Module 26 - Virology Flashcards
Differ between virus and virion.
While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles, also known as virions, consist of:
(i) the genetic material made from either DNA or RNA, long molecules that carry genetic information;
(ii) a protein coat, called the capsid, which surrounds and protects the genetic material; and in some cases
(iii) an envelope of lipids that surrounds the protein coat.
riDescribe the characteristics of the virion.
It contains a genome which may be DNA or RNA, single or double stranded, linear, circular or segmented.
Note: For RNA viruses, the genome can have + or - sense (+ sense is the same as mRNA)
Genome is packaged in a protective protein coat (capsid). Furthermore, it also has an envelope made of the host cell membrane modified to contain the virus proteins (which has glycoproteins responsible for cell interaction)
What is the function of a capsid? Explain the concept of capsid symmetry.
Capsid has the function to protect the genome form breakdown by nucleases. Capsid proteins are assembled symmetrically around the nucleic acid to form either:
- an icosahedral capsid (composed of capsomers)
- helical capsid (enclosed in modified cell membrane envelope)
Mention examples of viruses with icosahedral symmetry.
Adenovirus. Herpesvirus. Pappilomavirus
Mention examples of virus with helical symmetry.
Mumps virus, influenza virus
How are virus classified?
Viruses are classified into Families by
- Type of nucleic acid genome
- Strategy of replication (how is mRNA produced from genome?)
- Morphology of the virion
Families names end with …viridae
Genera, species, and types are distinguished by different arrangements of genes, or sizes of proteins, or serological reactions, and often on types of disease produced.
Explain the method of the plaque essay.
In a plaque essay, a virus is added to a monolayer cell culture. The plaque arises due to cell death because of the virus (after which it is washed and stained).
Agar layer ensures virus only spread to neighbouring cells and not spread indiscriminately
Describe the overview of replication cycle curve of virus.
Eclipse Period: a period where there is a decline of intracellular virus particle due to uncoating of virus during replication (no longer considered virus)
Latent period: period before lysis of cell or budding (releasing virus)
Mention the stages in viral replication.
Mention the two methods of which virus can enter a cell.
- Fusion with the plasma
- Endocytosis then endosome escape
- Fusion with endosome
- Genome passes through endosome bilayer
- Lysis of endosome
All require interaction with a receptor on the host cell.
All results in uncoating and release of genome to replicate.
Describe the Baltimore Classification
- RNA requires transport of RNA-dependent RNA polymerase (structural part of virion)
- RNA (=mRNA) transcripts and translates RdRP which synthesizes minus strans -> new + RNA strand
- Some +RNA uses rvt to revert to dsDNA
- RNA (=mRNA) transcripts and translates RdRP which synthesizes minus strans -> new + RNA strand
- ssDNA and dsDNA both can use available DdRP in the host cell to produce mRNA
How are envelopes on virus acquired?
Envelopes are acquired by budding through host cell membranes.
Note: Non-enveloped cells exits through lysis.
Mention the types of the transmission method of viral pathogen.
- Respiratory or salivary spread
- Facial-oral spread
- Venereal spread
- Vector
- Vertebrate resevoir
- Vector-vertebrate reservoir
Mention and describe an example of systemic respiratory tract viral infection.
Mumps virus (causes measles) replicates primarily in the epithelial cells of URT, bind to sialic acid receptors in the cell. Systemic infection ensues, involving virtually all organs including CNS (mild meningitis common). Characteristic salivary gland involvement (swelling).
Explain how viruses can enter and replicate in the alimentary tract.
Virus can enter through either through ingestion (oral) or rectally. Ingested viruses can either:
- replicate in mouth and pharynx but not further down (herpes simplex)
- infect oropharynx and swallowed (rotavirus)
- progress into the circulation (enterovirus)
Viruses that are swallowed are usually acid and bile resistant and do not have an envelope.
Mention the types of virus-induced changes in cells.
How do viral genomes continually change?
- Point mutation
or, if two closely related viruses infect the same cell
- Recombination: exchange of nucleic acid sequence
- Reassortment: swapping of segments for viruses that have segmented genome
Mention an example of a viral disease that reappears, often in a different form.
It happens in cases of latent infections, such as the case in the Varicella-zoster virus.
In some cases, after the initial infection (chickenpox), the virus is then cleared from the system. However, in the others, from the skin lesions, some of that virus travels up the nerve under the skin to the nerve bundle (dorsal root ganglion) in the form of its genome. When you get older and immune response wanes, it will emerge throughout all the nerve originating from that ganglion (herpes zoster/shingles).
What is the aim of vaccination?
Most vaccination aims to establish a poll of memory T and N cells for more rapid and greater immune responses when the pathogen is encountered again (long-lived antibody responses).
- Memory CD8+ T cells -> killing of virus-infected cells by CTL
- Memory B cells -> neutralisation of virus by antibody
- Neutralising AB directed against surface proteins
- prevent attachment/subsequent stages of infection
- promote opsonisation or ‘Ab+complement’ lysis
- Memory CD4+ T cells -> help B cells make antibody, help cytotoxic T cell develop into memory cells
Explain the mechanism of antiviral drugs.
Antiviral drugs work by inhibiting particular stages of viral replication.
How do nucleosides analogs work? What determines its specificity for virus-infected cells?
It is an acyclovir compound used for herpesvirus infections.
Acyclovir is a guanosine analogue (with an acyclic sugar). The phosphate group can be added to the hydroxyl group attached to the 5’ Carbon to be incorporated into DNA. But the 3’ hydroxyl group required to extend the DNA polymer is absent (terminating the DNA replication chain).
This is specific to virus-infected cells, as herpesvirus thymidine kinase enzyme is required to add phosphate to Acyclovir. Without this, it can’t be incorporated into DNA, hence not affecting normal cells.
Mention the routes of entry of viruses.
