Viruses have three major differences from bacteria: their genome organization, characteristic mode of replication, and effect on host cell (either death, fusion, or transformation). Viruses are not alive in and of themselves. Instead, they are obligate intracellular parasites capable of replication and reproducing within living cells. A complete virus particle or virion may be regarded as a block of genetic material (nucleic acid) surrounded by proteins (capsid) to protect the nucleic acid. In some viruses, the nucleocapsid (nucleic acid and capsid) is surrounded by an envelope (made of lipids, proteins and glycoproteins). In addition, most viruses are much smaller than bacteria. However, the largest viruses are the same size as the smallest bacteria.
Virus multiply in particular host cells. This is defined by the host cell range of the virus. In transfection, entry does not depend on specific receptors. That is why transfection can be performed on a large host cell range. Host cell range is determined by:
- Specificity of attachment (receptor on cell surface)
- Availability of cellular factors required for viral replication
Viral Life Cycle
|Attachment/Adsoprtion||viruses bind to cells via specific receptor on the surface of the cell. htese are generally proteins or glycoprtoeins. Cells naturally lacking poliovirus receptors or in which ireceptor for influenza viruss are enzymatically destroyed are resistant to infection from these viruses....for some viruses use both receptor and coreceptor.|
|Penetration||Viruses generally penetrate thoruhg one of the following means: direct fusion with cell membrane releaseing content of virus into cell, by being enveloped into a coated pit which is a vesicle whose outer walls are coated with protein clathrin.|
|Uncoating + Eclipse||Within the cell, the coated pit will fuse wiht lysosomes, and the lower pH inside the lysosome results in: (i) fusion of the viral emmbrane to the mebrane of the lysosome and (ii) partial uncoating of the capsid. the viral nucleocapsid is then released intot he cytoplasm of the cell. it may remain here for some viruses (cytoplasmic viruses) or migrate into the nucleus for tohers (nuclear viruses). Uncoating of the virion results in eclipse or loss of inefectivityy (failure to recover infectious virus from infected cell).|
Release of progeny virion. Capsid protein and nucleic acids are assembled into virion particles. There are 2 ways the virions can be released:
|Synthetic Phase||Review of gene expression..DNA replication, transcription, splicing and translation. In order to complete their replication, all viruses must accomplish a common series of events...(i) synthesis of mRNA (ii) translation of mRNA to virus-specific proeins (iii) replciation of viral genome....ss and dsDNA viruses replicate their DNA in a manner similar to cellular DNA replication semi-conservative.|
- Lysis: For some viruses primarily the non-enveloped and POX viruses, infected cells are lysed by a viral proces to liberate viral progeny.
- Budding: Enveloped viruses are released by a process known as budding through the cell membrane. viral envelope proteins are incorporated into specific regions of host cell membranes. the assembled nucleocapsid ithen buds though these patches containing viral envelope proteins, possibly some cellular proteins and lipids and becomes enveloped by them. thus budding forms viral envelope.
Assaying for Tumor Viruses
|Electron Microscopy (EM)||Within virions of a single type, their number can be determined unambiguously. However, their biological activity cannot be determined this way.|
|Immunological Assay||These assays are based on antibodies directed against viral components. For most diagnostic purposes, the antibodies present in the serum of infected individuals is directed against proteins on the virus' outer surface, envelope proteins in case of enveloped viruses, or capsid proteins for non-enveloped viruses.|
|ELISA||Commonly used for screening hepatitis, HTLV, and HIV. Virus proteins are mounted to a solid support (ie, bottom of a tube) and exposed to a serum contain antibodies (Ab1). Radiolabelled or enzyme-tagged antibodies are then directed against Ab1, The amount of enzyme or radioactivity is in proportion to viral antibodies present. To detect the enzyme, the support is incubated in presence of a substrate that, when cleaved by the enzyme, elicits a signal.|
|Western Blot||Most commonly used. An antigen is fixed to a plate and is washed with a labelled antibody.|
|Infectivity Assay||Mammalian cell cultures have allowed the discovery of viruses such as HIV. Many different types of cells from different organs can be grown in culture (monolayers are cell lawns only 1 cell thick). Infection of monolayers with lytic viruses produces plaques. Tumors viruses generally do not kill cells but transform them into immortal cells, resulting in a focus formation. Like plaques, the number of foci formed gives a direct measure of infectious virus particles.|
Combating Animal Viruses
- Live Attenuated (mutant non-virulent virus), such as Sabine oral vaccine for poliovirus.
- Killed virus (heat and formaldehyde treated), such as Salk's polio vaccine.
- Recombinant or subunit vaccine. Many viral proteins elicit formation of neutralizing antibodies have been identified, and are being researched as subunit vaccines.
|Drugs||Azidothymidine (AZT) inhibits viral reverse transcriptase. AZT is a thymidine analog (deoxythymidine) which gets incorporated into viral DNA and acts as a chain terminator, inhibiting nucleic acid function and virus multiplication.|