secondstrand

Secondstrand is a term used in molecular biology to refer to the complementary DNA strand that is synthesized to form a complete double-stranded molecule from a single-stranded template. In DNA replication, the double helix is unwound, and each parental strand serves as a template for the synthesis of a new strand. The strand synthesized continuously in the 5' to 3' direction on one template is often called the leading strand, while the complementary strand synthesized discontinuously as Okazaki fragments on the other template is referred to as the second strand or lagging strand. Synthesis of the second strand involves priming, extension by DNA polymerase, primer removal, gap filling, and ligation to produce a continuous double-stranded DNA molecule.

During replication, the second strand is synthesized in short segments (Okazaki fragments) on the lagging template because DNA polymerases can add nucleotides only in the 5' to 3' direction. Each fragment begins with an RNA primer laid down by primase; DNA polymerase extends it, primers are removed, remaining gaps are filled, and DNA ligase seals the nicks. The process ensures semi-conservative replication, yielding two daughter DNA molecules each consisting of one parental and one newly synthesized strand.

In laboratory contexts, second-strand synthesis also refers to converting single-stranded cDNA into double-stranded cDNA during sequencing library preparation. This typically involves DNA polymerase-mediated synthesis using primers or adapters, enabling downstream amplification and sequencing.

Overall, "secondstrand" denotes the complementary strand synthesized to complete a double-stranded DNA molecule, central to replication and many molecular biology workflows.