In genetics, the guanine-cytosine content (GC content) is the ratio of guanine and cytosine to the total number of nucleotides of a given genome. It is used, among other things, to easily distinguish between different species when a doubt remains. The GC-content of Yeast (Saccharomyces cerevisiae), for example, is 38%, that of a (rather boring) plant used in research (Arabidopsis thaliana) is 36%.

The GC content can be measured by several means but one of the simplest methods is to measure what is called the melting temperature of the DNA double helix with a spectrophotometer. The absorbance of DNA at a wavelength of 260 nm increases fairly sharply when the double-stranded DNA separates into two single strands when sufficiently heated.