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Eukaryotic Viruses Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Types of Viral Infections

There are four major types of viral infection. The most common type is an acute or lytic infection that causes farily rapid death of the host cell when it ruptures to release progeny virions. The second type involves the virus entering a dormant state similar to lysogeny of bacterial viruses (phages) as discussed earlier. This type of infection is called a latent infection. The third type involves virions that are slowly released from the cell surface without lysing or killing the host cell. This type of infection is termed persistent infection. Typically, as the virion buds from the cell, it acquires a lipid envelope of host membrane. Certain viral proteins become incorporated into the host cell membrane. Thus, the viral envelope contains viral proteins that enable the virion to attach to another host cell and spread the infection. The fourth type of viral infection involves transformation of a normal cell into a tumor cell, leading to cancer. This type of infection will be discussed later in the Cancer study guide.

DNA Viruses

A typical double-stranded DNA (dsDNA) virus attaches to a cell receptor and then is taken into the cell, where its capsid is removed (uncoated). The viral DNA is replicated using host-cell enzymes. The viral DNA is also transcribed by host enzymes into mRNAs, which in turn are translated (by the host's ribosomes and enzymes) into viral capsid proteins or (in some cases) into enzymes that favor viral DNA replication over that of hostDNA. The capsomeres become organized into a capsid around the viral DNAto form progeny virions. The virions are released from the host cell by lysis or by budding. Deviations from this generalized life cycle exist in the dsDNA hepatitis B virus and in the ssDNA parvoviruses.

RNA Viruses

Few host cells contain the enzymes necessary to replicate or repair RNA (rare exceptions are mentioned with viroids later in this chapter). Thus, the genes of RNA viruses have much higher mutation rates (e.g., 10–3 to 10sup>–4) than DNA viruses, and they must either code for these enzymes or carry these enzymes with them when they infect a host cell. RNA viruses with single-stranded genomes that function as mRNAs are said to have positive or plus (+) strand genomes specifying (minimally) the coat proteins and the enzyme(s) needed for replication. RNA viruses with negative or minus (–) strand genomes have DNA that is complementary to the genomic or mRNA strand, and so cannot be translated. Such viruses must, therefore, encode an RNA-dependent, RNA polymerase that can synthesize a (+)RNA strand from a (–) RNA template, and this enzyme must be packaged in the virion together with the viral RNA genome. For all RNAviruses except the retroviruses, double-stranded RNA is always an intermediate in viral RNA replication, even if the infective virion contains only single-stranded RNA (ssRNA). Double-stranded RNA is replicated in an analogous manner as DNA; i.e., each RNA strand serves as a template for making a complementary RNA strand. The viral enzyme that replicates viral RNA in this way is an RNA-dependent RNA polymerase called RNA replicase. Retroviruses contain a reverse transcriptase enzyme that copies their RNA genome into a DNA copy (cDNA). This cDNA can then be used for transcription or, in the case of retroviruses, can become integrated into the genome of the host cell.

Four model life cycles for the RNA viruses are easily recognized.

  1. Model 1. If the viral RNA is double-stranded (dsRNA), the (+)strand is transcribed to produce RNA replicase. This enzyme not only replicates viral dsRNA [using both (+)strands and (–)strands as templates] to form dsRNA progeny genomes, but also makes many (+)copies using the (–)strands as templates. These extra (+)strands are required as mRNA templates for translating viral proteins in a relatively short period of time.
  2. Model 2. If the viral RNAis a single (+)strand, the virion enters the host cell, becomes uncoated, and the (+)strand RNA is translated to produce an RNA replicase. The replicase then synthesizes a complementary (–)RNA strand using the (+)strand as a template, thereby forming a double-stranded RNA replicative intermediate. The (–)strands are needed as templates for the synthesis of (+)genomic strands of progeny virions. Some of the (+)strands are translated by the host cell's machinery into capsid proteins, membrane proteins, etc.
  3. Model 3. If the viral RNA is a single (–)strand, it cannot serve as a translational template (mRNA) for making RNA replicase. Hence, this enzyme must be brought into the host cell along with the viral RNA. The RNA replicase uses the (–)strand as a template to produce a complementary (+)strand. More (–)strands are produced using the (+)strand(s) as templates and more (+)strands are produced using the (–)strand(s) as templates. The (+)strands serve as mRNAs for making viral proteins. The (–)strands then associate with the capsid proteins and RNA replicase to be packaged into progeny virions.
  4. Model 4. Retroviruses contain single (+)strand RNAgenomesthat are not used asmRNA.They are first reverse transcribed into a cDNA copy. This cDNA integrates into the genome. The latent state begins. Upon activation, viral mRNAs are produced from the integrated viral DNA and viral-specific proteins are produced. Retroviruses are further discussed later in this chapter under the subject of cancer.
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