Virus Flashcards
Definition of virus
Packages of / infectious agents
that contain genetic material and proteins
Types of bacteriophages
Viral
- phage that reproduces only by lytic cycle
- e.g. T4 phage
Temperate
- phage that is capable of using both lytic and lysogenic modes of reproduction
- e.g. Lambda λ phage
Reproductive cycle of T4 phage
- Adsorption
- multiple tail fibres bind to specific receptor sites on bacterial cell surface
- base plate settles down on cell surface - Penetration
- conformational changes occur in tail sheath, causing it to contract and central tube to pierce through cell wall and membrane
- DNA is extruded from head, through tail tube into host cell
- capsid is left on outside of cell wall - Synthesis and replication
- viral DNA is transcribed to synthesise mRNA using host RNA pol
- viral enzymes coded by viral DNA take over host cell’s macromolecular synthesising machinery for its own use
- virus then uses host cell’s nucleotides and DNA pol to synthesise many copies of its DNA - Assembly
- Release
- host cell is lysed by lysozyme, which digests cell wall
- water enters cell by osmosis -> swell and burst
Reproductive cycle of lambda λ phage
- Adsorption
- single tail fibre binds to specific receptor site on bacterial cell surface
- base plate settles down on cell surface
2a. Penetration
- conformational changes occur in tail sheath, causing it to contract and central tube to pierce through cell wall and membrane
- DNA is extruded from head, through tail tube into host cell
- capsid is left on putside of cell wall
2b. Prophage formation
- genome circularises and inserts itself into a
specific site on the bacterial chromosome,
known as the prophage insertion site,
by genetic recombination
=> integrated phage known as prophage
- Viral DNA replicated along with chromosome each time cell divides and is passed on to generations of host daughter cells
=> give rise to a large population of bacteria carrying the viral DNA in prophage form
2c. Environmental trigger -> switch from lysogenic to lytic cycle
- Lysis genes which were repressed during lysogeny are activated, allowing the lambda (λ) phage genome to be excised from the bacterial chromosome to give rise to new active phages
- Synthesis and replication
- Assembly
- Release
- host cell is lysed by lysozyme, which digests cell wall
- water enters cell by osmosis -> swell and burst
Reproductive cycle of influenza virus
- Adsorption
- Haemagglutinin (HA) molecules on viral membrane bind to sialic acid containing receptors on membrane of host cell - Penetration
- virus taken in by receptor-mediated endocytosis -> form endocytic vesicle called endosome
- endosome fuses with acidic lysosome -> pH of vesicle lowered -> viral envelope fuses with endosome membrane -> viral RNAs released directly into cytoplasm of host cell
- viral RNAs transported into nucleus - Synthesis and replication
- viral replicase copies (-) sense RNA template into complementary (+) sense RNAs
- Viral nucleic acid synthesis: (+) sense RNAs used as templates for synthesis of full-length (-) sense RNAs by viral replicase
- Viral protein synthesis: (+) sense RNAs used as mRNA which are translated in cytoplasm by host protein synthesis machinery- free ribosomes synthesis 3 sets of proteins: enzymes (viral replicase), matrix and capsomeres
- rER-bound ribosomes synthesis viral transmembrane surface glycoproteins (HA, NA and M2) -> golgi apparatus for glycosylation
-> cell membrane via fusion of vesicle with host cell membrane
- Assembly
- Release
- virus released from host cell by budding, acquiring with it host membrane containing HA, NA and M2
- viral particle remains attached to host cell due to HA on viral envelope and sialic-acid containing receptors on host cell membrane
- Neuraminidase (NA) cleaves sialic acid residues on the cellular receptor -> virus is released
Reproductive cycle of HIV virus
- Adsorption
- glycoprotein gp120 on viral membrane binds to CD4 receptor on membrane of T helper cells and macrophages - Penetration
- after binding, gp120 undergoes conformational change -> bind to co-receptor on membrane of cell
(T helper cell: CXCR4
macrophage: CCR5)
- gp41 pulls virus closer to host cell and co-receptor facilitates entry of gp120-CD4 complex through cell membrane
- viral envelope fuses with host cell membrane
-> release viral contents into host cell - Synthesis and replication
- reverse transcriptase transcribes viral RNA into complementary DNA strands, newly synthesised DNA strand used as template for synthesis of other complementary DNA strand to form double-stranded DNA molecule
- DNA molecules passes through nuclear pore and enters host nucleus
integrase catalyses integration of viral DNA into DNA of host
-> provirus (latent state for years)
- host cell stimutated in immune response
- Viral nucleic acid synthesis: proviral DNA transcribed by host RNA polymerase into RNA
- Viral protein synthesis: proviral DNA transcribed into viral mRNA which is translated in cytoplasm by host protein synthesis machine
- free ribosomes synthesis single long chain of HIV proteins (reverse transcriptase, integrase, protease)
- rER-bound ribosomes synthesis viral transmembrane surface glycoproteins (gp120, gp41)
-> golgi apparatus for glycosylation
-> cell membrane via fusion of vesicle with host cell membrane - Assembly
- Release
- virus released from host cell by budding, acquiring with it glycoprotein studded host membrane
- viral matures when HIV protease cleaves single long chains of HIV
How virus challenges cell theory
- All known living organisms are made of one or more cells
- acellular and do not have protoplasm - metabolism occurs within cells
- lack the necessary molecular machinery to conduct many of the biochemical rxns a normal cell would need
- metabolically inert and does not carry out respiration or biosynthesis in extracellular virion state - all living cells arise from pre-existing cells
- cannot replicate unless they have entered a suitable host cell
+ fits cell theory by …
1. all living cells on Earth store genetic material
- contains genetic material necessary to form the next gen
Mechanism for genetic variation in virus
- Antigenic shift:
(1) sudden change in antigenicity
(2) owing to reassortment of segmented virus genome with another genome of a different antigenic type - Antigenic drift:
(1) gradual accumulation
(2) of minor mutations in viral genes,
(3) which lead to altered antigenicity
Differences between antigenic drift and antigenic shift
- no of viral strains involved
- 1 vs 2 or more - mechanism for change
- accumulation of pt mutations in gene vs reshuffling of genome
- minor alteration for 3D conformation vs dramatic alteration of type - rate of occurrence
- regular occurrence (to give rise to seasonal epidemics) vs occasional occurrence (to give rise to panemics) - effect of host immunity
- possibility of pre-existing immunity vs no immunity - cross-species transmission
- only effect individuals of same species vs can effect a new species
example of antigenic drift
- influenza
- viral genes coding for different types of hemagglutinin (HA) / neuraminidase (NA)
- due to mutations during synthesis of (-) sense RNA from (+) sense RNA
- factors affecting rate of mutation
- viral polymerases are prone to errors
- viral polymerases lack proofreading ability
(doesn’t apply to reverse transcriptase) - ssRNA lacking a complementary strand for polymerases to proofread during replication
similarities between binary fission and mitosis
- separation of DNA molecules
- formation of genetically identical daughter cells
differences between binary fission and mitosis
- genetic material
a. binary fission: involves a single bacterial chromosome
mitosis: involves multiple eukaryotic chromosomes
b. binary fission: involves circular, ds bacterial chromosome
mitosis: involves linear, ds eukaryotic chromosomes - DNA rep
- binary fission: involves semi-conservative dna rep
mitosis: semi-conservative dna rep occurs before mitosis - nuclear envelope and centrioles
- binary fission: no nuclear envelope and centrioles to …
mitosis: nuclear envelope … and centrioles … - chromosome behaviour
- binary fission: chromosomes attach to cell membrane
mitosis: chromosomes attach to microtubules in prophase
- binary fission: chromosomes pulled apart by cell elongation
mitosis: chromosomes pulled apart by microtubules shortening
