ssDNA

Single-stranded DNA (ssDNA) is a polymer of nucleotides linked by a sugar-phosphate backbone, with the bases adenine, cytosine, guanine, and thymine. In contrast to double-stranded DNA, ssDNA contains only one strand, though it can fold back on itself to form secondary structures through intra-strand base pairing such as hairpins.

In living cells, ssDNA is produced transiently during replication and DNA repair when the double helix is unwound. Replication protein A (RPA) and other single-stranded DNA-binding proteins stabilize exposed regions to prevent degradation and secondary structure formation. Telomeres generate long 3' ssDNA overhangs, and repair pathways resect DNA ends to create ssDNA before processing.

ssDNA is more flexible and less thermally stable than double-stranded DNA and is more prone to chemical damage and nuclease attack. Its ability to form secondary structures varies with sequence; guanine-rich regions can form G-quadruplexes, while general sequences can fold into hairpins that influence replication and transcription.

Several viruses possess single-stranded DNA genomes, including members of the Circoviridae and Parvoviridae. Their replication involves converting the genome to a double-stranded form as an intermediate before amplification by host or viral enzymes.

In biotechnology and research, synthetic ssDNA oligonucleotides serve as primers, probes, and templates for sequencing and amplification. ssDNA is also used in diagnostics, antisense approaches, aptamers, and DNA nanotechnology as scaffolds or components of strand-displacement circuits.