It is important to realize that viruses bear no real resemblance to cells and that they lack any of the synthetic machinery found in even the simplest cells (figure 1). The general plan of virus organization is very simple and compact. Viruses contain only those parts needed to invade and control a host cell: an external coating and a core containing one or more nucleic acid strands of either DNA or RNA. This pattern can be represented with a flowchart:

Fig1. Viruses bear little resemblance to cells. Transmission electron micrograph of a single Ebola virus particle reveals a structure that has little in common with the eukaryotic cell the virus infects (100,000×). Callista Images/Image Source

All viruses have a protein capsid, or shell, that surrounds the nucleic acid in the central core. Together, the capsid and the nucleic acid are referred to as the nucleocapsid. Viruses that consist of only a nucleocapsid are considered naked viruses (figure 2a). Other viruses are referred to as enveloped—that is, they possess an additional covering external to the capsid called an envelope, which is a modified section of the host’s cell membrane (figure 2b). As we shall see later, the enveloped viruses also differ from the naked viruses in the possible ways that they enter and leave a host cell.

Fig2.  Generalized structure of viruses. (a) The simplest virus is a naked nucleocapsid consisting of a geometric capsid assembled around one or more nucleic acid strands. (b) An enveloped virus is composed of a nucleocapsid surrounded by a flexible envelope. The envelope usually has special receptor spikes inserted into its membrane.

The Viral Capsid: The Protective Outer Shell When a virus particle is magnified several hundred thousand times, the capsid appears as the most prominent geometric feature (figure 2). In general, the capsid of any virus is constructed from a number of identical protein subunits called capsomers. The capsomers can spontaneously self-assemble into the finished capsid. The shape and arrangement of the capsomers result in the production of one of two different types of capsids: helical or icosahedral.

The simpler helical capsids have rod-shaped capsomers that bind together to form a series of hollow discs resembling a bracelet. During the formation of the nucleocapsid, these discs link together and form a continuous helix into which the nucleic acid strand is coiled. In electron micrographs, the appearance of a helical capsid varies with the type of virus. The nucleocapsids of naked helical viruses are very rigid and tightly wound into a cylinder-shaped package (figure 3). Enveloped helical nucleocapsids are more flexible and tend to be arranged as a looser helix within the envlope (figure4). This type of capsid is found in several enveloped human viruses, including those of influenza, measles, and rabies.

Fig3. Assembly of helical nucleocapsids. Capsomers (blue-green) are assembled into a hollow tube while the nucleic acid (yellow) is wound within the lengthening capsid.

Fig4. Helical nucleocapsids (a) Schematic view and (b) a colorized micrograph featuring a positive stain of the influenza virus (300,000×), an enveloped helical virus. This virus has a well-developed envelope with prominent spikes. (b): Cynthia Goldsmith/CDC

Several major virus families have capsids arranged in an icosahedron—a three-dimensional, symmetrical polygon, with 20 sides and 12 evenly spaced corners. The arrangements of the capsomers vary from one virus to another (figure5). Although the capsids of all icosahedral viruses have this sort of symmetry, they can vary in the number of capsomers; for example, a poliovirus has 32 capsomers and an adenovirus has 242. During assembly of the virus, the nucleic acid is packed into the center of this icosahedron, forming a nucleocapsid. Another factor that alters the appearance of icosahedral viruses is whether or not they have an outer envelope. Inspect figure 6 to compare a rotavirus and its naked nucleocapsid with herpes simplex (cold sores) and its enveloped nucleocapsid.

Fig5. The structure and formation of an icosahedral virus (adenovirus is the model). (a) A facet or “face” of the capsid Vertex is composed of 21 identical capsomers arranged in a triangular shape. A vertex or “point” consists of five capsomers arranged with a single penton in the center. Other viruses can vary in the number, types, and arrangement of capsomers. (b) An assembled virus shows how the facets and vertices come together to form a shell around the nucleic acid. (c) A three-dimensional model (640,000×) of this virus shows fibers attached to the pentons. (d) A negative stain of this virus highlights its texture and fibers that have fallen off. (d): Dr. Linda Stannard, UCT/Science Source

Fig6. Two types of icosahedral viruses, highly magnified. (a) Transmission electron micrograph of rotavirus, revealing its capsomers that look like spokes on a wheel. (b) A computer-generated illustration of rotavirus reveals the three-dimensional nature of the capsomers. (c) Herpes simplex virus, a type of enveloped icosahedral virus (300,000×). (a): Dr. Erskine Palmer & Byron Skinner/CDC; (b): CDC/Alissa Eckert, MS; (c): Eye of Science/Science Source

The Viral Envelope

When enveloped viruses (mostly animal) are released from the host cell, they take with them a bit of the host’s membrane system in the form of an envelope. Some viruses bud off the cell membrane; others leave via the nuclear envelope or the endoplasmic reticulum. Al though the envelope is derived from the host, it is different because some or all of the regular membrane proteins are replaced with special viral proteins during the virus assembly process. Some proteins form a binding layer between the envelope and capsid of the virus, and glycoproteins (proteins bound to a carbohydrate) remain exposed on the outside of the envelope. These protruding molecules, called spikes or peplomers, are essential for the attachment of viruses to the next host cell. Because the envelope is more supple than the capsid, enveloped viruses are pleomorphic and range from spherical to filamentous in shape.

Functions of the Viral Capsid/Envelope

The outermost covering of a virus is indispensable to viral function because it protects the nucleic acid from the effects of various enzymes and chemicals when the virus is outside the host cell. We see this in the capsids of enteric (intestinal) viruses such as polio and hepatitis A, which are resistant to the acid- and protein-digesting enzymes of the gastrointestinal tract. Capsids and envelopes are also responsible for helping to introduce the viral DNA or RNA into a suitable host cell, first by binding to the cell surface and then by assisting in penetration of the viral nucleic acid. In addition, parts of viral capsids and envelopes stimulate the immune system to produce antibodies that can neutralize viruses and protect the host’s cells against future infections.

Complex Viruses: Atypical Viruses

Two special groups of viruses, termed complex viruses (figure 7), are more intricate in structure than the helical, icosahedral, naked, or enveloped viruses just described. The poxviruses (including the agent of smallpox) are very large DNA viruses that lack a typical capsid and are covered by a dense layer of lipoproteins and coarse fibrils on their outer surface. Some members of another group of very complex viruses, the bacteriophages, have a polyhedral capsid head as well as a helical tail and fibers for attachment to the host cell.

Fig7.  Detailed structure of complex viruses. (a) Section through the vaccinia virus, a poxvirus, shows its internal components. (b) Diagram of a T4 bacteriophage of E. coli (280,000×). (c) Transmission electron micrograph of a bacteriophage which infects cyanobacteria (90,000×). (c): Bin Ni, Chisholm Lab, MIT

Nucleic Acids: At the Core of a Virus

The sum total of the genetic information carried by an organism is known as its genome. So far, one biological constant is that the genome of all organisms is carried by and expressed through nucleic acids (DNA, RNA). Even viruses are no exception to this rule, but there is a significant difference. Unlike cells, which contain both DNA and RNA, viruses contain either DNA or RNA, but not both, as the primary genetic material. Because viruses must pack into a tiny space all of the genes necessary to instruct the host cell to make new viruses, the number of viral genes is usually quite small compared with that of a cell. It varies from 9 genes in human immunodeficiency virus (HIV) to as many as 2,500 genes in pandoraviruses. By comparison, the bacterium Escherichia coli has approximately 4,000 genes, and a human cell has around 20,000 genes. Having a larger genome allows cells to carry out the complex metabolic activity necessary for independent life. Viruses typically possess only the genes needed to invade host cells and redirect their synthetic machinery to make new viruses.

In chapter 2, you learned that DNA usually exists as a double stranded molecule and that RNA is single-stranded. Although most viruses follow this same pattern, viruses exhibit variety in how their RNA or DNA is configured. DNA viruses can be single-stranded (ss) or double-stranded (ds). RNA viruses can be double-stranded but are more often single-stranded. Notable examples are the parvoviruses, which contain single-stranded DNA, and reoviruses (a cause of respiratory and intestinal tract infections), which contain double stranded RNA.You will learn in chapter 9 that all proteins are made by “translating” the nucleic acid code on a single strand of RNA into an amino acid sequence. Single-stranded RNA genomes that are ready for immediate translation into proteins are called positive-strand RNA. RNA genomes that have to be converted into the proper form for translation are called negative-strand RNA ( figure 8).

Fig8. Replication of the genome of RNA viruses. Successful replication of a virus requires that it produce many copies of both its viral genome (RNA molecules) and viral proteins. The structure of the RNA dictates how this is done. (Top) + single-strand RNA viruses [(+) RNA] can be immediately translated to produce viral proteins. To produce additional (+) RNA molecules, the virus first synthesizes a complementary strand of (−) RNA, which is then used as a template to produce many copies of the (+) RNA viral genome. (Middle) The genome of −ssRNA [(−) RNA] viruses is initially used to synthesize a (+) RNA molecule, which in turn is used to produce viral proteins as well as additional (−) RNA viral genomes. (Bottom) Double-stranded (+/−) RNA viruses first produce (+) RNA molecules, which are used to make viral proteins. Complementary (−) RNA is then synthesized to re-create the (+/−) RNA molecule for use as the genome of newly produced viruses. McGraw Hill

RNA genomes may also be segmented, meaning that the indvidual genes exist in separate RNA molecules, similar to the way that the human (DNA) genome is spread across 23 separate chromosomes. The influenza virus (an orthomyxovirus) is an example of this form. Another group of RNA viruses with unusual features are the retroviruses (a prime example being HIV), which convert their RNA to DNA inside the host cell.

Whatever the virus type, these tiny strands of genetic material carry the blueprint for viral structure and functions. In a very real sense, viruses are genetic parasites because they cannot multiply until their nucleic acid has reached their haven inside the host cell. At a minimum, they must carry genes for synthesizing the viral capsid, for regulating the actions of the host, and for packaging the mature virus.

Other Substances in the Virus Particle

 In addition to the protein of the capsid, the proteins and lipids of envelopes, and the nucleic acid of the core, viruses can contain enzymes for specific operations within their host cell. They may come with preformed enzymes that are required for viral replication. Examples include polymerases that synthesize DNA and RNA and replicase enzymes that copy RNA. The human immunodeficiency virus (HIV) comes equipped with reverse transcriptase for synthesizing DNA from RNA. However, viruses completely lack the genes for synthesis of metabolic enzymes. As we shall see, this deficiency has little consequence, because viruses have adapted to assume total control over the cell’s metabolic resources. Some vi ruses can actually carry away substances from their host cell. For instance, arenaviruses pack along host ribosomes, and retroviruses “borrow” the host’s tRNA molecules.

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مواضيع ذات صلة

 How Viruses Are Classified and Named

The General Structure of Viruses: Size Range

HIV Infections in Humans: Laboratory Diagnosis

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Viral Capsids Are Regular Arrays of One or a Few Types of Protein

Structure and Composition of Lentiviruses

HERPESVIRUSES