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Historically, the papillomaviruses and polyomaviruses have been discussed together in microbiology textbooks, lumped under the category of papovaviruses. Papovaviruses are now split into two separate families: Papillomaviridae and Polyomaviridae. The unique characteristics that distinguish them from each other are shown in Table 19–1.
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Papillomaviruses are small, naked capsid (unenveloped), double-stranded, circular DNA viruses exhibiting cubic (icosahedral) symmetry of 55 nm in diameter (Figure 19–1). The icosahedral capsid comprises two capsid (structural) proteins, L1 (major capsid protein) and L2 (minor capsid protein). The 8 kb, circular, double-stranded DNA genome of human papillomavirus (HPV) encodes seven or eight early genes (E1-E8) and two late structural capsid genes (L1 and L2). The early genes are required for regulation of viral replication and transformation. The virus does not encode any polymerases and, therefore, is dependent on host cell transcription and replication machinery. Based on DNA homology, there are over 100 genotypes of HPVs. Papillomaviruses cause epidermal papillomas and warts in a wide range of higher vertebrates. Different members of the group are generally species specific. For example, bovine papillomaviruses and HPVs infect only the hosts reflected in their names. In some cases, lesions caused by these agents can become malignant, and the role of these agents as causes of certain human cancers is increasingly recognized. Papillomaviruses have been difficult to grow in tissue culture, and most of the virologic information has been derived from molecular and gene expression studies.
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Naked capsid, double-stranded, circular DNA viruses
Difficult to propagate HPV in tissue culture composed of regular cell lines
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The genomes of many of the papillomaviruses have now been cloned and compared by restriction endonuclease ...