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Chapter 3 Viral Replication

Chapter 3 Viral Replication

Flg. 3-3 Receptor-mediated endocytosis: penetration by a togavirus (bar, lOOnm) (A) Attachment and movement into a ctathrin-coated pit (P) Endocytosis, producing a coated vesicle. (A, From E Fries and A Helenius, Ltir J Bttkhcm 8, 213 (1979), B, from K Simons, H Garoff, and A flelenius, Sei Am 246, 58 (1982), courtesy Dr A. I lolcnius ]

Flg. 3-3 Receptor-mediated endocytosis: penetration by a togavirus (bar, lOOnm) (A) Attachment and movement into a ctathrin-coated pit (P) Endocytosis, producing a coated vesicle. (A, From E Fries and A Helenius, Ltir J Bttkhcm 8, 213 (1979), B, from K Simons, H Garoff, and A flelenius, Sei Am 246, 58 (1982), courtesy Dr A. I lolcnius ]

This may allow the nucleocapsid to be released into the cytoplasm. A number of other enveloped viruses gain entry in similar fashion.

For viral genes to become available for transcription it is necessary that virions be at least partially uncoated. f n the case of enveloped RNA viruses that enter by fusion of their envelope with either the plasma membrane or an endoso-mal membrane, the nucleocapsid is discharged directly into the cytoplasm, and transcription commences from viral nucleic acid still associated with this structure. With the nonenveloped icosahedral reoviruses only certain capsid proteins are removed, and the viral genome expresses all its functions without ever being released from the core. For most other viruses, however, un-coating proceeds to completion. For some viruses that replicate in the nucleus the later stages of uncoating occur there, rather than in the cytoplasm.

Replication of most DNA viruses involves mechanisms that are familiar in cell biology: transcription of mRNA from dsDNA and replication of DNA (Fig 3-2) The situation is quite different for RNA viruses, which are unique in

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