GC Content Calculator for DNA & RNA Sequences

GC content is the fraction of a nucleic acid made of guanine and cytosine. It matters physically because a G-C pair forms three hydrogen bonds against two for A-T, and because purine-pyrimidine stacking energies differ, so GC-rich duplexes are more thermally stable. Genomic values span an extraordinary range, from about 19 percent in Plasmodium falciparum to above 72 percent in Streptomyces coelicolor, with the human genome near 41 percent. Melting temperature follows directly: this tool applies the Wallace rule of 2 times the A plus T count plus 4 times the G plus C count for sequences of 14 bases or fewer, and the Marmur-Doty style relation Tm equals 81.5 plus 0.41 times percent GC minus 675 divided by length for longer ones, both assuming standard salt and neither substituting for a nearest-neighbor calculation on a real primer. The derived metrics answer different questions. GC skew, defined as G minus C over G plus C, measures strand asymmetry rather than composition; in most bacterial chromosomes it is positive on the leading strand and flips sign at the replication origin and terminus, which is how origins are located computationally. The CpG observed-to-expected ratio compares actual CG dinucleotide counts against the product of C and G frequencies, and it is depressed across vertebrate genomes because methylated cytosine in a CpG context deaminates to thymine; a region above 200 bp with GC over 50 percent and a ratio above 0.6 meets the usual CpG island definition. Shannon entropy over the four base frequencies, scaled against a maximum of 2 bits, flags low-complexity or homopolymeric tracts. Sliding-window analysis exists because a single average conceals structure, with 100 to 1000 bp windows typical for genomic work and 10 to 50 bp for oligonucleotides.

Base composition was among the first quantitative facts known about DNA. Erwin Chargaff and his coworkers at Columbia University, using paper chromatography and ultraviolet spectrophotometry in the late 1940s, established both that adenine equals thymine and guanine equals cytosine, and that the GC proportion varies characteristically between species while staying constant within one. Published around 1950, the second observation established base composition as a species-specific character and made it a usable taxonomic marker years before sequencing existed. Measuring it required indirect physical methods. Cesium chloride density gradient centrifugation, developed by Matthew Meselson, Franklin Stahl, and Jerome Vinograd in the late 1950s, separated DNA by buoyant density, which rises with GC content. In 1962 Julius Marmur and Paul Doty published the linear relationship between melting temperature and GC fraction, giving the thermal denaturation method that let any laboratory with a heated spectrophotometer estimate composition, and supplying the empirical Tm formula still quoted for long sequences. Two later contributions defined the derived statistics computed here: Miriam Gardiner-Garden and Marianne Frommer set out the operational definition of CpG islands in the Journal of Molecular Biology in 1987, using the 200 bp length, 50 percent GC, and 0.6 observed-to-expected thresholds; and Jean Lobry described GC skew asymmetry around bacterial replication origins in 1996, turning a compositional curiosity into a genome annotation tool. Direct sequencing has since made composition trivial to compute, but the interpretive framework predates it entirely.