Virology 1 Flashcards
- Describe the important characteristics that make viruses different from bacteria, protozoa and fungi.
viruses are obligate intracellular parasites
grow by making copies not by enlarging and dividing
contain either RNA or DNA but not both
no capacity to synthesize proteins or generate energy on their own
- Describe the basic structure of a virus with respect to structural components, symmetry and the presence of an envelop.
capsids come in two distinct shapes, helical (capsid wraps around central axis with nucleic acid inside) or icosahedral (capsid forms 20-sided polygon with 3 axis of symmetry)
vertices play important role in non-enveloped viruses for attachment structures or the envelop itself in enveloped structures
- Describe the major characteristics used in classifying virus families. (bold table 2)
nature of genome (RNA/DNA)
structure of genome (single or double stranded, segmented or non-segmented)
envelop (present or absent)
capsid symmetry (helical or icosahedral)
- Define virion, capsid, nucleocapsid and envelope.
virion: combo of nucleic acid and capsid (virus particle)– naked or non-enveloped
capsid: protein shell surrounding the viral genome
nucleocapsid: combo of capsid and nuclei acid
envelop: bi-lipid membrane derived from host cell membrane containing the virus particle
- Describe the primary functions of the capsid and envelope for for naked and enveloped for viruses.
envelopes can contain envelop glycoproteins which can be important in adhesion (viral attachment proteins or complexes), envelop like the capsid functions to protect the viral genome from damage, capsid is another site where attachment proteins can be located
- Explain why viruses disappear and infectious virus can not be found shortly after infecting a cell. Why is this different from any other microbial pathogen.
the drop due to the fact that viruses fall apart once they enter the cell ignored to deliver their genome (this is distinct from other microbes because viruses do not enlarge and divide)
eclipse period occurs between start of infection and when intracellular titer begins to increase
- List the steps during the process of virus replication in a cell.
attachment
entry
uncoating (often simultaneous with entry)
expression of early viral genes (regulation and replication proteins)
replication of viral genome
expression of late genes (structural proteins for virus particle)
assembly
release
- Describe the importance of receptors for resistance, pathogenesis, and viral effects on specific tissues or organs.
specific cell surface receptors are involved in viral attachment (sometimes with co-receptors)
receptors dictate host-range and tropism (susceptibility and process of entering the cell)
receptors made from proteins, carbohydrates, glycolipids and others
- Describe the two basic ways viruses use to enter cells.
(1) direct fusion between the viral envelope and the plasma membrane of the cell (Conformation change exposes hydrophobic region (fusion peptide which then inserts into target (cell) membrane)
(2) receptor-mediated endocytosis use naturally existing host pathways (clathrin-coated pit endocytosis; acidification is critical for conformation change)
some viruses can use either
- Describe a generic growth cycle (genome replication) for a D(R)NA virus and a retrovirus.
DNA: viral genome must enter nucleus (not pox); large virus- IE genes are regulatory, DE genes are involved in DNA replication, late genes are responsible for structural proteins
*most current antivirals attack DNA replication
RNA viruses:
positive: genomic RNA functions as mRNA for synthesis polymerase and of viral proteins
negative: include premade viral polymerase which can synthesize mRNA for viral proteins
retroviruses: particles contain viral polymerase (structural component); RT copies the + strand into single then double DNA; integrase catalyses insertion of DNA in nucleus (provirus)*; transcriptional proteins bind to a sequence “long terminal repeat” which contains the promoter
**+ strand genomes can’t be translated directly, it does not function as mRNA
insertion can cause insertional mutagenesis
- Briefly describe how viruses solve the problem of monocistronic mRNA translation in eukaryotic cells to generate individual viral proteins (5) and the significance of internal ribosome entry site (IRES) elements.
normally only one protein can be made from a single RNA/ DNA, viruses need various proteins and that is accommodated by:
(1) mRNA splicing to generate different mRNA
(2) polyprotein digested by proteolytic cleavages
(3) separate mRNAs with distinct promoters
(4) segmental genome
(5) ribosomal frame shifting due to psuedoknot
IRES are elements in the mRNAs at sites where translation initiation is desired
- Recognize how knowledge of the viral life cycle can be used to pick appropriate targets for antiviral drug development.
understanding how viruses get into, reproduce within and are released from cells are all steps which can be interrupted antivirals
knowledge of viral life cycle can also be important in communication with patients, explaining illness and treatment
- Briefly describe the basic strategies used by icosahedral and helical symmetry viruses for assembly of nucleocapsids.
helical: capsid proteins attach in a linear function, curl around and bind in a helical structure
icosahedral: capsid proteins assemble into substructures then attach to the scaffold structure; proteolytic digestion and nucleic acid insertion follows
**envelops glycoproteins are formed and inserted in host cell membrane; nucleocapsids bind to eh cytoplasmic tails of the envelop proteins and the envelope wraps around the nucleocapsid (budding)
- Describe briefly the mechanisms used by viruses for release from cells.
the most common mechanism is cell lysis
cells can fall apart due to cellular damage due to virus or the viruses themselves can cause apoptosis cascades
