Poliovirus and its empirical formula. (A) Empirical formula of the organic matter of poliovirus (Molla et al, 1991). (B) Depiction of a poliovirus particle generated from X-ray crystallographic data (Hogle et al, 1985).

Genome sequence of poliovirus, an RNA virus (Kitamura et al, 1981; Racaniello & Baltimore, 1981a), shown here in the corresponding DNA sequence.

Synthesis of poliovirus in the absence of natural template. (A) Short complementary segments of synthetic DNA (oligonucleotides) are annealed, and enzymatically extended and ligated (connected). A full-length complementary DNA (cDNA) is assembled stepwise to represent the entire genetic information of the poliovirus RNA genome in the form of DNA. The cDNA is then transcribed into infectious viral RNA by a T7 RNA transcriptase. This RNA is used to seed a HeLa cell-free extract that will replicate, just like in intact cells, to form progeny virions (Cello et al, 2002; Molla et al, 1991). (B,C) Evidence for de novo synthesized virus is provided by plaque assays. Poliovirus plaques derived from synthetic virus (sPV1) and wild-type virus, respectively, are formed on monolayers of HeLa cells (Cello et al, 2002). Reproduced from Mueller et al (2005b), with permission.

Two different strategies for generating RNA viruses via complementary DNA (cDNA) intermediates. (A) Synthesis of cDNA catalysed with reverse transcriptase followed by transfection of the cDNA into host cells (Taniguchi et al, 1978; Racaniello & Baltimore, 1981b). (B) Chemical synthesis of cDNA followed by transcription and incubation of the infectious viral RNA in a cell-free extract (Cello et al, 2002; Molla et al, 1991).