DNA Base Pair & Molecular Weight Calculator

DNA Structure and Properties Guide

What's the key thing to understand about 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 (<30%) GC content as an adaptation to extreme temperatures, since G-C pairs (three hydrogen bonds) are more thermally stable than A-T pairs (two hydrogen bonds) — thermophilic organisms living near hydrothermal vents tend to have higher GC content to keep their DNA double helix stable at high temperature.

What should I know about 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 designed with a Tm several degrees above the hybridisation temperature used in the assay, so the probe binds stably and specifically to its target sequence without dissociating during the reaction. Mismatched or short primers with low Tm are a common cause of failed or non-specific PCR amplification.

What should I know about 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.

What should I know about 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 bias toward or away from GC pairs varies between lineages and is thought to reflect differences in DNA repair mechanisms and replication machinery — this drift in base composition over evolutionary time is one of the reasons genome GC content varies so widely between species, from around 25% in some AT-rich bacteria to over 70% in some GC-rich ones.