Eukaryotic Viruses Help (page 3)
Eukaryotic viruses differ in many respects from virulent bacteriophages; some of the more obvious differences are outlined in Table 11.2.
Some animal viruses cause relatively mild diseases such as the common cold; others cause more severe problems or even life-threatening conditions such as rabies, AIDS, and cancer.
Viral capsids are usually constructed from only one or a few types of proteins and thus do not require much coding information. The viruses that infect animal cells have been classified into four morphological types: (1) naked icosahedral, (2) naked helical, (3) enveloped icosahedral, and (4) enveloped helical. Naked viruses have a protein capsid but no lipid envelope. An envelope is a portion of the host-cell membrane acquired as the virus leaves the cell by a budding process. The envelope is derived from the cell membrane in two steps. First, glycoproteins specified by the viral genome are inserted into the membrane. Then the virion capsid attaches to the cytoplasmic ends of the glycoproteins, causing the membrane to adhere to the capsid. The enveloped virus pinches off from the cell surface without creating a hole in the cell membrane. To infect another cell, an infective virus particle attaches to a specific receptor on the host-animal cellmembrane, either by capsid proteins of a naked virion or by the viral glycoproteins extending from the surface of an enveloped virion. The attached virion is then engulfed by the host cell and the viral genome becomes uncoated (removal of the capsid) inside the cell.
The genomes of animal viruses may be DNA or RNA, single-stranded or double-stranded. Double-stranded DNA viral genomes may be either linear or circular. Circular double-stranded DNA may be covalently closed on one or both strands. All known double-stranded RNA viral genomes are segmented (i.e., consisting of multiple RNA molecules, each carrying a different set of genes). Some single-stranded RNA genomes are also known to be segmented. There are no known segmented DNA viruses or circular RNA genomes in eukaryotic viruses. Most DNAvirus genomes are copied in the nucleus, using the host's RNA polymerase and other enzymes for capping, splicing, and adding poly-A tails in processing their transcripts. Most RNA viruses (except influenza virus) replicate in the cytoplasm.
The great diversity of animal viruses has been classified into 15–20 viral families based on characteristics such as type and structure of nucleic acid, virion morphology, and common antigenic determinants. Because of the dependent relationship of the viral genome on its mRNAs for replication, animal viruses have been recognized as falling into seven groups as summarized in Fig. 11.5.
EXAMPLE 11.6 All adenoviruses have a double-stranded DNA genome of about 36 kb and a naked icosahedral capsid of 252 subunits with prominent spikes at the vertices. Their mRNAs are transcribed directly from their genomes and their genomes replicate directly from their double-stranded DNA templates.
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.
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.
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.
- 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.
- 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.
- 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.
- 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.
Plant viruses exist in rod and polyhedral shapes. Most plant viruses have genomes consisting of a single RNA strand of the (+)type. The best-known plant virus is the rod-shaped tobacco mosaic virus (TMV; Fig. 11.6 ), which has a single-stranded (+)RNA genome of 6395 nucleotides. Some viruses with (þ)genomes, however, cannot replicate unless the host cell is infected with two different virions. Such viruses are said to have segmented genomes. If the genomic fragments reside in different capsids, the virus is said to be heterocapsidic; if the fragments reside in the same capsid, the virus is said to be isocapsidic.
EXAMPLE 11.7 The heterocapsidic genome of the cowpea mosaic virus consists of two RNA chains, each encoding different proteins essential for replication. Each of these RNA chains is encapsulated into separate virions. The virus can only replicate in a host cell that has been infected by both kinds of virions.
Relatively few plant viruses have DNA genomes. There are only two classes of DNA plant viruses: those with a double-stranded DNA genome in a polyhedral capsule and those characterized by a connected pair of capsids, each containing a circular, single-stranded DNA molecule. The paired genomes may be identical in some viruses and markedly different in others.
Plant viroids have a very small RNA genome of 240–350 nucleotides in a single-stranded circle that can form extensive internal base pairing. This gives it essentially a stiff, double-helical structure and renders it resistant to digestion by ribonuclease enzymes that usually cut only at unpaired ribonucleotides. The genome is too small to code for any proteins. At least some plant cells (unlike animal cells) are known to contain enzymes capable of replicating RNA. Viroids are not encapsulated in a protein coat. They do not pass through a DNA stage in their life cycle and are not integrated into the host chromosomes.
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