section_path: Cause/Virology/Gene expression
section_name: Gene expression
paragraph_text: Gene expression
  
  Gene expression of variola virus occurs entirely within the cytoplasm of the host cell, and follows a distinct progression during infection. After entry of an infectious virion into a host cell, synthesis of viral mRNA can be detected within 20 minutes. About half of the viral genome is transcribed prior to the replication of viral DNA. The first set of expressed genes are transcribed by pre-existing viral machinery packaged within the infecting virion. These genes encode the factors necessary for viral DNA synthesis and for transcription of the next set of expressed genes. Unlike most DNA viruses, DNA replication in variola virus and other poxviruses takes place within the cytoplasm of the infected cell. The exact timing of DNA replication after infection of a host cell varies across the poxviridae. Recombination of the genome occurs within actively infected cells. Following the onset of viral DNA replication, an intermediate set of genes codes for transcription factors of late gene expression. The products of the later genes include transcription factors necessary for transcribing the early genes for new virions, as well as viral RNA polymerase and other essential enzymes for new viral particles. These proteins are then packaged into new infectious virions capable of infecting other cells.