第三章 病毒和亚病毒 108

Structure of the virus
Nucleic acid （DNA or RNA）
nucleocapsid
Viron Capsid （protein） Envelope （glycoprotein）
Fig. (a) unenveloped/helical (tobacco mosaic virus): (b) unenveloped/icosahedral (adenovirus); (c) enveloped/helical (paramyxovirus): (d) enveloped/ icosahedral (herpesvirus).
Immunoelectron microscopy
Methods of study（4）
Electron cryomicroscopy （电子低温显微术） reduces the risk of seeing artifacts as may be unavoidable by negative staining. High
A virus particle has thus been defined
as a structure which has evolved to transfer nucleic acid from one cell to another. The nucleic acid found in the particle is either DNA or RNA, is single- or double-stranded and linear or segmented. In some cases the nucleic acid may be circular.
Diagrammatic representation of the structure of virus particles
Glycoprotein
45nm
envelope
90nm
Icosahedral capsid containing DNA
herpes simplex (l) has a loose-fitting envelope, whereas adenovirus (R) is non-enveloped.
Electron cryomicroscopy
Methods f study（5）
X-ray diffraction of virus crystals is the ultimate in determining the ultrastructure of virion morphology. （是对病 毒形态和超显微结构分析的主要方法）At present, only simple viruses can be crystallized. More complex viruses are analyzed by attempting to form crystals of sub-particular molecules. The X-ray diffraction pattern of the virion particle allows mathematical processing, which can predict the molecular configuration（分子构型） of the virus particle .
Diagrammatic representation of the structure of virus particles
RNA
180nm
Reverse transcriptase 100nm
Lipid membrane
Rabies virus(l)and HIV(R) have tight-fitting envelopes.
viroid（类病毒） ——independent infectious RNA only virusoid（拟病毒）——non-independent infectious RNA only prion（朊病毒）——protein only
1. definitions
Viruses are obligate intracellular parasites which can only be viewed with the aid of an electron microscope. They vary in size from approximately 20-200 nm. In order to persist in the environment they must be capable of being passed from host to host and of infecting and replicating in susceptible host cells.
Chapter 3
virus and subvirus
Section 1
Virus structure
Euvirus ：at least contains two components—— nucleic acid and protein
Subvirus：molecular pathogen includes：
More complicated viruses have their nucleic acid surrounded by a protein coat which is further engulfed in a membrane structure, an envelope consisting of virally coded glycoproteins derived from one of several regions within the infected cell during the maturation of the virus particle. The genetic material of these complex viruses encodes for many dozens of virus specific proteins.
2. Methods of study（1）
While the electron microscope (EM) had been known for many years, the invention of the negative-staining technique in 1959 revolutionized studies on virus structure. In negative-contrast EM, virus particles are mixed with a heavy metal solution (e.g. sodium phosphotungstate磷钨酸盐) and dried onto a support film. The stain provides an electron-opaque background against which the virus can be visualized。
Electron microscopy
Methods of study（3）
Immunoelectron microscopy is used to study those viruses which may be present in low concentrations or grow poorly in tissue culture (e.g. Norwalk virus). These clumps of virus are more readily observed.
The complete fully assembled virus is termed the virion. It may have a glycoprotein envelope which has peplomers (包膜粒)(projections, spike突起,刺突) which form a „fringe边缘‟ around the particle. The protein coat surrounding the nucleic acid is referred to as the capsid . The capsid is composed of morphological units or capsomers（衣壳粒）. The type of capsomer depends on the overall shape of the capsid, but in the case of icosahedral capsids the capsomers are either pentamers or hexamers（五邻体或六邻体）. The combined nucleic acid-protein complex which comprises the genome is termed the nucleocapsid（核 衣壳）, which is often enclosed in a core within the virion.
X-ray diffraction
Summarize methods of studying
Negative-staining technique Electron microscopy Immunoelectron microscopy Electron cryomicroscopy X-ray diffraction
Negative-staining technique
Methodes of study （2）
Observation under the electron microscope has thus allowed the definition of virus morphology at the 5077A resolution level. In addition, negative staining of thin sections of infected cells has allowed definition of structures which appear during virus maturation and their interactions with cellular proteins.