What is GC content?
GC content is the percentage of bases in a DNA or RNA sequence that are guanine (G) or cytosine (C). It is calculated as (G + C) / (A + T + G + C) × 100. Ambiguous bases such as N are excluded from the denominator, so a stretch of unknown sequence does not distort the result.
GC pairs are joined by three hydrogen bonds rather than two, so GC-rich DNA is more thermally stable. GC content therefore matters for PCR primer design, sequencing bias and genome annotation.
Why use a sliding window?
A single percentage hides local variation. A sliding-window profile computes GC content over consecutive windows along the sequence, revealing CpG islands, GC-rich promoters, horizontally transferred regions in bacteria and regions that may sequence poorly.
Smaller windows show finer detail but are noisier; for a gene-length sequence a window of 50 to 100 bases is a reasonable start.
Typical values
Genome-wide GC content ranges from below 25% in some bacteria to above 70% in others. The human genome averages about 41%, while human coding regions are typically higher. Comparing a sequence against the background of its organism is more informative than an absolute value.