Page 263 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com • The classification of viruses is based primarily on the nature mebooksfree.com
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PART III Basic Virology
252
Arenaviruses
PEARLS
These are enveloped viruses with a helical nucleocapsid
and a single-stranded, circular, negative-polarity RNA in
two segments. (A part of both segments is positive-polarity
of the genome and whether the virus has an envelope.
RNA, and the term ambisense RNA is used to describe this
• Poxviruses, herpesviruses, and hepadnaviruses are DNA
unusual genome.) The term arena means “sand” and refers
to granules on the virion surface that are nonfunctional
viruses, papillomaviruses, and parvoviruses are DNA viruses
ribosomes. Two human pathogens are lymphocytic chorio-
without an envelope (i.e., they are naked nucleocapsid
meningitis virus and Lassa fever virus. viruses with an envelope, whereas adenoviruses, polyoma-
viruses). Parvoviruses have single-stranded DNA, whereas
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all the other families of DNA viruses have double-stranded
Bunyaviruses
DNA. The DNA of hepadnaviruses (hepatitis B virus) is
mostly double-stranded but has a single-stranded region.
These are enveloped viruses with a helical nucleocapsid
and a single-stranded, circular, negative-polarity RNA in
are RNA viruses without an envelope, whereas all the other
three segments. Some bunyaviruses contain ambisense
families of RNA viruses have an envelope. Reoviruses have
RNA in their genome (see Arenaviruses). The term bunya
double-stranded RNA; all the other families of RNA
refers to the prototype, Bunyamwera virus, which is named
viruses have single-stranded RNA. Reoviruses and influ-
after the place in Africa where it was isolated. These viruses
enza viruses have segmented RNA; all the other families of
cause encephalitis and various fevers such as Korean hem-
RNA viruses have nonsegmented RNA. Picornaviruses, hep-
orrhagic fever. Hantaviruses, such as Sin Nombre virus (see
eviruses, caliciviruses, flaviviruses, togaviruses, retroviruses,
Chapter 46), are members of this family.
and coronaviruses have positive-polarity RNA, whereas all
the other families have negative-polarity RNA.
Deltavirus
mebooksfree.com mebooksfree.com mebooksfree.com PRACTICE QUESTIONS: USMLE & mebooksfree.com
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Hepatitis delta virus (HDV) is the only member of this
genus. It is an enveloped virus with an RNA genome that is
a single-stranded, negative-polarity, covalently closed cir-
COURSE EXAMINATIONS
cle. The symmetry of the nucleocapsid is uncertain. It is a
defective virus because it cannot replicate unless hepatitis B
Questions on the topics discussed in this chapter can be found
virus (HBV) is present within the same cell. HBV is
in the Basic Virology section of Part XIII: USMLE (National
required because it encodes hepatitis B surface antigen
Board) Practice Questions starting on page 720. Also see Part
(HBsAg), which serves as the outer protein coat of HDV.
XIV: USMLE (National Board) Practice Examination starting
The RNA genome of HDV encodes only one protein, the
on page 751.
internal core protein called delta antigen.
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