Essentials of virology I Flashcards
What is a virus?
It is an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.
In what species can viruses be found?
Viruses can be found in different hosts:-
- Humans
- Animals (including insects)
- Plants
- Bacteria
Where a virus is found is called its “host range”.
In what tissues and organs can viruses be found?
- Viruses can be found in multiple tissues and organs of their hosts.
- The tissues that viruses can be found in can be referred to as their “tissue tropism”.
- Through the process of evolution viruses evolve to replicate in tissues that favour transmission of virus betwenn hosts and allow them to evade the immune system of the host.
How can viruses casue diseases? What diseases can it cause in humans?
- Viruses casue a range of diseases, either directly or indirectly.
- They can cause the disease directly as a product of their host range and tissue tropism.
- They can casue diseases indirectly by promotimg or inhibiting cell function.
- In humans it can cause:-
- Cancer
- Immunodeficiency
- Respiratory diseases
- The diseases can be acute or chronic.
Give information on bacteria, prions and viruses.
- Bacteria :-
- contain nucleic acid (DNA) covered in protein.
- They have a cell wall (a coat of sugae molecules)
- They replicate outside the cell.
- Prions :-
- They are proteins.
- They do not contain nucleic acid.
- They replicate inside the cell.
- Viruses :-
- They are nuleic acids (DNA or RNA) covered by proteins.
- They have no cell wall and may or may not have a lipid coat.
- They must replicate in the cell.
- They are “obligate cellular parasites” - (Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite’s reproduction is entirely reliant on intracellular resources).
Give information on virus.
- Different viruses have different structures but retain similar organization.
- Virus structure can dictate a host range and tissue tropism.
How and why does a DNA virus enter a cell?
The virus uses all of the components in the host cell to replicate its genome.
- Viruses have spikes (proteins) on its outer coat that senses and detects proteins on the host cell.
- The viruses do this to look for a suitable host cell to bind to (like the lock and key model) and enter into the cell.
- Viruses cannot generate their own nucleotides so they use the nucleotides belonging to the host cell for the syntheis of their own DNA.
- The virus then makes its protein coats (capsid) by making its own mRNA which encodes for its protein coats.
- The capsid molecules then packages or encapsulates all of the genome which is made in the host cell and new viruses come out if the cell.
- The movemet of the virus out of the host cell causes the cell damage.
How and why does a RNA virus enter the cell?
- The viral proteins on the surface of the RNA virus sample the surface of the host cells membrane looking for a cell they find favourable.
- The virus uses the host cells nucleotides to replicate its genome (it makes RNA).
- It then makes mRNA which makes the capsid proteins that make the capsule.
- The genome gets encapsulated inside the capsule.
- As it pushes its way out of the cell it disrupts the cell.
- Glycoprotein receptors attach around the surface of the virus.
What are the four different configurations of the viral genome?
- Single-stranded RNA (ssRNA)
- Double-stranded RNA (dsRNA)
- Single-stranded DNA (ssDNA)
- Double-stranded DNA (ddDNA)
Describe the structure of viral genomes.
- RNA genomes can be linear and segmented. i.e. more than one RNA per capsid.
- DNA genomes can be linear or circular.
- Genomes can be encoded in a positive or negetive sense. i.e. 5’ - 3’ or 3’ - 5’.
- Based on this, we can separate viruses into different families.
How do viruses use the Central Dogma to replicate?
DNA viruses
- DNA viruses uses DNA polymerase to replicate itself (forms new DNA).
- The DNA then forms RNA (transcription) using RNA polymerase.
- Translation of the RNA happens in the ribosomes results in the protein they wish to produce.
RNA viruses
- If they have positive sense RNA they can use RNA polymerase to replicate the + sense genome in the 5’ - 3’ direction and send it to the ribosome to produce the required proteins.
- If they have negative sense RNA (coded in the 3’ - 5’ direction), they use RNA-dependent RNA polymerase to produce positive sense RNA polymerase from the negetive sense.
- And from the newly made positive sense RNA polymerase you cam make the required proteins by transcription in the ribosome.
Reverse transcriptase is another enzyme encoded by viruses which can make the RNA can go back to form DNA.
Central dogma meaning - The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes flows into proteins: DNA → RNA → protein.
RNA-dependent RNA polymerase meaning - It is an enzyme that catalyzes the replication of RNA from an RNA template.
Give two examples of proteins that viruses can produce.
- POLYMERASE:
- Polymerase proteins are enzymes that are involved in the catalysis of nucleotides to nucleic acids.
- Viruses with DNA genomes produce DNA polymerases, while viruses with RNA genomes produce RNA polymerases.
- Polymerase proteins interact with nucleic acids in protein-nucleic acid interactions.
- STRUCTURAL PROTEINS:
- These are proteins that form the capsid into which nucleic acid is packed.
- These proteins interact with one another in protein-protein interactions.
How do viral proteins promote pathogenesis?
- As ‘obligate cellular parasites’, viral and cellular proteins shape the intracellular environment to facilitate viral replication, or evasion of an immune response.
- Commonly, changes to the intracellular environment that facilitates virus replication or evasion of an immune response lead to disease (pathogenesis).
Pathogenesis meaning - the manner of development of a disease.
How can virus pathogenesis lead to cancer?
- As cells divide, they go through the cell cycle.
- During the cell cycle, the cell produces factors that a virus might need.
- An example of this is the production of nucleotides that can be used by viral polymerases to make new viral genomes.
- Some viruses produce proteins that ensure that the cell continuously goes through the cell cycle.
- Cancer can be defined as an uncontrolled cell division.
- Thus, viruses can cause cancer by causing uncontrolled cell division that benefits their replication.
Give some examples of some viruses that can cause cancer, and describe how this could happen.
- Human Papilloma viruses (HPVs)
- Hepatitis B virus (HBV)
- Hepatitis C virus (HCV)
- Human Immunodeficiency virus (HIV)
How does HPV cause cancer?
- Human Papillomavirus (HPV) proteins E6 and E7 remove the ‘brakes’ of the cell cycle.
- E6 can remove/degrade p53, an important factor that halts the cell cycle, so it allows the cell cycle to go on.
- E7 binds to RB, preventing it from binding to E2F, once again allowing the cell cycle to go on.
Describe how HIV cause immunodeficiency.
- In an uninfected person, the thymus produced normal CD4 cells, some of which will die of apoptosis (natural occurrence, as it keeps all of the cells being replenished).
- In an infected person, some of the CD4 cells made by the thymus will be infected, and these, in turn, will cause a number of different molecular pathways to be activated, all of which cause cell death.
