Virology 2 Flashcards
What is the importance of virus infections for the health of their host
- Many viruses are pathogens.
- Viruses can cause epidemics.
- Spanish influenza outbreak in 1918 caused more than 500 million infections and approximately 50 million deaths worldwide.
- Human Immunodeficiency Virus is the causative agent of Acquired Immunodeficiency Syndrome which has killed 35 million people since the start of the epidemic.
- Foot and mouth disease outbreak of 2001. Disease of cattle, sheep, and pigs. Causes painful blisters in mouth and on feet, high fever, and weight loss. Highly contagious. In 2001 more than 6 million animals were culled to contain the disease.
Outline the virology research which has taken place
- End of 19th century: bacteria smallest microorganism known. Could be seen under light microscope and grown in simple or defined nutrient media.
- 1884: Louis Pasteur worked with rabid dogs. Unable to detect bacteria in infectious material. Speculated disease was caused by infinitely small microorganisms.
- 1884: Chamberland Filter created which had a pores too small to filter bacteria. Breakthrough in discovery of viruses as filtrate was bacteria-free.
- 1898: Friedrich Loeffler and Paul Frosch discovered foot and mouth disease virus. First animal virus. Filterable small particle (able to pass through Chamberland Filter) and able to cause disease, even when diluted (not toxin).
1915: Frederick Twort and Felix d’Herelle discovered bacteriophages. - Development of tissue culture (TC) cell lines. In 1949 John Enders grew poliomyelitis virus in TC cells. In 1955 Wilton Earle created a reliable artificial medium for cultured cell growth. Led to a golden era of virology.
What is the basis of tissue culture
- Once confluent (continuous monolayer of cells on base of flask),
- Cells can be infected with virus of interest.
- Virus growth changes shape of cells: cytopathic effect (CPE).
- Infected cells round up and eventually die and detach from base of flask.
- Some viruses cause cells to fuse e.g. Paramyxoviruses.
What are the benefits of being able to grow viruses in tissue culture cells lines
Allows to:
- establish association with diseases.
- purify viruses
- study the structure and biochemistry of viruses.
- study viral infection and replication in molecular detail.
Allows for progress in molecular biology:
- develop antivirals.
- isolation and manipulation of viral genomes.
- attenuate viruses or genetically engineer vaccines.
- develop gene therapy or destroy cancer cells.
Describe the general features of viruses
- Viruses are infectious, obligate, intracellular parasites comprising genetic material surrounded by a protein coat and/or a membrane.
- Use host cell machinery for replication of the viral genome and for production of viral proteins.
- They assemble from proteins and genomes formed in infected cells.
- Do not replicate by division.
- Outside of host cells they are inert, possess few enzymes, and cannot reproduce.
- Inside host cells their nucleic acids can transform host cells into virus factories.
Describe the structure and composition of virus particles
- All contain a viral genome surrounded by a capsid.
- The capsid is a protein coat which protects the viral genetic material and aids its transfer between host cells.
- Some capsids are surrounded by an envelope (lipid bilayer derived from host).
- Nucleocapsid = capsid + viral nucleic acid.
- Virion = complete infectious virus particle.
- Generally smaller than bacteria.
Describe the capsid structure
- Self-assemble from multiple copies of one or more structural proteins.
- Individual protein subunit of a capsid = protomer.
- Can be helical, icosahedral, or complex.
- Shaped like hollow tubes with protein walls.
- Can be rigid or flexible.
- Icosahedron: 20 triangular faces arranged around surface of a sphere. Most efficient way to enclose a space. Constructed from ring- or knob-shaped assemblages of 5 or 6 protomers. Example = Picornaviruses (Poliovirus or Rhinovirus).
Describe the structure of virus envelopes
- Lipoprotein bilayer derived from the host.
- Obtained when virus particles bud through host cell membranes (often plasma membrane).
- Viral proteins get embedded in the envelope (spikes).
- Spikes are involved in viral attachment to host cell antigenic site.
State where virion enzymes are found, what they are involved in, and give an example of a virus which contains enzymes and the enzymes it contains
- Mostly within the capsid.
- Involved in nucleic acid replication.
- Example: retroviruses (HIV)
Reverse transcriptase: converts viral RNA genome into DNA.
Integrase: integrates viral DNA into the host genome.
Describe the viral genome
- Can be DNA or RNA.
- Can be single- or double-stranded.
- Single-stranded viral genome can be positive of negative stranded.
- Positive single strands have nucleic acid strands which have the exact same base sequence as the mRNA they produce.
- Negative single strands have viral nucleic acid strands which have a base sequence complementary to that of the mRNA they produce.
- Can be linear or circular.
- Can be continuous or segmented.
Describe how the types of DNA viruses produce positive mRNA to synthesise protein
ds (+/-) DNA
- DNA-dependent RNA polymerase uses the DNA template strand to transcribe a strand of +mRNA.
- Examples: Herpes virus, Adenovirus, Poliovirus.
ss (+) DNA
- Use single strand of DNA as a template strand and use DNA polymerase to produce a double strand of DNA.
- DNA-dependent RNA polymerase uses one of the DNA strands as a template strand for transcription to produce +mRNA.
- Examples: Adeno-associated virus.
Describe how the types of RNA viruses produce positive mRNA to synthesise protein
ds (+/-) RNA
- Positive and negative strands of RNA are separated.
- Positive RNA strand can readily serve as mRNA to translate proteins.
- Negative RNA strand is used as a template strand while RNA-dependent RNA polymerase carries out transcription to produce more positive RNA strands which can serve as mRNA.
- Examples: Reovirus.
ss (+) RNA
- Can serve readily as mRNA.
- +RNA strands act as a template strand to produce -RNA strands by transcription (using RNA-dependent RNA polymerase).
- -RNA strands used as templates to produce many +RNA strands. Some +RNA strands are used as viral genetic material and some are used as mRNA to translate proteins.
- Examples: Hepatitis A, Hepatitis C, and Poliovirus.
ss (-) RNA
- -RNA used as a template for transcription (using RNA-dependent RNA polymerase) to produce +RNA which can serve as mRNA for translation of proteins.
- +RNA produced can also serve as a template to produce more -RNA by transcription which serves as viral genetic material.
- Example: Influenza virus.
Describe how the types of retro-transcribing viruses produce positive mRNA to synthesise protein
ss (+) RNA Retroviruses
- +RNA used as a template to produce a single strand of DNA using reverse transcriptase.
- This produces a DNA/RNA hybrid.
- Ribonuclease is used to remove the RNA strand from the hybrid.
- The DNA strand is used as a template strand to produce a double strand of DNA using reverse transcriptase.
- The DNA template strand undergoes transcription using DNA-dependent RNA polymerase to produce mRNA.
- DNA integrates into host cell genome.
- Example: HIV.
ds (+/-) DNA Reverse Transcribing DNA Viruses
- Reverse transcriptase used to produce lots of DNA.
- DNA becomes integrated into host cell genome.
- Translated into protein products by host cell enzymes.
Pregenome
- DNA template strand undergoes transcription to produce +RNA.
- Reverse transcriptase converts +RNA into -DNA.
- Reverse transcriptase produces double stranded DNA from the -DNA.
- Double-stranded DNA can be used as viral genetic material.
Name the steps of the replication cycle of viruses
1) Attachment to host
2) Entry into host
3) Uncoating
4) Synthesis of viral nucleic acids and proteins
5) Self-assembly of nucleocapsids
6) Release from host
How can viruses a) enter humans, and b) be transmitted to humans
Routes of virus entry in humans:
- Conjunctiva of eyes.
- Skin (bite of vertebrae or scratch injury).
- Respiratory tract.
- Gastrointestinal tract.
- Urogenital tract.
- Placenta.
Virus transmission
1) Airborne: Virus suspended in airborne droplets. Saliva or mucus propelled from mouth or nose of infected person (sneezing or coughing). Infection of recipient mucosa (nose, mouth, or eyes). Directly or indirectly via hands.
2) Person-to-person contact: Touching, kissing, and sexual contact. Contact with lesions, secretions, body fluids, and mucosa.
3) Faecal-oral transmission: Viruses that are ingested, replicate in gastrointestinal tract, and excreted in faeces. Infection can arise from contaminated hands, sewage-contaminated drinking water, or washing of food with contaminated water.
4) Vector-borne transmission: Transmission via arthropod vectors (Arboviruses). Insects/ticks transfer virus from infected to non-infected individual via bites.
5) Animal to human transmission (zoonotic viruses).
