Virus particles viral nucleic acid

Virus maturation and assembly at the cell membrane or the nuclear membrane has long been seen as a potential target for antiviral compounds. For the virus to mature and be released in a conformation that will insure stability and survival of the viral genome in the exttacellular enviromnent, the protein subunits of the capsid or nucle-ocapsids have to be transported to the assembly point where they will form the final particles around the viral nucleic acid. If this process does not occur in an orderly and programmed manner, the capsid subunits will not form the required multimers and the viral components will become targets for the cellular disposal mechanisms. 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

As we have noted, the outcome of a virus infection is the synthesis of viral nucleic acid and viral protein coats. In effect, the virus takes over the biosynthetic machinery of the host and uses it for its own synthesis. A few enzymes needed for virus replication may be present in the virus particle and may be introduced into the cell during the infection process, but the host supplies everything else energy-generating system, ribosomes, amino-acid activating enzymes, transfer RNA (with a few exceptions), and all soluble factors. The virus genome codes for all new proteins. Such proteins would include the coat protein subunits (of which there are generally more than one kind) plus any new virus-specific enzymes. 

Subsequently, similar experiments were done with viral nucleic acids. The pure viral nucleic acid, when added to cells, led to the synthesis of complete virus particles the protein coat was not required. This process is called transfection. More recently, DNA has been used in cell-free extracts to program the synthesis of RNA that functions as the template for the synthesis of proteins characteristic of the DNA... 

Penetration. After fusion of viral and host membranes, or uptake into a phagosome, the virus particle is carried into the cytoplasm across the plasma membrane. This penetration process is an active one that requires expenditure of energy by the cell. At this stage the envelope and the capsid are shed, and the viral nucleic acids are released. The uncoating of virus accounts for the drop in infectious virus assayed, because the uncoated virus cannot withstand the assay conditions. 

Unlike in bacteria and fungi, viruses do not have a protective coat that separates essential proteins and nucleic acids from the environment. The majority of viruses consist of nucleic acid polymers (DNA or RNA) enclosed within a protein coat (capsid). Sometimes, viruses pick up a lipid membrane (envelope) from the host cell that surroimds the capsid. The average size of viral particles is in the range 10-300 nm. The most common... 

Viral replication consists of several steps (Figure 49-1) (1) attachment of the vims to receptors on the host cell surface (2) entry of the virus through the host cell membrane (3) uncoating of viral nucleic acid (4) synthesis of early regulatory proteins, eg, nucleic acid polymerases (5) synthesis of new viral RNA or DNA (6) synthesis of late, structural proteins (7) assembly (maturation) of viral particles and (8) release from the cell. Antiviral agents can potentially target any of these steps. 

During inactivation steps, viral infectivity is reduced by treatment with chemicals and/or physical methods. Remnants of virus particles (e.g., viral nucleic acids) may remain in the product-containing fraction but are not infectious. Chemical methods of virus inactivation, such as treatment with solvent-detergent or acetone, must be placed upstream, since subsequent steps are needed to remove or reduce the levels of the toxic chemicals. Terminal inactivation is often achieved using physical methods, such as heat and low pH, because these methods leave no chemical residues. After treatment, the final products are delivered to patients, so aseptic processing conditions must be maintained throughout terminal inactivation steps and the parameters for virus inactivation must be balanced with the conditions to preserve product quality and yield. 

UV irradiation can be used to eliminate viruses, especially airborne particles, by damaging viral nucleic acid, although again, small non-enveloped particles might be more resistant. Virus destruction can also be achieved by exposure to ionizing radiation (e.g. y-rays, accelerated electrons), which is used in terminal sterilization processes applied to pharmaceutical and medical products (Chapter 20). 

Interferon is a low molecular weight protein, produced by virus-infected cells, that itself induces the formation of a second protein inhibiting the transcription of viral mRNA. Interferon is produced by the host cell in response to the virus particle, the viral nucleic acid and non-viral agents, including synthetic polynucleotides such as polyinosinic acid and polycytidylic acid (poly I C). There are two types of interferon. 

The occurrence of virus protein cleavages and the possible role of these reactions are summarized in Table I. The most common observation is that limited proteolysis occurs at the level of maturation (formation of Infectious virus particles). Mechanistically, the virus structural protein precursor is cleaved, and activated to a form which makes it recognizable to viral nucleic acid and/or cellular membrane receptors. The cleavages occur most commonly within the Infected cells, but in several examples final proteolytic "tailoring" occurs after virus has exited the cell, and is in the serum or some other extracellular environment. 

Once inside the host cell, the vims must replicate its own nucleic acid. To do this, it often uses part of the normal synthesizing machinery of the host cell. If the vims is to continue its growth cycle, viral nucleic acid and viral protein must be propedy transported within the cell, assembled into the infective vims particle, and ultimately released from the cell. All of these fundamental processes involve an intimate utilization of both cellular and viral enzymes. Certain enzymes that are involved in this process are specifically supplied by the invading vims. It is this type of specificity that can provide the best basis for antiviral chemotherapy. Thus an effective antiviral agent should specifically inhibit the viral-encoded or virus-induced enzymes without inhibition of the normal enzymes involved in the biochemical process of the host cell. Virus-associated enzymes have been reviewed (2,3) (Table 1). 

In the lytic pathway, the viral nucleic acid is replicated in the host cell and packaged into new virus particles that lyse the host cell. In the lysogenic pathway, the viral DNA is incorporated into the host DNA. 

The whole virus or its genetic material alone (DNA or RNA) enters the cell s cytoplasm (penetration and uncoating). A virus may have different penetration mechanisms in the host cell. For enveloped virus, fusion of membrane sometimes occurs. Most viruses are introduced into the cell by a kind of phagocytosis named viropexis. Virus particles are transported along the network of cytoplasmic microtubules to a specific cell site where subsequent replication takes place. Uncoating results in the liberation of viral nucleic acids into the cell which makes them sensitive to nucleases. 

Unlike bacteria, virus particles (virons) are capable of replication only within a host cell. Once within the host cell, the viral nucleic acid directs the synthesis of specific enzymes needed to replicate itself, and directs the synthesis of the viral-coating protein (capsid). The capsid protects the nucleic acid from enzyme attack, and in the absence of high temperatures (above about 70°C the virus may remain intact for several decades. Separation of the nucleic acid from the capsid is necessary in order that the virus becomes active, but the separated components appear to have the capacity for self-assembly to reconstruct the virus particle. 

Viruses consist of nucleic acid molecules (RNA or DNA) encased in a protein coating. Virns capsids (protein shells) can be near spherical or rod-like (helical). Spherical viruses often have an icosahedral structure (a polyhedron with 20 triangular faces, Fig. 6.25a). Within each face, the snbunits of the viral capsid have different symmetries. Many common viruses including rhi-novirus (responsible for the common cold) and herpes simplex virus have icosahedral structures. In contrast, the first virus to be discovered, tobacco mosaic virus, has a helical structure (Fig. 6.25b), leading to a rod-shaped particle 300 nm long and 18 nm in diameter. Here the proteins are wrapped around RNA. 

A viral infection begins when an enzyme in the protein coat of the virus makes a hole in the host cell, allowing the viral nucleic acids to enter and mix with the materials in the host cell (see Figure 17.17). If the virus contains DNA, the host cell begins to replicate the viral DNA in the same way it would replicate normal DNA. Viral DNA produces viral RNA, and a protease processes proteins to produce a protein coat to form a viral particle that leaves the cell. The cell synthesizes so many virus particles that it eventually releases new viruses to infect more cells. 

FIGURE 1.25 The virus life cycle. Viruses are mobile bits of genetic iuformatiou encapsulated in a protein coat. The genetic material can be either DNA or RNA. Once this genetic material gains entry to its host cell, it takes over the host machinery for macromolecular synthesis and subverts it to the synthesis of viral-specific nucleic acids and proteins. These virus components are then assembled into mature virus particles that are released from the cell. Often, this parasitic cycle of virus infection leads to cell death and disease. 

Viruses are infectious particles formed by nucleic acid, proteins, and in some cases lipids. As viruses (for example, retro- and adenoviruses) transfer viral genes into cells with high efficiency, modified forms are sometimes used as vectors for gene transfer. However, procedures using virus-based vectors are often significantly more complicated and time-consuming than other transfection methods. In addition, viral vectors are potentially hazardous, and biological safety issues need to be considered carefully. Therefore, techniques that combine... 

Virases are much simpler organisms than bacteria, and they are made from protein substances and nucleic acid. A single nucleoprotein molecule formed from molecules of nucleic acid that are chemically bound to a bulky protein molecule can be considered a simple viral particle. The protein molecule plays the role of a protective membrane. Thus the virus can be schematically described as a nucleic acid insert that is protected by a protein covering. A virus can contain either ribonucleic acid or deoxyribonucleic acid, but it never contains both of them together. The type of nucleic acid is the basis of one of the classifications of viruses. Viruses are obligatory intracellular parasites, which, upon entering a cell (i.e. after being infected) use many biochemical systems of the host cell. 

Viruses are obligate intracellular parasites that use many of the host cell s biochemical mechanisms and products to sustain their viability. A mature virus (virion) can exist outside a host cell and still retain its infective properties. However, to reproduce, the virus must enter the host cell, take over the host cell s mechanisms for nucleic acid and protein synthesis, and direct the host cell to make new viral particles. 

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