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One type of nucleic acid: RNA or DNA, surrounded by a protein coat.
Some have an envelope (lipid membrane) around the protein coat
Obligate intracellular parasites-
Multiply only within living cells. Has few enzymes of its own.
Lacks ability to make proteins or ATP. Takes over the cell's "synthetic machinery".
The host range of viruses is very specific.
a complete virus particle
A single type (DNA or RNA, never both). Single stranded (ss) or double stranded (ds).
Linear, circular, or segmented (multiple strands)
ssRNA virus may be + or -.
Plus sense RNA (+RNA, or sense) can be used directly as mRNA and translated to protein.
Minus sense RNA (-RNA, or anti-sense) cannot be used as mRNA. It is complementary to mRNA.
Nucleic acid (NA)
the protein coat. Protects the nucleic acid.
Protein subunits that make up the capsid
The envelope is derived from the host cell plasma membrane or nuclear membrane as virus "buds out".
Consists of lipid, polysaccharide, and plasma membrane proteins (host and viral encoded).
Some types are enveloped; others are non-enveloped (naked viruses).
Roughly on the basis of NA type, morphology, and envelope.
Respiratory diseases, animal tumors.
may cause warts, epithelial abnormalities, and cancer.
Papovavirus: (papilloma, polyoma, and vacuolation viruses
Mostly animal infections.
May cause fetal deaths and gastroenteritis in humans. Needs Adenovirus co-infection.
Cold sores and genital herpes, chickenpox (and shingles), infectious mononucleosis, Burkitt's lymphoma.
the Hepatititis B Virus (HBV). Has an RNA dependent DNA polymerase (reverse transcriptase). Causes Hepatitis B and liver cancer.
Skin lesions (pus filled)
enteric diseases, including polio and hepatitis A
Arthropod borne equine encephalitis; rubella.
Hepatitis C, yellow fever, dengue, West Nile encephalitis.
Common cold and severe acute respiratory syndrome (SARS).
RNA dependent RNA polymerase. Rabies and other animal diseases.
Ebola, Marburg and other hemorrhagic fevers.
Reverse transcriptase. (is a DNA polymerase enzyme that transcribes single-stranded RNA into double-stranded DNA.)
becomes integrated in host genome. Tumors, animal leukemias, AIDS, HIV.
Respiratory enteric orphans
suspension cultures, solid media. Grown on bacterial cells.
Grown in animals, eggs, cell lines. Ex: Influenza grown in chicken eggs.
Attachment: Receptor mediated, hydrogen bonding of phage tail fibers and bacterial cell wall
Penetration: Virus enzyme makes a hole, tail penetrates and DNA is directly injected.
Biosynthesis: Host synthesis is blocked or stopped and synthesis of viral protein and NA occurs.
Early genes code for enzymes to replicate the genome; late genes code for capsid proteins
Maturation: Assembly of virus particles. Appears to be spontaneous
Release: Lysis of cell. Plasma membrane erupts and virus is released
Bacteriophage: Lytic cycle
host cell is not lysed. Virus integrates in bacterial chromosome (Prophage).
Host cell is resistant to further infection by the same phage. Virus may remain inactive for a long time.
Host cell may exhibit new characteristics. Transduction may occur. Can return to lytic cycle
Bacteriophage: Lysogenic cycle
Similar to bacteriophage. An additional step is required.
Attachment- Viral surface proteins bind cell plasma membrane proteins and glycoproteins (receptors)
Penetration- Endocytosis of the entire virion (non-enveloped), or fusion of the virus envelope with the host cell’s plasma membrane occurs. The virus capsid is then released into the host cell's cytoplasm.
Uncoating- NA is not injected, so the capsid must be removed. May be accomplished by viral or host enzymes.
Biosynthesis: There are variations, depending on the type of viral NA.
DNA viruses: DNA synthesis usually occurs in the nucleus, with viral enzymes.
Protein synthesis is in the cytoplasm, with host enzymes. (Fig.13.15)
RNA viruses: A variety of unique strategies and enzymes for replicating genome
+RNA viruses: Synthesize -RNA to serve as a template to make new genomes and mRNA
-RNA viruses: Synthesize +RNA which is mRNA and a template for -RNA genomes
Retrovirus: Synthesizes DNA from RNA and must integrate to host chromosome
Maturation: Capsid assembly is usually spontaneous.
In nucleus for most DNA viruses; in cytoplasm for RNA viruses
Release: By rupture of plasma membrane (lysis) or by budding, host cell may survive a long time.
Animal cell viruses replication
A multi-step process.
Progression (2nd genetic change occurs, irreversible) transformed into cancer cells.
Result is an inability of cells to recognize signals telling them to stop growing.
The uncontrolled growth leads to tumor formation.
Alteration of genetic material to make a normal cell cancerous
are host genes that are sites of transforming alterations. These genes are typically necessary for normal cellular function. Most code for proteins involved with signal transduction. Virus infection may result in activation of proto-oncogenes to oncogenes.
DNA: Adenovirus, Herpesvirus, Poxvirus, Papovavirus, Hepadnavirus;
Virus groups associated with cancer
Duplications of a gene could result in overexpression
-mutation is the only mechanism of oncogene activation that is independent of virus.
- Proteinaceous infectious agent
- No nucleic acid
- Abnormal form of a protein acts as a template to convert normal to abnormal