The requirement for cleavage affects which tissues can produce infectious virus. The cleaved protein needs to then undergo a conformational change, usually caused by exposure to a acidic endosome environment when it infects the next cell, before it can cause fusion.
NA probably helps the virus leave the cell by removing sialic acid from receptors. NA may also help the virus penetrate mucus to reach epithelial cells of the respiratory tract by enabling it to dissociate from sialic acid-containing receptors in the mucus by destroying them. The neuraminidase does not prevent the virus infecting new cells because endocytosis is presumably faster than receptor removal.
There are similarities and differences between the Paramyxovirus family and the Orthomyxovirus family, members of both are enveloped, both contain negative sense, single stranded RNA, have helical nucleocapsids. However, the two families are very different. There is NO immunological relationship between the two families. Need for mRNA primer. Movies were created by Stephan Spencer. Bluetongue virus the members of the Coltivirus family e. Colorado tick fever virus Figure 22 Structure of a typical reovirus Adapted from Joklik et al.
Zinsser Microbiology 20 th Ed. Reoviruses have icosahedral symmetry and a multiple layered capsid inner and outer capsid figure 22 The RNA is double stranded.
There are segments depending on the genus of the Reovirus family to which the virus belongs figure There are some significant differences in the life cycle of members of the reovirus family and of the rotavirus family. Due to their clinical importance in humans, we shall focus on rotaviruses. Adsorption, penetration and uncoating It is still not clear what exactly what happens in vivo during the entry of rotaviruses into the cell. There appears to be a need for a protease to remove some of the outer layer of the capsid and to generate an "intermediate sub-viral particle" ISVP before the virus can enter the cytoplasm.
A viral attachment protein is then exposed on the ISVP, probably at the vertices, and binds to host cell receptors. The activated ISVP enters the cytoplasm directly or via endocytosis. In the cytoplasm, the virion RNA is copied by the viral RNA polymerase while still in a nucleocapsid that has fewer proteins associated with it than are associated with the ISVP or the virion.
The RNA is capped and methylated by virion packaged enzymes. It is then extruded from the vertices of the capsid. Assembly More proteins are made and eventually the immature capsids bud into the lumen of the endoplasmic reticulum. In doing so, they acquire a transient envelope which is lost as they mature. This is a very odd feature of the rotaviruses. Release This probably occurs via cell lysis. Note: The entire replication cycle occurs in the cytoplasm Return to the Virology section of Microbiology and Immunology On-line.
Some differences between rhabdoviruses and paramyxoviruses Rhabdovirus Paramyxovirus Shape bullet bacilliform round pleomorphic Glycoproteins One has both attachment and fusion activities Two one attachment and one fusion Fusion pH acidic neutral physiological.
Examples: influenza virus orthomyxovirus measles virus , mumps virus , paramyxoviruses rabies virus rhabdovirus Double-stranded RNA viruses The virion genomic RNA is double stranded and so cannot function as mRNA; thus these viruses also need to package an RNA polymerase to make their mRNA after infection of the host cell.
Infectivity of RNA. Initial event in cell. Used with permission. It functions as mRNA immediately upon infection Prototype member: poliovirus figure 1 and 2 Adsorption and penetration A viral protein recognizes a receptor on the host cell membrane this is important in the tropism of virus.
Synthesis of viral proteins Poliovirus virion RNA functions as an mRNA but does not have the methylated cap structure typical of eucaryotic mRNAs - it has a "ribosome landing pad" known as the internal ribosome entry site or IRES which enables ribosomes to bind without having to recognize a 5' methylated cap structure figure 3.
Figure 4 Adapted from Schaechter et al. Figure 5 Replication of Picornaviridae viral genome. RNA replication We now have newly made viral proteins to support replication. It may be translated into polyprotein In this case VPg is usually removed prior to translation Assembly When sufficient plus-sense progeny RNA and virion proteins have accumulated, assembly begins.
Figure 7 Structure of a typical rhabdovirus Figure 7b Rabies virus budding from an inclusion Negri body into the endoplasmic reticulum in a nerve cell. The RNA genome: is single stranded is negative minus sense codes for 5 proteins Attachment, penetration and uncoating The virus adsorbs to cell surface. Translation Messenger RNAs are translated on host ribosomes and all five viral proteins made at the same time.
New negative strands may: i. Figure 9 Transport of glycoproteins from the endoplasmic reticulum to the plasma membrane. Assembly The virus consists of two "modules" - the envelope and the nucleocapsid: Envelope Transmembrane proteins are made on ribosomes bound to the endoplasmic reticulum.
Figure 10 Rhabdovirus assembly. Nucleocapsid Synthesis of the nucleocapsid was described above. Note: The entire life cycle occurs in the cytoplasm RNA polymerase and RNA modification enzymes are virally-coded and present in the virus particle virion There is no division between early and late stages. The envelope contains two virally coded glycoproteins: The F protein and the attachment protein The F protein has fusion activity The attachment protein binds to receptors on the host cell This protein may have: Hemagglutinating activity and neuraminidase activity HN protein or hemagglutinating activity alone H protein or neither G protein.
Figure 12 Structure of a typical paramyxovirus. HN, F. Figure 13 Attachment and endocytosis of paramyxoviruses. Figure 14 Transcription and replication of paramyxovirus RNA.
Transcription, translation and replication of RNA Events inside the cell are very similar to rhabdoviruses figure 14 : Viral multiplication occurs in the cytoplasm. Assembly Both viral glycoproteins i. Role of neuraminidase In those paramyxoviruses which have it, the neuraminidase may facilitate release. Activation of the F protein The F protein needs to be cleaved before it can function in facilitating fusion when the virus binds to another cell figure Some differences between rhabdoviruses and paramyxoviruses.
Virions contain RNA polymerase packaged within the virus particle These viruses are enveloped and have two membrane glycoproteins figure 19 : HA - hemagglutinin - This is the attachment and fusion protein NA - neuraminidase - This is important in release. Figure 19 Structure of a typical orthomyxovirus. Adsorption and penetration The virus adsorbs to receptors on the cell surface and is internalized by endocytosis.
At acid pH of an endosome, HA undergoes a conformational change and fusion occurs. Nucleocapsids are released to cytoplasm. Figure 20 Transcription of orthomyxoviridae RNA. Transcription, translation and replication Nucleocapsids are transported into the nucleus. Note: HA needs to be cleaved before it can promote fusion. RNA synthesis. Syncytia formation. Colorado tick fever virus. Figure 22 Structure of a typical reovirus Adapted from Joklik et al. Figure 24 Replication of reoviridae.
The mRNAs are translated and the resulting viral proteins assemble to form an immature capsid. The mRNAs are packaged into the immature capsid and are then copied within the capsid to form double stranded RNAs It is not known how the virus ensures that each particle gets one copy of the 11 different mRNAs figure More mRNAs are now made by the newly formed immature capsids.
Note: The entire replication cycle occurs in the cytoplasm. Viruses also reproduce, but they cannot do so on their own. They use the replication apparatus of the host cells, and have additionally developed a number of special characteristics.
Scientists differentiate viruses according to the genome type — there are DNA and RNA viruses: viruses may have single-stranded or double-stranded linear RNA, single-stranded or double-stranded linear DNA, single-stranded or double-stranded circular DNA and other variations. Some viruses contain some of the enzymes required for their replication, for example the influenza virus, whose envelope not only contains an RNA genome but also an RNA polymerase.
When the virus enters the host cell, the enzyme RNA polymerase starts to replicate the viral genome. The synthesis of the genome of DNA viruses usually begins at a replication origin that binds specific initiator proteins, which recruit replication enzymes of the host cell which then replicate the viral genome. The HI virus is a retrovirus and thus a very exotic case. The virus got its name due to the fact that it reverses the normal process of transcribing DNA into RNA transcription during reproduction.
The virus has a single-stranded RNA genome and an enzyme called reverse transcriptase. Once the viral genome is integrated into the host genome, it can be transcribed into RNA by the host enzymes at which point it can reproduce.
Since viruses are able to use a broad range of replication mechanisms to reproduce, scientists working on the development of anti-viral drugs need to specifically investigate the individual viruses one by one. Many of the currently used drugs interfere with viral replication, for example the so-called nucleoside analogues which are used against the hepatitis B virus.
Replication is also of great interest in the field of medicine, in particular in the fight against cancer. Cancer cells are body cells that no longer behave normally - they replicate their genome and proliferate far more often than healthy cells. Researchers and physicians exploit this behaviour in their work on substances designed to interfere with cancer growth.
Some substances inhibit replication, which prevents tumour growth. Modern chemotherapy uses alkylating agents e. These substances bind to DNA via alkyl groups. Since these groups have two binding sites, the genome is joined together, thereby preventing it from replicating.
Another example is platinum analogues that are among the most effective chemotherapeutic drugs. These substances have a platinum atom that binds to the DNA and joins it e. Substances like anthracyclins and the antibiotic doxorubicin intercalate DNA and make it unaccessible to DNA polymerase, thereby preventing the enzyme from synthesising DNA.
The principle of replication was used for the first time in the s by molecular biologists in the laboratory. When researchers are investigating DNA they usually have very small amounts of the molecule available. In order to be able to make effective statements, the quantity of DNA needs to be increased.
The DNA fragments of interest are used as a template, to which free DNA constituents nucleotides and specific primer sequences are added. The PCR process consists of several cycles of repeated heating and cooling of the reaction in order to melt and enzymatically replicate the DNA. As the reaction progresses, the DNA generated is used as a template for further amplification. If the researchers do not know the sequence of the DNA template to be replicated, the primers used can help indicate the sequence as they consist of a specific sequence of nucleotides that enable the scientists to draw conclusions on the amplified material.
PCR has been further developed in many ways and has now become a standard automated method in molecular biology laboratories around the world. What made autonomously replicating systems develop? Many scientists assume that RNAs were the first replicating macromolecules. RNAs, just like DNA, are in principle able to create negative blueprints of themselves through spontaneous base-pairing.
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SARS-coronavirus replication is supported by a reticulovesicular network of modified endoplasmic reticulum.
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