GC Content Calculator
Calculate the GC content of a DNA sequence.
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What it does
The GC content calculator reports what fraction of a DNA sequence is made up of guanine and cytosine, together with the raw counts of G+C, A+T and total bases. It is one of the first numbers molecular biologists check when designing PCR primers, annotating a plasmid, or comparing genomes between species. Only A, C, G and T are counted (case-insensitive); other characters such as N or gaps are excluded from the percentage.
How it works
GC content is a simple composition ratio: GC% = (G + C) / (A + T + G + C) x 100. It matters biologically because a G-C base pair is held together by three hydrogen bonds while an A-T pair has only two, so GC-rich duplexes are more thermally stable and melt at a higher temperature. This is why GC content correlates with melting temperature (Tm), with the stringency needed during PCR, and with structural features such as CpG islands. Because ambiguous codes carry no defined base, they are left out of both numerator and denominator, so the denominator is the number of unambiguous bases rather than the raw string length.
Worked example
Take the 20-mer 5'-GGCATCGCTAGCCATGGTAC-3'. Counting bases gives G = 6, C = 6, A = 4 and T = 4, so G + C = 12 out of 20 total bases. GC% = 12 / 20 x 100 = 60.0%, and the A+T count is 8 (40.0%). A sequence at 60% GC sits at the upper edge of the usual 40-60% window for primers, so it would amplify well but may need a slightly higher annealing temperature.
When to use it
When designing a PCR primer pair, check that each primer falls in the 40-60% GC range: below that the primer binds weakly, above it the primer tends to form stable mispriming products and hairpins. When you receive a new bacterial contig or 16S amplicon, comparing its GC content with the expected value for that taxon (Escherichia coli is about 51%, Streptomyces about 72%) is a quick way to spot contamination or a mis-assigned read. Before ordering a difficult amplicon, a GC value above roughly 65% warns you that the template will need additives such as DMSO or betaine and a hot-start polymerase.
FAQ
- What is a good GC content for PCR primers?
- Aim for 40-60% GC, with about 50% being ideal. Primers below 40% bind too weakly and need a low annealing temperature, while primers above 60% often anneal at partially matched sites and can form stable secondary structures. Also try to keep the GC content of the forward and reverse primer similar so their melting temperatures match.
- Why does GC content affect melting temperature?
- Guanine and cytosine pair through three hydrogen bonds, whereas adenine and thymine pair through two, and GC stacking interactions are also stronger. More G-C pairs therefore mean more energy is needed to separate the two strands, so Tm rises as GC content rises. That is the basis of rules of thumb such as the Wallace formula, which counts 4 degrees per G or C but only 2 degrees per A or T.
- Are N and other ambiguous bases counted in GC content?
- No. Only A, C, G and T are counted, and any other character is excluded from both the base total and the percentage, so a sequence with many Ns will report a GC percentage based on its unambiguous bases only. That is usually what you want, but if a large share of the sequence is ambiguous the percentage becomes less reliable and the sequence should be re-checked.
- What is the GC content of the human genome?
- The human genome averages about 41% GC, but the value varies widely along chromosomes, from AT-rich gene-poor regions near 35% to GC-rich isochores and CpG island promoters well above 60%. Bacterial genomes span a far wider range, from under 20% in some obligate intracellular species to about 75% in some actinobacteria, which is why GC content has long been used as a taxonomic descriptor.