehydrated conditions): right-handed; more compact; 11 bp/turn; deeper, narrower major groove; found in DNA-RNA hybrids and in RNA double-strands. Z-DNA (alternating purine-pyrimidine sequences, high salt): left-handed; zigzag backbone; 12 bp/turn; biological role controversial; may regulate gene expression. Cruciform DNA: forms at palindromic sequences (inverted repeats) under supercoiling stress → hairpin structures.

G-C base pairs form 3 hydrogen bonds (vs. 2 for A-T) → require more energy to melt. Higher GC content → higher Tm. Simple approximation for short oligos: Tm ≈ 2°C×(A+T) + 4°C×(G+C). For longer DNA: Tm = 81.5 + 16.6×log[Na⁺] + 0.41×%GC − 675/length. Practical implications: PCR amplification of GC-rich regions (> 65% GC) requires: higher denaturation temperature (95–98°C); longer denaturation times; additives (DMSO, betaine, GC-enhancer) to reduce secondary structure. DNA melting analysis (HRMA, high-resolution melt): uses Tm difference to distinguish DNA variants including SNPs and methylation differences.

UV absorbance (NanoDrop): A₂₆₀ = 1.0 corresponds to ~50 μg/mL dsDNA. Concentration = A₂₆₀ × 50 × dilution factor. Purity: A₂₆₀/A₂₈₀ ≈ 1.8–2.0 (pure dsDNA); < 1.7 = protein or phenol contamination; A₂₆₀/A₂₃₀ ≈ 2.0–2.2 (< 1.8 = organic contamination). Limitations: RNA and ssDNA also absorb at 260 nm; overestimates DNA if RNA is present. Fluorometric (Qubit, PicoGreen): PicoGreen dye binds specifically to dsDNA → fluorescence; highly sensitive (0.01–100 ng/μL); not affected by RNA or ssDNA contamination; preferred for downstream applications (NGS, sensitive cloning).