Characteristics of Viruses
- Contain DNA or RNA
- Protein coat
- Some have envelope
- Some have spikes
- Most infect only specific types of cells in one host
Host Range
Determined by specific host attachment sites and cellular factors
Viron Structure
- Genetic info (DNA or RNA)
- Protein coat (capsomere)
Viron
Complete infectious particle and all the parts it needs to be so
Capsid
Entire coating of capsomeres + nucleic acid
Virus Morphology
1) Helical
2) Polyhedral
3) Enveloped
4) Complex
Helical virus
Long string and nucleic acids wrapped around it w/the capsomeres on the outside. Forms a helical, tube structure
- Ebola
Polyhedral virus
Looks like soccer ball. Capsomeres make a polyhedral soccer ball shape w/nucleic acid stuffed inside
- Mastadenovirus (also has spikes)
Enveloped virus
Whether helical or polyhedral, have an envelope. Envelope does not always have spikes. Most common in human viruses
- Influenza virus (helical w/envelope), Herpes virus (polyhedral w/envelope)
Complex virus
If not helical or polyhedral, considered complex virus. Most common is the bacteriophage
- Orthopox virus
Growing and Culturing viruses
1) On agar plate w/bacteria on it. Bacteriophages form plaques on a lawn of bacteria. Viruses must be grown in living cells
2) Animal viruses grown in living animals or embryonated eggs
3) Animal and plant viruses grown in cell culture like human tissue cells
Virus Identification
1) Cytopathic effects (CPE): cells start to look strange after being infected by a virus, CPE is looking at whether cells look normal or abnormal
2) Serological tests. Serum will be tested for antibodies against viruses
3) Nucleic Acids (PCR)
Lytic Cycle
1) Attachment: phage attaches by tail fibers to host cell
2) Penetration: phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell. Virus itself does not enter cell, just injects its genetic info
3) Biosynthesis: production of phage DNA and proteins. Once genetic info enters cell, takes over cell and uses it to replicate virus. Makes all components necessary for the viron
4) Maturation: assembly of phage particles. Putting together all components to make mature virons
5) Release: phage lysozyme breaks cell wall. Massive release of virus that is detectable
One-Step/Acute Viral Infection
Happens during Step 5 Release when virus breaks out of human cell killing it and there is a lot of detectable virus
Lysogenic Cycle
- Virus can choose to pause before Step 3 Biosynthesis until it is triggered
1) Immune to reinfection: virus combines its genetic info w/host cell DNA
2) Phage conversion. Can give host cell different attributes
3) Specialized Transduction: virus takes specific genes from a host cell w/it each time it infects a new host cell
Differences btwn Lytic Cycle and Lysogenic Cycle
- Lytic cycle, phage causes lysis and death of host cell
- Lysogenic cycle, virus cells are latent
- Viruses can go from Lysogenic cycle to Lytic cycle at any time
Generalized Transduction
Virus is not taking specific genes from host cell when it leaves to infect a new host cell, it is just by accident
Specialized Transduction
Virus, every time it leaves a host cell, is taking specific genes from the host cell with it
Animal Virus Multiplication
- Pathogens that infect humans
- Most have an envelope and an extra step called Uncoating
- Virus actually penetrates into the cell, not just inject its genetic info
1) Attachment: virus attaches to cell membrane
2) Penetration: by endocytosis or fusion
3) Uncoating: by viral or host enzymes. Virus loses its envelope so the capsid can undergo biosynthesis
4) Biosynthesis: production of nucleic acids and proteins
5) Maturation: assembly of nucleic acid and capsid proteins
5) Release: by budding (enveloped viruses) or rupture. Virus lost its envelope after Step 3 Uncoating but needs an envelope again to exit host cell. Uses host cell’s plasma membrane – buds off and takes host cell’s plasma membrane as its envelope
Types of Penetration
1) Pinocytosis: triggers human cell to allow it entrance where it then invaginates into the human cell
2) Fusion: fuse viral envelope w/human plasma membrane b/c they are both phospholipid bilayers. Becomes absorbed
Multiplication of a Retrovirus
- Retroviruses are RNA viruses, but get converted into a DNA version in the cell and inserted into the chromosome
- Human cells do not have an enzyme to go from RNA to DNA (can only go from DNA to RNA), so the virus must carry its own enzyme – Reverse Transcriptase
- Important b/c if Reverse Transcriptase can be removed from a retrovirus, the virus is dead and cannot replicate (Ex: HIV fighting drugs do this)
Cancer
Cell loses its ability to do. Can happen by Spontaneous or Induced mutations or Viruses
Oncogenes
If activated, transform normal cells into cancerous cells. Control cellular growth. If infected by a virus, can lead to cancer
Transformed Cells
Increased growth, loss of contact inhibition, tumor-specific transplant antigens and T antigens
