GC含量是在所研究的对象（例如放线菌）的全基因组中，（鸟嘌呤）（Guanine）和胞嘧啶（Cytosine）所占的比例。一种生物的基因组或特定DNA、RNA片段有特定的GC含量。

在DNA链中G和C是以三个氢键相连，而T和A则是两个氢键相连的。氢键的多少体现连接的能量，氢键多的不容易被打断。

在双链DNA中，腺嘌呤与胸腺嘧啶（A/T）之比，以及鸟嘌呤与胞嘧啶（G/C）之比都是1。但是，（A+T）/（G+C）之比则随DNA的种类不同而异。GC含量愈高，DNA的密度也愈高，同时热及碱不易使之变性，因此利用这一特性便可进行DNA的分离或测定。

In molecular biology and genetics, GC-content (or guanine-cytosine content) is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine (from a possibility of four different ones, also including adenine and thymine). This may refer to a specific fragment of DNA or RNA, or that of the whole genome. When it refers to a fragment of the genetic material, it may denote the GC-content of part of a gene (domain), single gene, group of genes (or gene clusters), or even a non-coding region. G (guanine) and C (cytosine) undergo a specific hydrogen bonding, whereas A (adenine) bonds specifically with T (thymine).