rcDNA

rcDNA, or relaxed circular DNA, is a form of viral DNA that is partially double-stranded and circular. It is best known in the Hepadnaviridae family, particularly hepatitis B virus (HBV). The HBV genome is about 3.2 kilobases, and the rcDNA form is packaged into mature virions and produced during replication as an intermediate within nucleocapsids. Structurally, rcDNA contains a complete minus strand that is covalently attached to the viral DNA polymerase at the 5' end, and a shorter, incomplete plus strand that leaves a single-stranded region and a gap. The molecule is circular but relaxed rather than highly supercoiled due to the single-stranded segment.

Function and lifecycle: After infection, rcDNA is delivered to the hepatocyte nucleus, where host DNA repair enzymes convert it into covalently closed circular DNA (cccDNA). cccDNA forms a chromatinized minichromosome that serves as the template for transcription of viral RNAs. The formation of cccDNA is a key step in HBV persistence and chronic infection, and is a major target for antiviral research. rcDNA itself is a replication intermediate and a marker of viral replication activity; it can be detected in virions and in assembling nucleocapsids.

Other pararetroviruses in which rcDNA is relevant share similar structural features, but the term is most closely associated with HBV. Understanding rcDNA structure and the repair pathways that convert it to cccDNA is important for the study of HBV biology and antiviral strategies.