18nt

18nt refers to an oligonucleotide that is 18 nucleotides long, and it can be composed of DNA or RNA. In molecular biology and biotechnology, 18-nucleotide sequences are commonly used as primers for PCR and sequencing, as probes in hybridization assays, and as antisense or guiding oligos in various applications. The designation describes length rather than function, and the same term can apply to short DNA or RNA fragments synthesized for a range of experiments.

Synthesis and handling are straightforward. 18nt oligonucleotides are produced by solid-phase synthesis and typically purified by PAGE or HPLC. They may be used with or without chemical modifications, such as fluorescent labels for detection, phosphate or phosphorothioate backbones for stability, or other protective groups depending on the intended assay. The exact sequence determines properties like stability and binding behavior.

Thermodynamics and design considerations are important for effective use. The melting temperature (Tm) of an 18nt oligo depends on its sequence and the salt conditions; GC content strongly influences stability, with higher GC content increasing Tm. For primers and probes, a balanced GC content (roughly 40–60%) and avoidance of strong secondary structures or self-complementarity improve performance. Longer sequences can offer greater specificity, while shorter ones may bind more broadly, increasing the risk of off-target interactions.

Related concepts include other short oligonucleotides of similar length (such as 17mer or 19mer) and standard primers or probes of 18–22 nt that are widely used across molecular biology workflows.