Penetration results in the internalization of the virus or its nucleic acid.

1) Viruses penetrate cells through direct penetration such as the parvoviruses.
2) Fusion can also occur at the plasma membrane.

The retroviruses and the paramyxoviruses fuse with the membrane and are resopnseible for sycnctyium formation which is doue to the fusion of many adjacenet infected cells to generaget a giant cell containing many nuclei.

3) Receptor mediated endocytosis—this occurrs with the flu virus, rhabdovirus, and non-envoeloped viruses. Glycoproteins are used in this process and they can not get in unless the pH is low. This is facilitated by the fusion of the lysosome with the endosome—the pH drop allows for the release of the virus from the vesicle.

Segmented Negative strand RNA viruses

1. the negative strand enters the cell—the first event to occur is the transcription to produce the plus strand

2. the plus strand is then used to replicate the negative strand genome and

3. plus strands are also used to make proteins through translation—and viral proteins.

--this process occurs for all RNA segments in the genomes

Non-segmented Negative strand RNA viruses

1. the negative strand is transcribed to make a positive strand

2. the positive strands are used to make copies of the negative strand

3. the positive strands are used to make viral proteins that are subsequently cleaved to make the subunits of the virus capsid.

--for both of the above, the RNA-dependent RNA polymerase is used to make the positive strand RNA

Ambisense RNA Viruses

1. they have a genome that is one molecule of RNA that is both + and – in polarity

2. RNA-dependent RNA polymerase makes positive strands from the negative side that is used for translation

3. The same enzyme makes a positive copy of the negative side of the original.

4. From the positive copy made above, negative stands are used to make positive RNA copies for the genome.

Positive Stranded RNA viruses

1. the RNA can be directly translated

2. a copy of negative RNA is transcribed

3. From the negative copy, + copies of the genome are made

In other cases

1. the + strand is directly translated

2. a negative strand is made

3. from there copies of the genome are made as well as more positive transcripts that are used for translation.

Double stranded viruses

1. a positive strand is transcribed form the negative side

2. this is used for replication and for translation.

Retrovirusese—have two copies of the Same RNA

they are transcribed to produce DNA that is double stranded
this DNA is then transcribed to produce RNA again

Viral mRNA is translated into protein by host cell translation machinery.

Polyproteins are processed by viral as well as cellular proteases.

To enhance tran slation of their genomes, viruses employ several mechanisms- they have the internal ribosome entry sites (IRES) they cleave the CAP binding protein—at the 5’ RNA.

This host cell binds those caps on cellular mRNAs. By cleaving this protein, the ribosomes won’t pick up the cellular mRNA as much as it usually does. This makes it easier for the viral RNA to compete. they cleave the cellular mRNA’s

Viral proteins undergo a variety of post-translational modifications including polyprotein processing, phosphorylation and acylation.

Assembly and budding—non-enveloped viruses are generally assembled as empty capsids and the genome is then packaged. For enveloped viruses the matrix protein promotes the interaction of the viral nuclecapsid with the viral glycoprotein modified membrane for the assembly. Non-enveloped viruses are usually released by lysis of the host cell. Envoloped viruses are usually released by budding.

Category: Microbiology Notes