CELL BIOLOGY OF VIRUSES Flashcards
MEANING OF THE WORD VIRUS?
LATIN FOR POISON
VIRUSES WERE FIRST DISCOVERED BECAUSE OF THEIR ROLE IN?
ROLE IN PRODUCING DISEASE IN AGRICULTURALLY IMPORTANT PLANTS AND ANIMALS
WORK ON WHICH DISEASES FIRST LEAD TO THE WORD VIRUS TO MEAN ANY INFECTIOUS AGENT SO SMALL THAT IT COULD BE NEITHER OBSERVED BY LIGHT MICROSCOPY NOR REMOVED FROM A SOLUTION BY FILTRATION THROUGH THE SMALLEST PORES THAT HUMANS COULD MANUFACTURE?
TOBACCO MOSAIC DISEASE
FOOT-AND-MOUTH DISEASE
ONE OF THE FIRST DEFINITIONS OF VIRUSES?
ANY INFECTIOUS AGENT SO SMALL THAT IT COULD BE NEITHER OBSERVED BY LIGHT MICROSCOPY NOR REMOVED FROM A SOLUTION BY FILTRATION THROUGH THE SMALLEST PORES THAT HUMANS COULD MANUFACTURE
WHAT IS A CHAMBERLAND FILTER AND HOW DOES IT RELATE TO VIRUSES?
- NAMED AFTER CHARLES CHAMBERLAND (1851-1908)
- C.C. DISCOVERED THAT WARM PORCELAIN CAN RETAIN FINE PARTICLES IN SOLUTION AND, ALONG WITH LOUIS PASTEUR, SHOWED THAT BACTERIA CAN BE FILTERED OUT OF A SOLUTION USING A PORCELAIN FILTER IN A GLASS TUBE
- VIRUSES WERE FIRST UNDERSTOOD AS MICROBES SO TINY THAT THEY COULD PASS THROUGH A CHAMBERLAND FILTER
USUAL SIZE OF VIRUSES?
20-100 nm IN DIAMETER
MICROSCOPY FOR OBSERVING VIRUSES?
- MOST VIRUSES ARE TOO SMALL TO BE SEEN USING LIGHT MICROSCOPY
- LIGHT MICROSCOPY USEFUL FOR STUDYING VIRAL EFFECTS ON CELLS
- VIRUSES ARE USUALLY VISUALISED USING ELECTRON MICROSCOPY OR ATOMIC FORCE MICROSCOPY
IS HERPESVIRUS A SMALL OR A LARGE VIRUS?
LARGE (200nm)
WHICH OF THESE CANNOT BE USED FOR OBSERVING VIRUSES - LIGHT MICROSCOPY, ATOM FORCE MICROSCOPY, ELECTRON MICROSCOPY
LIGHT M.
NAME OF A LARGE VIRUS COVERED BY SURFACE FIBERS, WITH A DISTINCTIVE STAR-SHAPED DEPRESSION?
MIMIVIRUS
very big, has a huge genome, initially mistaken for bacteria
WHAT ARE CYTOPATHIC EFFECTS AND WHAT DO THEY DEPEND ON?
Cytopathic effect or cytopathogenic effect (abbreviated CPE) refers to structural changes in host cells that are caused by viral invasion.
DEPENDS ON THE SPECIFIC HOST CELL AND THE VIRUS
WHAT ARE VIRUS-INDUCED SYNCITIA (SYNCYTIA)?
LARGE, MULTINUCLEATE CELLS
Viral protein mediates fusion of an infected cell with neighboring cells leading to the formation of multi-nucleate enlarged cells called syncytia.
EXAMPLES OF CYTOPATHIC EFFECTS DUE TO VIRAL INFECTIONS?
- MANY VIRUSES CAUSE INFECTED CELLS TO ROUND UP AND DETACH FROM TISSUE FLASK AS THEY DIE (E.G. PANCREATIC NECROSIS VIRUS)
- SOME VIRUSES CAUSE CELLS TO FORM SYNCITIA (LARGE, MULTINUCLEATE CELLS)
4 TRAITS THAT DEFINE VIRUSES?
1) VIRUSES ARE OBLIGATE INTRACELLULAR PARASITES THAT HAVE AN INFECTIOUS EXTRACELLULAR STAGE, CALLED VIRION
2) ALL VIRUSES ENCODE AT LEAST ONE CAPSOMERE PROTEIN, CAPSOMERES COVER AND PROTECT THE NUCLEIC ACID IN A VIRION, AN ALL VIRUSES CARRY GENETIC INSTRUCTIONS FOR SYNTHESIZING AT LEAST ONE CAPSOMERE
3) ALL VIRUSES REPLICATE, NOT BY GROWING LARGER AND DIVIDING AS DO CELLS, BUT BY ASSEMBLY (CELLS INFECTED BY VIRUSES SYNTHESIZE THE COMPONENT PARTS OF THE VIRION, AND THEN HE PARTS, ONCE SYNTHESIZED, SPONTANEOUSLY ASSEMBLE INTO NEW VIRIONS WITHOUT FURTHER INPUT OF ENERGY
4) ALL VIRUSES HAVE THE CAPACITY TO EVOLVE (BECAUSE THE MINIMUM COMPOSITION OF VIRUSES IS NUCLEIC ACIDS AND CAPSOMERES, VIRAL POPULATIONS CAN CHANGE THROUGH TYPICAL EVOLUTIONARY PROCESSES THAT ALTER THOSE NUCLEIC ACIDS IN A HERITABLE WAY
WHAT IS THE MINIMUM COMPOSITION OF VIRIONS?
NUCLEI ACIDS AND CAPSOMERES
HOW TO VIRAL POPULATIONS USUALLY EVOLVE?
THROUGH TYPICAL EVOLUTIONARY PROCESSES THAT ALTER THEIR NUCLEIC ACIDS IN A HERITABLE WAY
NAME AND EXPLANATION OF THE PROCESS VIRUSES USE TO REPLICATE?
ASSEMBLY
CELLS INFECTED BY VIRUSES SYNTHESIZE THE COMPONENT PARTS OF THE VIRION, AND THEN THE PARTS, ONCE SYNTHESIZED, SPONTANEOUSLY ASSEMBLE INTO NEW VIRIONS WITHOUT FURTHER INPUT OF ENERGY
WHICH TYPE OF PROTEIN IS ENCODED BY EVERY VIRUS?
AT LEAST ONE CAPSOMERE PROTEIN IS ENCODED BY EVERY VIRUS
WHAT ARE CAPSOMERES?
STRUCTURES THAT COVER AND PROTECT THE NUCLEIC ACIDS (GENETIC MATERIAL) IN VIRUSES
WHAT IS THE EXTRACELLULAR INFECTIOUS STAGE OF VIRUSES CALLED?
VIRION
WHAT IS CAPSID?
PROTEIN COAT OF CAPSOMERES THAT SURROUND THE NUCLEIC ACIDS IN VIRUSES
EXPLAIN THE TERM NUCLEOCAPSID?
IN SOME VIRUSES, THE NUCLEIC ACID GENOME AND THE PROTECTIVE STRUCTURES AROUND IT, CAPSOMERES, ARE SO INTIMATELY ASSOCIATED THAT THEIR JOINT STRUCTURE IS CALLED NUCLEOCAPSID (e.g. SARS CoV 2)
DIFFERENCE BETWEEN ENVELOPED AND NAKED VIRIONS?
ENVELOPED VIRIONS HAVE A LIPID BILAYER, WHEREAS NAKED VIRIONS DO NOT
DESCRIBE THE SPHERICAL CAPSIDS IN VIRUSES?
- ICOSAHEDRONS (POLYHEDRONS WITH 20 FACES) CONSTRUCTED FROM REPEATED UNITS, APPROXIMATING A SPHERE
- SOMEWHAT RIGID, COMPARED TO E.G. HELICAL CAPSIDS THAT CAN BE RIGID OR FLEXIBLE
WHAT IS THE BALTIMORE CLASSIFICATION SCHEME?
- FIRST DEFINED IN 1971
- THE SCHEME DERIVED FROM THE MECHANISM BY WHICH THE VIRAL GENOME IS RELATED TO mRNA (CLASSES BASED ON THE WAY THE VIRUSES USE AND PRODUCE RNA)
- CLASSIFICATION SYSTEM THAT PLACES VIRUSES INTO ONE OF SEVEN GROUPS DEPENDING ON A COMBINATION OF THEIR NUCLEIC ACID (DNA OR RNA), STRANDEDNESS (SINGLE OR DOUBLE STRANDED), SENSE, AND METHOD OF REPLICATION
- GROUPS DESIGNATED BY ROMAN NUMERALS
- E.G. GROUP I ARE DS DNA VIRUSES, GROUP II SS DNA VIRUSES, III:DS RNA…
(it does NOT reflect evolutionary relatedness of different groups of viruses)
WHAT ARE THE MOST ABUNDANT EVOLVING ENTITIES?
VIRUSES
NUMBER OF VIRUSES COMPARED TO THE NUMBER OF BACTERIAL AND ARCHEAL CELLS ON EARTH?
10x MORE VIRUSES
ALL LIVING MICROBES SHARE A TRAIT OF HAVING ENERGISED MEMBRANES; EXPLAIN THIS TRAIT + ARE VIRUSES ALIVE BASED ON THIS DEFINITION?
One trait shared by all living microbes, is that they have an energized membrane in which the concentration of ions is different in the two compartments separated by that membrane. For a microbial cell, dissipation of the ion gradients is the definitive sign that the cell is no longer alive.
By this definition there are no living viruses because even though some viruses have a lipid bilayer, called a viral envelope, the viral envelope is not energized!!!
WHAT IS THE VIRAL LIPID BILAYER THAT SOME VIRUSES HAVE CALLED?
VIRAL ENVELOPE
WHAT ARE SPIKE PROTEINS (PEPLOMERS)?
In virology, a spike protein or peplomer protein is a protein that forms a large structure known as a spike or peplomer projecting from the surface of an enveloped virus. The proteins are usually glycoproteins that form dimers or trimers. Often the term “spike protein” refers specifically to the coronavirus spike protein, one of the four major structural proteins common to all coronaviruses, which gives rise to the distinctive appearance of these viruses in electron micrographs (projections from a spherical capsid). These proteins typically have a role in viral entry.
% OF THE HUMAN GENOME THAT IS MADE UP OF 100,00 PARTIAL SEGMENTS OF HUMAN ENDOGENOUS RETROVIRUSES (and their role?)
8%
(Endogenous retroviruses are associated with major evolutionary events, including the origin of Homo sapiens; retroviral infections are likely causing evolutionary change today)
HUMAN ENDOGENOUS RETROVIRUSES IN THE GENOME MAY BE ASSOCIATED WITH THE FORMATION OF WHICH HUMAN ORGAN?
PLACENTA
WHAT IS RAPID EVOLUTIONARY RADIATION?
AN EVOLUTIONARY PROCESS IN WHICH MANY SPECIES SEEM TO APPEAR FROM A COMMON ANCESTOR IN A SHORT PERIOD OF TIME ON AN EVOLUTIONARY SCALE
HOW ARE HUMAN ENDOGENOUS RETROVIRUSES IN THE HUMAN GENOME ASSOCIATED WITH PLACENTA DEVELOPMENT?
- THE DEVELOPMENTAL PROGRAM TO BUILD MAMMALIAN PLACENTA DEPENDS ON SYNCYTIN, WHICH IS THE ENVELOPE GENE OF HUMAN ENDOGENOUS DEFECTIVE RETROVIRUS, HERV-W
- PLACENTAL SYNCYTIOTROPHOBLASTS (MULTINUCLEATED CELLS THAT ORIGINATE FROM FETAL TROPHOBLASTS) ARE MAJOR SITES OF SYNCYTIN EXPRESSION
- EXPRESSION OF RECOMBINANT SYNCYTIA IN A WIDE VARIETY OF CELL TYPES INDUCES FORMATION OF A GIANT SYNCYTIA
(INFECTION BY A RETROVIRUS MADE THE EMERGENCE OF PLACENTAL MAMMALS POSSIBLE BY PROVIDING A VIRAL PROTEIN THAT ENABLS FORMATION OF THE PLACENTA)
VIRIONS?
THE EXTRACELLULAR INFECTIOUS PARTICLES THAT CARRY VIRAL NUCLEIC ACIDS FROM ONE HOST CELL TO ANOTHER
- ALL ARE MADE UP OF, AT A MINIMUM, PROTEIN CAPSOMERES ENCLOSING NUCLEIC ACIDS
TYPES OF VIRIONS BASED ON SHAPE AND ENCLOSURE?
VIRUSES CAN BE ICOSAHEDRAL OR HELICAL = NAKED OR ENVELOPED
DESCRIBE VIRAL GENOME DIVERSITY?
- CAN BE COMPOSED OF DNA OR RNA
- CAN BE ARRANGED AS LINEAR OR CIRCULAR MOLECULES THAT ARE EITHER DOUBLE OR SINGLE STRANDED AND EITHER SEGMENTED OR NON SEGMENTED
STAGES OF THE VIRAL REPLICTION CYCLE?
1) ATTACHMENT (THE VIRION ATTACHES TO THE HOST CELL)
2) PENETRATION AND UNCOATING (THE VIRION’S GENOME ENTERS THE HOST CELL: IN DOING SO, THE VIRUS AS A DISCRETE VIRION CEASES TO EXIST, BEGINNING THE ECLIPSE PERIOD) —> THE GENOME SOMETIMES ENTERS WITH THE CAPSID OR PIECES OF THE CAPSI
3) SYNTHESIS OF EARLY PROTEINS (THE PROTEINS EXPRESSED EARLY IN AN INFECTION OFTEN GIVE ONE OF 3 FUNCTIONS: SHUT DOWN HOST PROTEIN SYNTHESIS, REGULATE XPRESSION OF VIRAL GENES, SYNTHESISE VIRAL NUCLEIC ACIDS)
4) SYNTHESIS OF NEW VIRL GENOMES AND LATE PROTEINS (LATE PROTEINS ARE THOSE EXPRESSED AFTER GENOME REPLICATION HAS BEGUN, USUALLY STRUCTURAL PROTEINS, MEANING THEY WILL BECOME COMPONENTS OF THE PROGENY VIRIONS)
- ————– STAGES 3 AND 4 OVERLAP IN SOME VIRUSES, ESP THOSE WITH SMALL GENOMES
5) ASSEMBLY (COMPONENT PART OF THE VIRION ASSEMBLE INTO COMPLETED VIRIONS)
6) RELEASE (THE RELEASE OF THE NEWLY ASSEMBLED VIRUSES, THE LAT STEP, CAN BE SUDDEN OR GRADUAL, ULTIMATELY KILLING CELLS)
WHAT IS THE VIRAL ECLIPSE PERIOD?
the time between infection by (or induction of) a bacteriophage, or other virus, and the appearance of mature virus within the cell; an interval of time during which viral infectivity cannot be recovered
- begins after virion’s genome enters a host cell (AND THE VIRUS AS A DISCRETE VIRION CEASES TO EXIST)
WHT ARE EARLY PROTEINS IN VIRAL REPLICATION?
- PROTEINS EXPRESSED EARLY IN AN INFECTION
USUALLY HAVE ONE OF 3 FUNCTIONS: - SHUT DOWN THE SYNTHESIS OF HOST PROTEINS
- REGULATE EXPRESSION OF VIRAL GENES
- SYNTHETISE VIRAL NUCLEIC ACIDS
WHAT ARE LATE PROTEINS IN VIRAL REPICTION?
- PROTEINS EXPRESSED AFTER THE GENOME REPLICATION HAS BEGUN
- USUALLY STRUCTURAL PROTEINS, HICH WILL ACT AS COMPONENTS OF FUTURE VIRIONS
EXAMPLES OF VIRUSES WITH SUDDEN VS GRADUAL RELEASE OF NEWLY SYNTHESISED VIRIONS AFTER INFECTED THE HOST?
- BACTERIOPHAGES RELEASE IS SUDDEN BECAUSE LYSIS OF THEIR HOST CELLS MUST OCCUR IN ORDER FOR VIRUS TO ESCAPE
- SHEDDING OF VIRUSES FROM ANIMAL CELLS CAN BE GRADUAL AND MAY NOT LYSE THE HOST CELLS
DESCRIBE THE ONE-STEP GROWTH CURVE OF VIRAL GROWTH
- AFTER A SINGLE VIRUS IS ADDED TO ITS HOST CELL, EXTRACELLULAR VIRUSES BECOME UNDETECTABLE DURING THE ECLIPSE PERIOD
- THEN, IN AN ABRUPT CHANGE, THE NUMBER OF EXTRACELLULAR VIRUSES INCREASES EXPONENTILLY
- EVENTULLY THE NUMBER OF VIRUSES LEVELS OFF, AND THERE IS NO FURTHER INCREASE IN THE POPULATION SIZE
GRAPH DESCRIPTION: - NUMBER OF VIRUSES = 0 (Y=0), THE POINT JUST MOVING ALONG THE X AXIS (TIME)
- AFTER A CERTAIN X HAS BEEN REACHED THERE IS A SHARP EXPONENTIAL RISE ALONG THE Y AXIS, WHICH SOON REACHES A PEAK AND REMAINS CONSTANT
(once virion is taken up by the cell no change is detected for awhile and then there is a sudden sharp rise in the number of viruses after replication in the cell)
DESCRIBE THE ORDER OF SYNTHESI OF VIRAL COMPONENTS DURING AN INFECTION?
FROM 1ST O LAST TO START FORMING:VIRAL MRNA, VIRAL PROTEINS, VIRAL GENOMES, INTRACELLULAR VIRUSES, EXTRACELLULAR VIRUSES (THEY STOP BEING PRODUCED IN THE SAME ORDER, AS THEY ARE REQUIREMENTS FOR ONE ANOTHER)
INFLUENZA VIRUS OVERVIEW
Influenza type A viruses are RNA viruses categorized into subtypes based on the type of two proteins on the surface of the viral envelope:
H = hemagglutinin, a protein that causes red blood cells to agglutinate.
N = neuraminidase, an enzyme that cleaves the glycosidic bonds of the monosaccharide sialic acid (previously called neuraminic acid).
The hemagglutinin is central to the virus’s recognizing and binding to target cells, and also to its then infecting the cell with its RNA. The neuraminidase, on the other hand, is critical for the subsequent release of the daughter virus particles created within the infected cell so they can spread to other cells.
Different influenza viruses encode for different hemagglutinin and neuraminidase proteins. For example, the H5N1 virus designates an influenza A subtype that has a type 5 hemagglutinin (H) protein and a type 1 neuraminidase (N) protein. There are 18 known types of hemagglutinin and 11 known types of neuraminidase, so, in theory, 198 different combinations of these proteins are possible.
Variants are sometimes named according to the species (host) in which the strain is endemic or to which it is adapted. The main variants named using this convention are:
Bird flu Human flu Swine influenza Equine influenza Canine influenza Bat influenza
INFLUENZA A VIRUS BASED ON ITS DNS?
negative-sense, single-stranded, segmented RNA viruses
positive vs negative stranded viruses?
Positive-stranded RNA is just like mRNA can be read natively by the host’s translational machinery to make proteins
Negative-stranded RNA is complementary to mRNA AND must be transcribed into a positive strand by the virus’s own RNA-dependent RNA polymerase (this enzyme is carried in the capsid of all negative-stranded RNA viruses)
Most DNA viruses have both a positive and negative strand
WHAT DOES THE TERM ‘POLYCISTRONIC’ REFER TO WHEN DESCRIBING MRNA? IS IT TYPICAL OF PROKARYOTES OR EUKARYOTES?
The term “polycistronic” describes the situation in which two (bicistronic/dicistronic), three (tricistronic), or more separate proteins are encoded on a single molecule of messenger RNA (mRNA): thy have separate start and stop codons. In prokaryotes, polycistronic expression is common. (EUKARYOTIC MRNA ENCODES JUST ONE PROTEIN WITH A WINGLE START AND STOP CODON)
WHAT ARE VIRUS REPLICATION COMPLEXES (VRC)?
- ASSEMBLY FACTORIES IN EUKARYOTIC VIRUSES
- ASOCIATED WITH VARIOUS MEMBRANES, SUCH AS ER OR GOLGI BODY, IN ADDITION TO THE PLASMA MEMBRANE
HOW DOES THE HERPESVIRUS EXIT THE NUCLEUS?
BY BUDDING (Budding enables viruses to exit the host cell and is mostly used by enveloped viruses which must acquire a host-derived membrane enriched in viral proteins to form their external envelope. Viruses can bud at every stage in the ER-Golgi-cell membrane pathway.)
PROCESS BY WHICH BACTERIOPHAGES PERFORM VIRAL RELEASE IN HOST CELLS IS CALLED?
OSMOTIC LYSIS (BACTERIOPHAGES ENCODE CELL-WALL DEGRADING ENZYMES SO THAT THE PHAGES CAN ESCAPE LEADING TO THE OSMOTIC LYSIS)