Animal virus

The fact that spherical plant viruses and some small single-stranded RNA animal viruses build their icosahedral shells using essentially similar asymmetric units raises the possibility that they have a common evolutionary ancestor. The folding of the main chain in the protein subunits of these viruses supports this notion. 

The coat proteins of many different spherical plant and animal viruses have similar jelly roll barrel structures, indicating an evolutionary relationship... 

Figure 16.13 The known subunit structures of plant. Insect, and animal viruses are of the jelly roll antiparallel p barrel type, described in Chapter 5. This fold, which is schematically illustrated in two different ways, (a) and (b), forms the core of the S domain of the subunit of tomato bushy stunt virus (c). [(b), (c) Adapted from A.J. Olson et al., /. Mol. Biol. 171 61-93, 1983.1...

Required events for transmission of animal viruses to man by wate... 

Baltimore D (1971) Expression of animal virus genomes. Bacteriol Rev 35 235-241 Baltimore D (1988) Gene therapy. Intracellular immunization. Nature 335 395-396 Bauer DJ (1985) A history of the discovery and clinical application of antiviral drugs. Br Med Bull... 

Many animal viruses, particularly the oncogenic viruses—either directly or, in the case of RNA viruses such as HIV that causes AIDS, their DNA transcripts generated by the action of the viral RNA-dependent... 

The multiple sites that serve as origins for DNA replication in eukaryotes are poorly defined except in a few animal viruses and in yeast. However, it is clear that initiation is regulated both spatially and temporaUy, since clusters of adjacent sites initiate rephcation synchronously. There are suggestions that functional domains of chromatin replicate as intact units, implying that the origins of rephcation are specificaUy located with respect to transcription units. 

There exists in one species of animal viruses (retroviruses) a class of enzymes capable of synthesizing a sin-... 

Helical symmetry was thought at one time to exist only in plant viruses. It is now known, however, to occur in a number of animal virus particles. The influenza and mumps viruses, for example, which were first seen in early electron micrographs as roughly spherical particles, have now been observed as enveloped particles within the envelope, the capsids themselves are helically symmetrical and appear similar to the rods of TMV, except that they are more flexible and are wound like coils of rope in the centre of the particle. 

Viruses are classified initially on the basis of the hosts they infect. Thus we have animal viruses, plant viruses, and bacterial viruses. Bacterial viruses, sometimes called bacteriophages (or phage for short, from the Greek phago meaning to eat), have been studied primarily as convenient model systems for research on the molecular biology and genetics of virus reproduction. Many of the basic concepts of... 

When a virus multiplies, the genome becomes released from the coat. This process occurs during the infection process. The present chapter is divided into three parts. The first part deals with basic concepts of virus structure and function. The second part deals with the nature and manner of multiplication of the bacterial viruses (bacteriophages). In this part we introduce the basic molecular biology of virus multiplication. The third part deals with important groups of animal viruses, with emphasis on molecular aspects of animal virus multiplication. 

The complete complex of nucleic acid and protein, packaged in the virus particle, is called the virus nucleocapsid. Although the virus structure just described is frequently the total structure of a virus particle, a number of animal viruses (and a few bacterial viruses) have more complex structures. These viruses are enveloped viruses, in which the nucleocapsid is enclosed in a membrane. Virus membranes are generally lipid bilayer membranes, but associated with these membranes are often virus-specific proteins. Inside the virion are often one or more virus-specific enzymes. Such enzymes usually play roles during the infection and replication process. 

As we have noted, viruses can be classified into broad groups depending on their hosts. For instance, there are plant viruses, animal viruses, and bacterial viruses. A number of viruses infecting insects are also known and although viruses are known for fungi, protozoa, and algae, these viruses have been so little studied that no classification has been developed. In the present chapter, we discuss only the animal (primarily mammalian) and bacterial viruses, and we discuss here briefly how these two groups of viruses are classified. 

Virus genera are designated by terms ending in - virus. Thus, among the Poxviridae those poxviruses which infect fowl are called by the genus name Avipoxvirus. Note that frequently in the animal viruses, the genus is defined based on the host which the virus infects. 

With animal viruses, the initial host may be a whole animal which is susceptible to the virus, but for research purposes it is desirable to have a more convenient host. Many animal viruses can be cultivated in tissue or cell cultures, and the use of such cultures has enormously facilitated research on animal viruses. 

Plaques may be obtained for animal viruses by using animal cell-culture systems as hosts. A monolayer of cultured animals cells is prepared on a plate or flat bottle and the virus suspension overlayed. Plaques are revealed by zones of destruction of the animal cells. 

The eclipse is the period during which the stages of virus multiplication occur. This is called the latent period, because no infectious virus particles are evident. Finally, maturation begins as the newly synthesized nucleic acid molecules become assembled inside protein coats. During the maturation phase, the titer of active virus particles inside the cell rises dramatically. At the end of maturation, release of mature virus particles occurs, either as a result of cell lysis or because of some budding or excretion process. The number of virus particles released, called the burst size, will vary with the particular virus and the particular host cell, and can range from a few to a few thousand. The timing of this overall virus replication cycle varies from 20-30 minutes in many bacterial viruses to 8-40 hours in most animal viruses. We now consider each of the steps of the virus multiplication cycle in more detail. 

We have discussed in a general way the nature of animal viruses in the first part of this chapter. Now we discuss in some detail the structure and molecular biology of a number of important animal viruses. Viruses will be discussed which illustrate different ways of replicating, and both RNA and DNA viruses will be covered. One group of animal viruses, those called the retroviruses, have both an RNA and a DNA phase of replication. Retroviruses are especially interesting not only because of their unusual mode of replication, but because retroviruses cause such important diseases as certain cancers and acquired immunodeficiency syndrome (AIDS). 

Before beginning our discussion of the manner of replication of animal viruses, we should mention first the important differences which exist between animal and bacterial cells. Since virus replication makes use of the biosynthetic machinery of the host, these differences in cellular organization and function imply differences in the way the viruses themselves replicate. 

We might also note another important difference between animal and bacterial cells. Bacterial cells have rigid cell walls containing peptidoglycan and associated substances. Animal cells, on the other hand, lack cell walls. This difference is important for the way by which the virus genome enters and exits the cell. In bacteria, the protein coat of the virus remains on the outside of the cell and only the nucleic acid enters. In animal viruses, on the other hand, uptake of the virus often occurs by endocytosis (pinocytosis or phagocytosis), processes which are characteristic of animal cells, so that the whole virus particle enters the cell. The separation of animal virus genomes from their protein coats then occurs inside the cell. 

Classification of animal viruses Most of the animal viruses which have been studied in any detail have been those which have been amenable to cultivation in cell cultures. As seen, animal viruses are known with either single-stranded or doublestranded DNA or RNA. Some animal viruses are enveloped, others are naked. Size varies greatly, from those large enough to be just visible in the light microscope, to those so tiny that they are hard to see well even in the electron microscope. In the following sections, we will discuss characteristics and manner of multiplication of some of the most important and best-studied animal viruses. 

Figure 5.31 Possible effects that animal viruses may have on cells they infect, humans has, in most cases, been uncertain. It is difficult to prove the viral origin of a human cancer because of the difficulties of carrying out the necessary experimentation. However, it is now well established that certain specific kinds of human tumors do have a viral origin. A summary of some of the human cancers with definite viral origins is given in Table.

Figure 5.32 One-step growth curve of animal viruses.

Dimmock, N. J. (1993), Neutralization of Animal Viruses, Springer-Verlag, Berlin, p. 149. 

Animal uptake, of herbicides, 23 310 Animal viruses, 3 135-136 Animal waxes, 26 203, 206 Anion binding, in supramolecular chemistry, 24 43-47 Anion-exchange membranes, 15 836 Anion exchangers, organic fouling of, 24 416... 

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