Page 237 - Review of Medical Microbiology and Immunology ( PDFDrive )

mebooksfree.com

mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com H A P mebooksfree.com mebooksfree.com
28
C
T
R
E

Structure

mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com Atypical Virus-Like Agents mebooksfree.com mebooksfree.com
CHAPTER C ONTENT S

Size & Shape of Viruses
Pearls
Viral Nucleic Acids
Self-Assessment Questions
Viral Capsid & Symmetry
Practice Questions: USMLE & Course Examinations
Viral Proteins
Viral Envelope

SIZE & SHAPE OF VIRUSES VIRAL CAPSID & SYMMETRY
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com somer, consisting of one or several proteins, can be seen in mebooksfree.com
mebooksfree.com
Viruses range from 20 to 300 nm in diameter; this corre-
The nucleic acid is surrounded by a protein coat called a
sponds roughly to a range of sizes from that of the largest
capsid made up of subunits called capsomers. Each cap-
protein to that of the smallest cell (see Figure 2–2). Their
shapes are frequently referred to in colloquial terms (e.g.,
the electron microscope as a spherical particle, sometimes
with a central hole.
spheres, rods, bullets, or bricks), but in reality they are
The structure composed of the nucleic acid genome and
complex structures of precise geometric symmetry (see
later). The shape of virus particles is determined by the
the capsid proteins is called the nucleocapsid. The arrange-
arrangement of the repeating subunits that form the pro-
ment of capsomers gives the virus structure its geometric
symmetry. Viral nucleocapsids have two forms of symme-
tein coat (capsid) of the virus. The shapes and sizes of some
important viruses are depicted in Figure 28–1.
in 20 triangles that form a symmetric figure (an icosahe-
dron) with the approximate outline of a sphere; and (2)
VIRAL NUCLEIC ACIDS try: (1) icosahedral, in which the capsomers are arranged
helical, in which the capsomers are arranged in a hollow
mebooksfree.com mebooksfree.com mebooksfree.com sid are enclosed by an outer membrane called an envelope mebooksfree.com
mebooksfree.com
mebooksfree.com
coil that appears rod-shaped. The helix can be either rigid
The anatomy of two representative types of virus particles
or flexible. All human viruses that have a helical nucleocap-
is shown in Figure 28–2. The viral nucleic acid (genome) is
located internally and can be either single- or double-
(i.e., there are no naked helical viruses). Viruses that have
1
stranded DNA or single- or double-stranded RNA.
an icosahedral nucleocapsid can be either enveloped or
Only viruses have genetic material composed of single-
naked (see Figure 28–2).
stranded DNA or of single-stranded or double-stranded
The advantage of building the virus particle from identi-
RNA. The nucleic acid can be either linear or circular. The
cal protein subunits is twofold: (1) it reduces the need for
DNA is always a single molecule; the RNA can exist either
genetic information, and (2) it promotes self-assembly (i.e.,
as a single molecule or in several pieces. For example, both
influenza virus and rotavirus have a segmented RNA
particles have been assembled in the test tube by combining
genome. Almost all viruses contain only a single copy of
the purified nucleic acid with the purified proteins in the
their genome (i.e., they are haploid). The exception is the no enzyme or energy is required). In fact, functional virus
absence of cells, energy source, and enzymes.
retrovirus family, whose members have two copies of their
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com VIRAL PROTEINS mebooksfree.com mebooksfree.com
RNA genome (i.e., they are diploid).
Viral proteins serve several important functions. The cap-
sid proteins protect the genome DNA or RNA from degra-
1
The nature of the nucleic acid of each virus is listed in Tables 31–1 and
dation by nucleases. The proteins on the surface of the
31–2.
226
mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com