Viruses and their Structure Flashcards
define virus
an infectious, obligate intracellular parasite comprising genetic material (DNA or RNA) surrounded by a protein coat and/or an envelope derived from a host cell membrane
define genome in the case of viruses
encodes for viral proteins: 2 types
1. structural proteins: capsid proteins contained within the virion
2. non-structural proteins: essential to complete the replication cycle, block type I IFN response in host cells to avoid detection, viral encoded polymerase genes to replication genome
define capsid; give function
comprised of multiple copies of a single (or a few different) capsomere proteins (capsomeres are encoded by the viral genome); main function is to protect the fragile nucleic acid genome from physical damage (sheering and breaking of genome), chemical damage (UV, disinfectants), and enzymatic damage;
for naked viruses, parts of the capsid serve as ligand to bind to receptors on a host cell
define envelope
the nucleocapsid (just a capsid coated with lipid) of some viruses is surrounded by this lipid bilayer envelope (stole the lipid bit from the host they replicated in), which contains at least one viral-encoded membrane protein that serves as a ligand to interact with a receptor on host cells; for enveloped viruses, loss of the envelope = loss of infectivity
define naked virus
naked viruses are coated with protein that can be highly stable, meaning that they are very hardy and can be dried out and still remain infectious (ex. many GI viruses as they can survive through stomach acid and bile enzymes)
define enveloped virus
lipid (fat) coated, sensitive to drying out and therefore less environmentally stable; if lipid layer is removed, virus is no longer infectious; these viruses are sensitive to soap (detergent) and alcohols (hand sanitizer)
explain why viruses are considered parasites
viruses cannot synthesize their own proteins and are absolutely dependent on the host cell’s ribosomes to synthesize viral protein and also to provide energy (ATP) for viral replication
describe the basic anatomy of a virus and how naked and enveloped viruses differ
shape is determined by
nucleic acid genome content: -RNA viruses have helical nucleocapsids, most other types (DNA and +RNA) are icosahedral; the size of the viral genome, the capsid symmetry (helical, icosahedral, or complex), determine shape, and enveloped viruses have an additional lipid bilayer envelope, whereas naked viruses are coated in protein
describe the sizes of viruses in comparison to other microorganisms
most viruses are much smaller than a typical bacterium and cannot be easily visualized under a microscope
describe how viruses are classified (Baltimore’s system)
based on 4 criteria:
- the relation of the viral genome to how it produces mRNA
- genome: DNA or RNA?
- strandedness: single or double? if single, is it positive or negative?
- the method of replication it follows
describe the difference between +RNA and -RNA
mRNA is designated as + stranded; other single stranded RNA is classified as - stranded
list and describe the main steps in the viral replication cycle
- adsorption/attachment: animal cells do not have specific receptors for viruses, but viruses exploit molecules on cell surfaces for entry
- virion entry
- uncoating: occurs when viral nucleic acid genome and capsid proteins separate; capsid proteins may be enzymatically digested or destabilized to release the nucleic acid genome but nucleocapsids tends to remain on the genome and help in replications; this is the beginning of the eclipse phase of viral replication, as viral particles can no longer be detected since they are inside the cell
- synthesis of viral nucleic acid and proteins
- virion assembly: assembly-line fashion like making a car since viruses do not grow
- virion release: a whole bunch made from just one initial virus
