How GC content affects PCR and sequencing

The ratio of G/C to A/T bases is not just a number — it changes how DNA behaves in a tube and on a sequencer. Extreme regions amplify and sequence unevenly, even with perfect input.

What GC content is

GC content is the percentage of bases in a sequence that are G or C. Most genomes sit somewhere around 40–60%, which is considered balanced. Anything much above that — say 70%+ — is GC-rich, and anything well below — under about 30% — is AT-rich. Both extremes cause trouble, for opposite reasons.

Why the extremes are hard

G and C bases pair with three hydrogen bonds; A and T pair with two. So GC-rich DNA is held together more tightly: it is harder to denature, and it tends to fold into stable secondary structures that block polymerases — which is why GC-rich templates are notoriously hard to amplify. AT-rich DNA has the opposite problem: it melts easily, so primers bind more weakly and less specifically, which can reduce yield and raise mispriming.

Why this shows up as uneven coverage

During the amplification steps of library prep and cluster generation, regions that amplify efficiently end up over-represented and regions that resist amplification end up under-represented. The result is GC bias: even starting from equal amounts of DNA, GC-rich and AT-rich regions are sequenced less efficiently than balanced ones, so their read depth sags — sometimes all the way to dropout.

A concrete contrast

Take a balanced region like AATTGGCC (50% GC) and a GC-rich region like GGCCGCGATA (70% GC). On paper they are just sequence; in practice the 50% region amplifies and sequences cleanly at roughly the depth you planned, while the 70% region is more likely to show reduced, patchy coverage — a visible dip in the depth track, or in the worst case a gap. If you measure depth across many regions, you typically see coverage fall off as GC content climbs toward either extreme.

GC content also feeds directly into melting temperature, so a GC-rich primer runs hotter than an AT-rich one of the same length — another reason to check it during primer design.