06: An Introduction to Viruses, Viriods, and Prions Flashcards
Viruses in the Biological Spectrum
- Inactive macromolecules outside the host cell and active only inside host cells
- Basic structure consists of protein shell (capsid) surrounding nucleic acid core
- Nucleic acid of the viral genome is either DNA or RNA but not both
- Nucleic acid can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA
- Molecules on virus surface impart high specificity for attachment to host cell
General Size of Viruses
- Megaviruses
- Pandaviruses
Viral Structure
- Viruses bear no resemblance to cells
- Lack protein-synthesizing machinery
- Viruses contain only the parts needed to invade and control a host cell
- Virus particle
- Covering
- Capsid
- Envelope (not found in all viruses)
- Central core
- Nucleic acid molecule(s) (DNA or RNA)
- Matrix proteins Enzymes (not found in all viruses)
Capsids
- protein coats that enclose and protect their nucleic acid
- All viruses
- Nucleocapsid
- capsid together with the nucleic acid
- Envelope
- Some viruses have an external covering
- those lacking an envelope are naked
- Capsomers
- Each capsid is made of identical protein subunits
Naked Nucleocapsid Virus
Enveloped Virus
- Protects the nucleic acid when the virus is outside of the host cell
- Helps the virus bind to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell
Poxviruses
- Complex
- Atypical
- Lack a typical capsid
- Covered by a dense layer of lipoproteins
Bacteriophages
- polyhedral nucleocapsid along with a helical tail and attached fibers
Complex Viruses
- poxvirus
- large DNA virus
- flexible-tailed bacteriophage
Enveloped Viruses
- With a helical nucleocapsid
- mumps virus
- rhabdovirus
- with an icosahedral nucleocapsid
- herpes virus
- HIV (AIDS)
Naked Viruses
- Helical capsid
- plum poxvirus
- Icosahedral capsid
- poliovirus
- papillomavirus
How would you describe this virus?
A. Helical and Naked
B. Icosahedral and Naked
C. Complex and Naked
D. Helical and Enveloped
E. Icosahedral and Enveloped
F. Complex and Enveloped
E. Icosahedral and Enveloped
Viral Genome
- Either DNA or RNA
- but never both
- Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses
- Number of genes varies for each type of virus
- few to hundreds
DNA Viruses
- Usually double stranded (ds)
- but may be single stranded (ss)
- Circular or linear
RNA Viruses
- Usually single stranded
- may be double stranded
- may be segmented into separate RNA pieces
- ssRNA genomes ready for immediate translation
- positive-sense RNA
- •ssRNA genomes that must be converted into proper form
- negative-sense RNA
Pre-formed enzymes required for viral replication
- Polymerases
- synthesize DNA or RNA
- Replicases
- copy RNA
- Reverse transcriptase
- synthesis of DNA from RNA (HIV virus)
Modes of Viral Multiplication
- Adsorption
- Penetration
- Uncoating
- Synthesis
- Assembly
- Release
Adsorption
binding of virus to specific molecules on the host cell
Penetration
Genome enters the host cell
Uncoating
Viral nucleic acid is released from the capsid
Synthesis
Viral components are produced
Assembly
New viral particles are constructed
Release
Assembled viruses are released by budding (exocytosis) or cell lysis
Host Range
- Spectrum of cells a virus can infect
- Virus coincidentally collides with a susceptible host cell
- adsorbs specifically to receptor sites on the membrane
- Hepatitis B
- human liver cells
- Poliovirus
- primate intestinal and nerve cells
- Rabies
- various cells of many mammals
- Hepatitis B
Viruses commonly contain both DNA and RNA
A. True
B. False
B. False
Cytopathic Effects
- cell damage altering microscopic appearance
- Disorientation of individual cells
- Gross changes in shape or size
- Intracellular changes (inclusion bodies, syncytium)
Persistent Infections
cell harbors the virus and is not immediately lysed
Latent State
- Can last weeks or host’s lifetime
- several can periodically reactivate
- Measles virus
- may remain hidden in brain cells for many years
- Herpes simplex virus
- cold sores and genital herpes
- Herpes zoster virus
- chickenpox and shingles
Oncogenic
- Some animal viruses enter the host cell and permanently alter its genetic material resulting in cancer (transformation)
- Transformed cells have
- increased rate of growth
- alterations in chromosomes
- capacity to divide for indefinite time periods resulting in tumors
- Oncoviruses
- Mammalian viruses capable of initiating tumors
- Papillomavirus – cervical cancer
- Epstein-Barr virus – Burkitt’s lymphoma
- Mammalian viruses capable of initiating tumors
Bacteriophages
- bacterial viruses (phages)
- Most widely studied are those that infect
- Escherichia coli – complex structure, DNA
- Multiplication goes through similar stages as animal viruses
- Only the nucleic acid enters the cytoplasm
- uncoating is not necessary
- Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle
Steps in Phage Replication
- Adsorption
- Penetration
- Replication
- Assembly
- Maturation
- Lysis and Release
Phage Absorption
Binding of virus to specific molecules on host cell
Phage Penetration
Genome enters host cell
Phage Replication
Viral components are produced
Phage Assembly
Viral components are assembled
Phage Maturation
Completion of viral formation
Phage Lysis and Release
The lytic cycle involves full completion of viral infection through lysis and release of virions
Lysogeny
- Not all phages complete the lytic cycle
- Temperate phages
- undergo adsorption and penetration but don’t replicate
- The viral genome inserts into bacterial genome and becomes an inactive prophage – the cell is not lysed
- Prophage is retained and copied during normal cell division
- resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny
- Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis
Lysogeny
- Lysogeny results in the spread of the virus without killing the host cell
- lysogenic conversion
- Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology
- Corynebacterium diphtheriae
- Vibrio cholerae
- Clostridium botulinum
- Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology
Which of the following is a step found in animal virus multiplication but not in bacteriophage replication?
A. Adsorption
B. Penetration
C. Uncoating
D. Assembly
E. Release
C. Uncoating
In vitro
- Cell (tissue) cultures
- cultured cells support viral replication
- permit observation of cytopathic effects
In vivo
- Bird embryos –
- intact and self-supporting unit
- complete with its own sterile environment and nourishment, with embryonic tissues that support viral multiplication
- Live animal inoculation
- The animal is exposed by injection of a viral preparation or specimen into the brain, blood, muscle, body cavity, skin, or footpads
Prions
- misfolded proteins
- contain no nucleic acid
- Extremely resistant to usual sterilization techniques
- Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases
- Common in animals:
- Scrapie in sheep and goats
- Bovine spongiform encephalopathies (BSE) mad cow disease
- Wasting disease in elk
- Humans – Creutzfeldt-Jakob Syndrome (CJS)
Satellite Viruses
- dependent on other viruses for replication
- Adeno-associated virus
- replicates only in cells infected with adenovirus
- Delta agent
- naked strand of RNA expressed only in the presence of hepatitis B virus
Viroids
- Short pieces of RNA
- No protein coat
- Only been identified in plants
Exposure to Nucleases that degrade DNA and RNA would damage all of the following EXCEPT
A. Animal Viruses
B. Bacteriophage
C. Prions
D. Satellite Viruses
E. Viroids
C. Prions
