DNA Base Pair & Molecular Weight Calculator
Calculate DNA properties including molecular weight, GC content, melting temperature, and base pair composition from sequence length or actual sequence.
DNA Structure and Properties Guide
DNA Base Pairing Rules
DNA follows Watson-Crick base pairing: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds. Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds. In RNA, Uracil (U) replaces Thymine. Chargaff's rules: in any double-stranded DNA, %A = %T and %G = %C. If GC content is 60%, then G = C = 30% each, A = T = 20% each. The human genome has approximately 40% GC content. Highly expressed genes tend to have higher GC content. Bacteria in extreme environments often have very high (>70%) or very low
DNA Melting Temperature
The melting temperature (Tm) is the temperature at which 50% of double-stranded DNA is single-stranded. G-C pairs have 3 hydrogen bonds (stronger than A-T's 2), so higher GC content raises Tm. Simple approximation: Tm = 4(G+C) + 2(A+T) in °C (for short oligonucleotides). For longer sequences with salt correction: Tm = 81.5 + 16.6 × log[Na⁺] + 0.41 × (%GC) − 675/length. Applications: PCR primer design requires primers with Tm in a similar range (typically 55-65°C). Hybridisation probes must be de
DNA Molecular Weight
Average molecular weight per base pair (dsDNA): approximately 650 Da (daltons) or 0.65 kDa. For dsDNA: MW ≈ 650 × bp (g/mol). More precisely: the four deoxyribonucleotide monophosphates have weights of 308 (dA), 332 (dC), 347 (dG), 323 (dT). Sodium salt form (common in molecular biology): add 22 Da per residue for Na⁺. Typical molecular biology sizes: PCR product 500 bp → 325 kDa. Gene 3,000 bp → 1.95 MDa. Human chromosome (average) → 150 MDa. E. coli genome 4.6 Mb → 3 GDa.
GC Content and Biology
GC content has important biological effects: gene expression: promoters and regulatory regions often have distinctive GC patterns. GC-rich regions form more stable secondary structures in RNA. Methylation: CpG dinucleotides (CG sequences) are methylation sites in mammals — CpG islands near gene promoters are often unmethylated in active genes. Codon usage: organisms with high GC content use different codons to code for the same amino acid (codon degeneracy allows this). Evolution: mutational bia
Recommended for this calculator