Viruses dna rna

Sharing is Caring!! Newsletter Updates Subscribe to our newsletter and never miss an important update!! Make a Voluntary Contribution. Compounds April 29, 1 Comment. Trending now. Shopping Cart. No products in the cart. Log in. Login with Google Login with Facebook. Don't have an account? Email Login with Google Login with Facebook. Back to login. Know More. They are larger in size as compared to virus. Have no metabolism of their own. Have metabolism of their own. Take no food by any method.

Take food by absorption. Command the host cell to produce virus. They can reproduce by their own. They have more complex DNA. Some common examples of DNA viruses are parvovirus, papillomavirus, and herpesvirus. DNA viruses can affect both humans and animals and can range from causing benign symptoms to posing a very serious health risk.

This coating of viral DNA is known as a capsid. The capsids accumulate inside the cell until the cell reaches capacity and bursts open, releasing the newlyformed viruses to infect new host cells. Some examples of retroviruses are hepatitis viruses and HIV. This process, called reverse transcription, enables the virus to inject its genetic material into the host cell and use the host's biochemical machinery, similar to a DNA virus. Often, retroviruses use an enzyme, called integrase, to insert the retroviral DNA into the genome of the host cell.

Translation of this mRNA generates proteins required for replication and viral encapsidation. As such, many dsRNA viruses undergo replication within their icosahedral capsids. The replicating RNA polymerases are located within the capsid and produce mRNA strands that are extruded from the particle. Replication occurs in the cytoplasm. The viral RdRP complex is assumed to be the same for both replication and transcription and can switch off functions as required.

Of note, two genome subgroups can be distinguished in this group: nonsegmented and segmented. Viruses with segmented genomes replicate in the nucleus, and the RdRp produces one monocistronic mRNA strand from each genome segment. The mode of transcription is similar to eukaryotic transcriptional events in which the process is divided into three steps: 1 the initiation step, when a transcription initiation complex is assembled at the promoter region located upstream of the transcriptional start site, allowing for the recruitment of the RNA polymerase, 2 the elongation step, in which, the polymerase is recruited to template DNA, is activated by phosphorylation of the c arboxy- t erminal d omain CTD , and proceeds to transcribe the template DNA to RNA, and 3 the termination step, which involves the recognition of specific signals, including the polyadenylation signal.

For productive infection, viruses must then utilize this machinery, and remain both stable and undetected in the cell. Furthermore, while the great majority of cellular mRNAs are monocistronic, viruses must often express multiple proteins from their mRNAs. As a result, viruses have evolved a number of mechanisms to allow translation to be customized to their specific needs.

Straightforward exploitation of the cellular capping machinery is typical of DNA viruses that replicate in the nucleus. Other strategies used by viruses include internal initiation of translation of uncapped RNAs in picornaviruses and their relatives, snatching of capped oligonucleotides from host pre-mRNAs to initiate viral transcription in segmented negative-strand RNA viruses, and recruitment of genes for the conventional, eukaryotic-type capping enzymes that apparently occurred independently in diverse groups of viruses flaviviruses, reoviridae, poxviruses, asfarviruses, some iridoviruses, phycodnaviruses, mimiviruses, baculoviruses, nudiviruses.

For instance, flaviviruses e. Other examples that follow the same strategy include rotaviruses, barley yellow dwarf viruses, and possibly Hepacivirus C HCV. Since eukaryotic cells are not equipped to translate polycistronic mRNA into several individual proteins, DNA viruses overcome this limitation by using the cellular mechanism of splicing of their polycistronic mRNA to monocistronic mRNA.

RNA viruses, on the other hand, that mostly replicate in the cytoplasm, do not have access to these host mechanisms and consequently produce monocistronic sgRNAs e. However, the use of these mechanisms is not without consequences: 1 some viral proteins may be expressed from sgRNAs but the components of the replication complex that are needed early in infection must still be translated from the genomic RNA, 2 viruses with segmented genomes have to ensure the correct packaging of the different segments, and 3 polyprotein expression represents an inefficient use of host cell resources as all virus proteins are produced in equal amounts, even though catalytic proteins are often required in much smaller quantities than the structural proteins.

Alternative and more efficient mechanisms of expressing multiple proteins from a single viral mRNA involve internal ribosome entry, leaky scanning, ribosome shunting, reinitiation, ribosomal frameshifting, and stop codon read-through.

Viral gene expression is facilitated by the possession of regulatory signals within viral mRNAs that are recognizable by the host cell. These signals ultimately enable the virus to shut off host gene expression to ensure preferential viral gene expression.

The strategies are reviewed in the section that follows. Transcription can be viewed as a highly regulated 3-phase process: initiation, elongation, and termination. Initiation of transcription requires the recruitment and assembly of a large multiprotein DNA-binding transcription initiation complex.

During the course of evolution, several viruses have developed strategies that affect the loading of host transcription initiation factors into transcription complexes, which ultimately shuts down host protein synthesis Fig. Viral mRNA transcripts compete against cellular mRNAs and preferentially gain access to the cellular gene expression machinery.

Different strategies used by viruses to down regulate host transcription. The capping apparatus can be either host- or virus-derived. If virus-encoded, cleavage is carried out by the endonuclease activity of the viral RdRp. Sequence complementarity shared between the nucleotides within the cleavage site of the donor mRNA and the viral RNA facilitates successful cap snatching.

The cap-stealing mechanisms used by segmented RNA viruses to generate their mRNAs circumvent this innate detection system. Cap snatching of cellular mRNA.

Downstream hairpin loops are RNA structures that facilitate initiation of cap-dependent translation in the absence of eIF2 translation initiation factors. In addition, the physiological state of the infected cell dictates whether host mRNA transcripts undergo cap-dependent translation or cap-independent translation.

When the cell exhibits normal housekeeping functions, translation of cellular mRNAs is carried out by a cap-dependent mechanism; however, under stressful conditions, such as heat shock, viral infection, hypoxia, and irradiation, the translation mechanism switches from cap dependency to IRES-driven mechanisms.

Infection by a range of viruses induces the activation of the ER stress response, resulting in the stimulation of IRES-dependent translation. As such, viruses containing IRES are able to efficiently benefit from the host cells ER stress response for their own multiplication.

This is the site of binding of poly A binding protein in the cytoplasm. Viral mRNAs are synthesized without this signal sequence. Stuttering occurs at a site containing a slippery sequence mononucleotide repeats and involves 1-base repeated frameshifts on the mRNA strand Fig. Stuttering mechanism. This mechanism, also observed in some eukaryotes, allows RNA viruses except dsRNA viruses to produce multiple proteins from a single gene.

In these viruses, the RNA polymerase reads the same template base more than once, creating insertions or deletions in the mRNA sequence, thereby generating different mRNAs that encode different proteins.

There are two kinds of mRNA editing: 1 cotranscriptional editing through polymerase slippage and 2 posttranscriptional editing. RNA editing in members of the Ebolavirus genus increases their genome coding capacity by producing multiple transcripts encoding variants of structural and nonstructural glycoproteins from a single gene, ultimately increasing its ability for host adaptation.

Also observed in many cellular organisms, alternative splicing allows production of transcripts having the potential to encode different proteins with different functions from the same gene Fig.

The sequence of the mRNA is not changed as with RNA editing; rather the coding capacity is changed as a result of alternative splice sites. Alternative splicing is regulated by cellular and viral proteins that modulate the activity of the splicing factors U1 and U2, both of which are components of the spliceosome. Activation of the spliceosome is facilitated by cis-acting signals in the mRNA sequence. While only mature, spliced mRNA transcripts are exported out of the nucleus, hepadnaviruses and retroviruses are able to export nonspliced mRNA transcripts out of the nucleus for translation.

On the other hand, the NS1 protein n onstructural p rotein 1 of influenza viruses can interact with multiple host cellular factors via its effector- and RNA-binding domains. It is capable of associating with numerous cellular spliceosome subunits, such as U1 and U6 snRNAs, and can inhibit cellular gene expression by blocking the spliceosome component recruitment and its transition to the active state.

Alternative splicing. Alternative splicing is common in parvovirus pre-mRNA transcript processing and allows for the generation of different proteins from a specific nucleotide sequence on the viral mRNA strand.

Dotted lines indicate alternative splice sites. Therefore, viruses can induce preferential induction of viral mRNA splicing by the cellular splicing machinery. Knowledge concerning the coordination between cellular and viral genome splicing comes from adenoviruses and retroviruses, but only limited data are available for other viruses, for example, influenza viruses.

This is also referred to as stop codon read-through, and is a programmed cellular and viral-mediated mechanism used to produce C-terminally extended polypeptides, and in viruses, it is often used to express replicases. Termination of translation occurs when one of three stop codons enters the A-site of the small 40S ribosomal subunit.

Stop codons are recognized by release factors eRF1 and eRF3 , which promote hydrolysis of the peptidyl-tRNA bond in the peptidyl transferase center P-site of the large ribosomal subunit. Read-through occurs when this leaky stop codon is misread as a sense codon with translation continuing to the next termination codon.

Read-through signals and mechanisms of prokaryotic, plant, and mammalian viruses are variable and are still poorly understood. Programmed ribosomal frameshifting is a tightly controlled, programmed strategy used by some viruses to produce different proteins encoded by two or more overlapping open reading frames Fig. Ordinarily, ribosomes function to maintain the reading frame of the mRNA sequence being translated.

However, some viral mRNAs carry specific sequence information and structural elements in their mRNA molecules that cause ribosomes to slip, and then readjust the reading frame. This ribosomal frameshift enables viruses to encode more proteins in spite of their small size. Viruses are classified on the basis of morphology, chemical composition, and mode of replication. The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.

In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.

Such nucleocapsids form rigid, highly elongated rods or flexible filaments; in either case, details of the capsid structure are often discernible by electron microscopy. In addition to classification as flexible or rigid and as naked or enveloped, helical nucleocapsids are characterized by length, width, pitch of the helix, and number of protomers per helical turn.

The most extensively studied helical virus is tobacco mosaic virus Fig. Many important structural features of this plant virus have been detected by x-ray diffraction studies. Figure shows Sendai virus, an enveloped virus with helical nucleocapsid symmetry, a member of the paramyxovirus family see Ch.

The helical structure of the rigid tobacco mosaic virus rod. About 5 percent of the length of the virion is depicted. Individual 17,Da protein subunits protomers assemble in a helix with an axial repeat of 6. Each more Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope. The intact nucleocapsid is about 1, nm long and 17 nm in diameter; its pitch more An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.

Lines through the centers of opposite triangular faces form axes of threefold rotational symmetry; twofold rotational symmetry axes are formed by lines through midpoints of opposite edges. An icosaheron polyhedral or spherical with fivefold, threefold, and twofold axes of rotational symmetry Fig.

Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. These axes are perpendicular to the plane of the page and pass through the centers of each figure.

Both polyhedral upper and spherical lower forms more Viruses were first found to have symmetry by x-ray diffraction studies and subsequently by electron microscopy with negative-staining techniques. In most icosahedral viruses, the protomers, i.

The arrangement of capsomeres into an icosahedral shell compare Fig. This requires the identification of the nearest pair of vertex capsomeres called penton: those through which the fivefold symmetry axes pass and the distribution of capsomeres between them. Adenovirus after negative stain electron microscopy. A The capsid reveals the typical isometric shell made up from 20 equilateral triangular faces. The net axes are formed by lines of the closest-packed neighboring capsomeres.

In adenoviruses, the h and k axes also coincide with the edges of the triangular faces. This symmetry and number of capsomeres is typical of all members of the adenovirus family.

Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species. The larger viruses contain in a core the nucleic acid genome complexed with basic protein s and protected by a single- or double layered capsid consisting of more than one species of protein or by an envelope Fig. Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins.

SU stands for more Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses. Mutation rates of 10 -4 lead to the continuous generation of virus variants which show great adaptability to new hosts.

The viral RNA may be single-stranded ss or double-stranded ds , and the genome may occupy a single RNA segment or be distributed on two or more separate segments segmented genomes. In addition, the RNA strand of a single-stranded genome may be either a sense strand plus strand , which can function as messenger RNA mRNA , or an antisense strand minus strand , which is complementary to the sense strand and cannot function as mRNA protein translation see Ch. Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins.

Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components. Schemes of 21 virus families infecting humans showing a number of distinctive criteria: presence of an envelope or double- capsid and internal nucleic acid genome. DsRNA viruses, e. Each segment consists of a complementary sense and antisense strand that is hydrogen bonded into a linear ds molecule. The replication of these viruses is complex; only the sense RNA strands are released from the infecting virion to initiate replication.

The retrovirus genome comprises two identical, plus-sense ssRNA molecules, each monomer 7—11 kb in size, that are noncovalently linked over a short terminal region.