In CRISPR guide RNA design, why is GC content considered?

• A. It controls the plasmid copy number
• B. It affects binding stability with the target DNA
• C. It fixes off-target effects completely
• D. It determines the color of the fluorescent tag

Answer: (B)

GC content matters because it governs how stably the guide RNA pairs with its target DNA. Guanine–cytosine pairs form three hydrogen bonds, while adenine–thymine pairs form two, so higher GC content raises the melting temperature of the RNA–DNA duplex. That means guides with more GC tend to bind more tightly, which can improve cleavage efficiency by keeping the Cas9–guide complex securely attached to the target. At the same time, too much GC can cause the guide RNA to fold into secondary structures or bind too aggressively, reducing accessibility to the target and potentially increasing off-target risks. So designers aim for a moderate GC content to balance strong enough binding with good availability of the target site. This concept is why GC content is considered in guide RNA design.