singlestranded

Single-stranded describes a molecule composed of a single nucleic acid chain, as opposed to a double-stranded molecule that contains two complementary strands. The term is most often applied to single-stranded DNA (ssDNA) and single-stranded RNA (ssRNA).

In biology, ssDNA occurs transiently during replication and transcription when the double helix temporarily unwinds. It is stabilized by single-stranded DNA-binding proteins and by intramolecular base pairing that can form secondary structures such as hairpins. Compared with double-stranded DNA, ssDNA is more chemically reactive and more susceptible to nuclease attack. Single-stranded RNA is the primary form of most cellular RNA and serves as the genetic material for many viruses; it can fold into complex secondary and tertiary structures that influence function and regulation. Positive-sense ssRNA genomes can function directly as mRNA, while negative-sense ssRNA genomes require transcription before translation.

Laboratory and clinical contexts frequently rely on single-stranded nucleic acids. Synthetic oligonucleotides are used as primers for polymerase chain reactions and sequencing, probes for hybridization assays, and as antisense or aptamer agents. Denaturation and controlled annealing of nucleic acids exploit their single-stranded states, with stabilization provided by binding proteins or chemical modifications when needed.

Examples of pathogens with ssRNA genomes include many picornaviruses and coronaviruses, while ssDNA viruses include families such as Circoviridae and Parvoviridae. The concept of single-stranded nucleic acids contrasts with double-stranded forms, which are typically more stable and less prone to rapid degradation.