Their mRNA is produced by transcription in much the same way as with cellular DNA, using the enzymes of the host cell. Multiple, full-length RNA strands of negative polarity complementary to the positive-stranded genomic RNA are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.
In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome. Group VI viruses have diploid two copies ssRNA genomes that must be converted, using the enzyme reverse transcriptase , to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome.
The characteristics of each group in the Baltimore classification are summarized in Figure with examples of each group. Viruses are tiny, noncellular entities that usually can be seen only with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate either by using the replication proteins of a host cell or by using proteins encoded in the viral genome.
Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.
Figure Which of the following statements about virus structure is true? The first electron micrograph of a virus tobacco mosaic virus was produced in Before that time, how did scientists know that viruses existed if they could not see them? Viruses pass through filters that eliminated all bacteria which were visible in the light microscopes at the time. As the bacteria-free filtrate could still cause infections when given to a healthy organism, this observation demonstrated the existence of very small infectious agents.
These agents were later shown to be unrelated to bacteria and were classified as viruses. Varicella-zoster virus is a double-stranded DNA virus that causes chickenpox. How does its genome structure provide an evolutionary advantage over a single-stranded DNA virus? Classify the Rabies virus a rhabdovirus family member and HIV-1 with both the Baltimore and genomic structure systems.
Compare your results. What conclusions can be made about these two different methods? The genome structure system classifies both viruses as single-stranded RNA viruses with linear genomes. Baltimore classification sorts Rabies virus and HIV-1 into two different groups, indicating that the two viruses have very different life cycles. However, genome structure classification does not distinguish between the two viruses. This leaves out important information regarding virus function and survival.
Use this tool to review the glossary terms above. Feel free to change the study mode and other options. Increase Font Size. Biology Go to home Viruses Previous: Perspectives on the Phylogenetic Tree. Next: Virus Infections and Hosts. The tobacco mosaic virus, seen here by transmission electron microscopy left , was the first virus to be discovered.
The virus causes disease in tobacco and other plants, such as the orchid right. Learning Objectives By the end of this section, you will be able to do the following: Describe how viruses were first discovered and how they are detected Discuss three hypotheses about how viruses evolved Describe the general structure of a virus Recognize the basic shapes of viruses Understand past and emerging classification systems for viruses Describe the basis for the Baltimore classification system.
Discovery and Detection Viruses were first discovered after the development of a porcelain filter—the Chamberland-Pasteur filter—that could remove all bacteria visible in the microscope from any liquid sample.
Most virus particles are visible only by electron microscopy. In these transmission electron micrographs, a a virus is as dwarfed by the bacterial cell it infects, as b these E. Nataro and S. Other variations occur between the orders, for example, Nidovirales are isolated for their differentiation in expressing structural and non-structural proteins separately. However, this system of nomenclature differs from other taxonomic codes on several points. A minor point is that names of orders and families are italicized, as in the ICBN.
The recognition of orders is very recent and has been deliberately slow; to date, only three have been named, and most families remain unplaced. Approximately 80 families and species of virus are known. Holmes used Carolus Linnaeus system of binomial nomenclature classification system to viruses into 3 groups under one order, Virales.
They are placed as follows:. It is as follows:. Casjens and Kings classified virus into 4 groups based on type of nucleic acid ,presence of envelope,symmetry and site of assembly. Satellites depend on co-infection of a host cell with a helper virus for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it's then called a satellite virus.
Prions, named for their description as " pr oteinaceous and i nfectious particles," lack any detectable as of nucleic acids or virus-like particles. They resist inactivation procedures which normally affect nucleic acids. If purified virions are gently lysed under appropriate buffer conditions, with the addition of NTPs, mRNAs will be transcribed in the test tube.
However, genome RNA will not be synthesized under these conditions Table Schematic representation of replication of genomes of minus-strand RNA viruses. Upon entry into the cell, the active transcription complex synthesizes mRNAs. This process can also occur in a test tube see Fig. Translation of mRNAs produces proteins required for genome replication. Thus if protein synthesis is blocked in the infected cell, mRNAs continue to by synthesized but genome replication does not occur.
Newly synthesized proteins provide the switch from transcription to genome replication. The genomes of viruses in the order Mononegavirales are unsegmented, negative-strand RNA. Note that genome synthesis does not occur under these conditions. In contrast, viruses that use an ambisense coding strategy transcribe some mRNAs from the copy genome. There are virus families in which some members are considered negative-strand RNA viruses while others use an ambisense strategy.
Thus these two strategies are closely related. Some ambisense viruses package copy genomes that can be used as templates for transcription, such that the full complement of viral genes can be transcribed soon after infection.
It should be noted that packaged copy genomes are not mRNAs and are not translated. Reoviruses also have segmented genomes, packaging 11—12 segments of dsRNA. Reoviruses are nonenveloped and particles consist of two or three concentric icosahedral capsid layers.
A unique feature of the reovirus replication cycle is that the genome segments are transcribed from within the capsid. The genomes of RNA viruses have some common general features. Obviously there are one or more open reading frames that encode the viral proteins. But there are also regions of RNA that do not code for protein. These non-coding regions NCRs or untranslated regions UTRs are often highly conserved within a virus family, indicating that they have important functions. NCRs may have specific, critical nucleotide sequences but in some cases they are regions of the genome that fold into conserved structures, and structure may be more critical than a specific sequence.
Of course a source of RdRp must be supplied. RdRp may be encoded in the minigenome or may be supplied in trans by using a cell line stably expressing the viral RdRp, for example. The sequences required to direct RNA replication are often fairly simple and can be linked to virtually any RNA sequence to drive its replication. These promoter sequences can be rather short but provide a means to direct the RdRp to internal sites on the genome. There may also be specific RNA sequences that signal polyadenylation.
There are a variety of different strategies that RNA viruses use to regulate transcription and genome replication, but all involve RNA sequences found in the genome. The RNA genomes of some viruses are highly structured and extensively base paired. The IRES serves as a platform for ribosome assembly.
Promoters can be quite long and complex and promoter regions themselves are not transcribed. It is particularly important, in the case of genome synthesis, that genetic information not be lost or modified; however, mRNAs are often capped and polyadenylated. Are the methods for priming viral mRNA synthesis the same or different from the methods of priming genome replication? The RNA viruses seem to have experimented widely. For example, the picornaviruses use poly A tracts encoded in the genome.
The hepadnaviruses contain a DNA genome that is partially double-stranded, but contains a single-stranded region.
The pregenome is used as a template for the reverse transcriptase to produced minus-strand DNA genomes, with a small piece of pregenome used as a primer to produce the double-stranded region of the genomes. Double-stranded RNA viruses infect bacteria, fungi, plants, and animals, such as the rotavirus that causes diarrheal illness in humans.
But cells do not utilize dsRNA in any of their processes and have systems in place to destroy any dsRNA found in the cell. Thus the viral genome, in its dsRNA form, must be hidden or protected from the cell enzymes. Cells also lack RNA-dependent RNA-polymerases , necessary for replication of the viral genome so the virus must provide this enzyme itself.
For the rotavirus, the viral nucleocapsid remains intact in the cytoplasm with replication events occurring inside, allowing the dsRNA to remain protected. Messenger RNA is transcribed from the minus-strand of the RNA genome and then translated by the host ribosome in the cytoplasm. Viruses with plus-strand RNA, such as poliovirus, can use their genome directly as mRNA with translation by the host ribosome occurring as soon as the unsegmented viral genome gains entry into the cell.
Translation of the poliovirus genome yields a polyprotein , a large protein with protease activity that cleaves itself into three smaller proteins. Additional cleavage activity eventually yields all the proteins needed for capsid formation, as well as an RNA-dependent RNA-polymerase. Other possibilities include:.
Minus-strand RNA viruses include many members notable for humans, such as influenza virus, rabies virus, and Ebola virus. Upon entrance into the host cell, the plus-strand RNAs generated by the polymerase are used as mRNA for protein production. Instead, the virus uses its reverse transcriptase to synthesize a piece of ssDNA complementary to the viral genome. This allows the virus to insert its genome, in a dsDNA form, into the host chromosome, forming a provirus.
Unlike a prophage, a provirus can remain latent indefinitely or cause the expression of viral genes, leading to the production of new viruses. Excision of the provirus does not occur for gene expression.
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