Viral cycle Flashcards
What is horizontal transmission?
Respiratory route transfers viruses like rhinovirus, influenza, measles and coronavirus. Droplets are directly inhaled if they are light, infect the conjunctiva, or fall onto surfaces if they are larger and be transferred by the fingers to the mouth.
Faecal-oral route (vomiting, diarrhoea and fever) includes hepatitis A leads to jaundice, poliovirus 1% of the time leads to muscle paralysis.
Sexual transmission includes HIV, hepatitis B, human papilloma (HPV).
Urine transmission is rare but passes poliovirus.
Mechanical transmission includes passing of virus between medical tools. Includes passing of blood-borne viruses like HIV and hepatitis B.
What is transmission?
This is the movement of a virus from one host to another, this is necessary for the viruses life as it can only survive within another organism.
Transmission can be horizontal, vertical, or zoonosis.
When a virus is within another host, it has to find the appropriate cell within the body, dock onto it by interaction with host cell receptors, penetrate the cell and become uncoated, and then get back out of the cell before cell death.
What is vertical transmission?
mother to baby during birth.
- transplacental e.g. rubella
- birth (perinatal) e.g. herpes
- postnatal e.g. HIV-1 in breast milk
What is zoonosis?
Transmission of virus from animals to human. The animal transmitting the virus is known as the vector.
How does HIV penetrate the host cell?
The HIV surface has a glycoprotein with 2 subunits (gp41 and gp120). The HIV recognises the CD4 binding protein on the surface of a target cell. The CD4 will bind to the gp120 subunit, resulting in a conformational change of the gp120 glycoprotein, exposing a second binding site which will bind to the CCR5 (chemokine co-receptor 5) on the host cell surface. The binding of both receptors on the target cell causes further conformational change of the gp41 protein, allowing it to now interact with the host cell membrane.
The viral envelope will then fuse with the membrane and release the nuclear capsid into the cytoplasm of the host.
What is internalisation, and what are the types?
This is how the viral particle gets into the host cell.
1. fusion from without
- enveloped viruses
2. receptor mediated endocytosis
- enveloped
- non-enveloped
What does fusion from without mean?
This only occurs to enveloped viruses. The envelope will bind to the host membrane (as explained with HIV), and the nuclear capsid is released into the host.
How does receptor mediated endocytosis mean for an enveloped virus?
Endocytosis = bringing in of matter/ particle into itself.
The viral cell will dock onto the surface of the host which endorses endocytosis of the viral particle. The plasma membrane will engulf the whole viral particle, and will enter the cytoplasm as a vesicle. The vesicle will fuse with an enzyme inside the cell that will drop the pH within the vesicle, leading to conformational changes in viral surface proteins. The viral envelope will fuse with the inner membrane of the vesicle, and the nuclear capsid will be released into the cytoplasm.
How does receptor mediated endocytosis mean for a non-enveloped virus?
It is the only was a non-enveloped virus can enter the cell. The viral particle will bind to the cell membrane, get engulfed by it into the cytoplasm as a vesicle. The vesicle will fuse with an endosome and the pH will drop, resulting in a change of viral proteins shape which will create a pore in the vesicle and the viral genome will enter the cytoplasm.
What is biosynthesis?
genome replication + mRNA synthesis + translation.
DNA virus genomes can be replicated and transcribed by host polymerases, and some DNA viruses can encode their own DNA and RNA polymerases.
RNA virus genomes can only be replicated by their own polymerases because host cell DNA and RNA polymerases can only use DNA as a template as there are no cells in our body that can recognise and make RNA from RNA.
RNA viruses which have to make their own RNA polymerases that can use RNA as a template are classed as RNA dependent.
Viruses dont carry their own ribosomes and must fight for the host cell ribosomes.
What are the 2 classes of virus proteins that are made from the viral genome?
Structural = form part of the virus particle
Non-structural = such as enzymes used in transcription.
How and why is protein synthesis inhibited in virally infected cells?
This is either directly by the virus or production of interferon. The bodies response to viral infection is to produce interferon which will supress protein synthesis inside the infected cell, ultimately causing death of the cell.
Through the evolution of viruses, they often take control of protein synthesis within the host. There are many ways they can do this, one of which is some viral RNAs can be translated via a cap independent mechanism (IRES) which provides the same function as the 5’ cap so viruses can continue translating their proteins even if the host cell is shutting down.
How does the viral particle leave the host cell?
In the endoplasmic reticulum, the viral mRNA is translated into viral glycoproteins which become inserted into the membrane of the ER. in vesicles, they bud off and move to the golgi. Glycosylation occurs as they move through the golgi. The vesicles pinch off and move to the plasma membrane, fusing with it, and causing a mix of both viral and normal host proteins on the surface of the cell. The virus buds off from the cell.
Non-enveloped cells can only be released via lysis.
How does the virus spread throughout the body?
Primary site which is local. Some viruses shed here e.g. influenza (lungs), rotaviruses (gut), warts (skin). Or moves to a secondary site via lymph nodes and blood called viraemia. Secondary sites are the liver, bone marrow, spleen and blood vessel endothelium. Then they can replicate and move through the blood (secondary viraemia) to target organs such as the lungs, skin or brain.
What is M13 filamentous bacteriophage?
commonly used in the lab to help develop sequencing technology. They have circular ssDNA and are released from the cell via budding. Bacteriophage turn bacteria virulent.
