Viruses Flashcards
Describe the characteristics of viruses
- Small; sizes range from 10nm to 300nm
- Considered as living or nonliving
- Nonliving: acellular, no organelles, no cytoplasm, outside host, no metabolic processes e.g. respiration
> Enclosed by protein coat called capsid, made of capsomeres
> Geometric shape, exist as crystalline forms outside host - Living: hv genetic material e.g. RNA/DNA, can reproduce and replicate in host, use host cell’s enzymes like RNA polymerase for transcription and ribosome for translation & use host resources to replicate
⇒ obligate parasites
Describe the structure of viruses (general)
Genome - Nucleic acid coding for synthesis of viral components and enzymes for viral replication & assembly - DNA/RNA, ss/ds - Few genes Capsid+genome→ nucleocapsid/viral core
Capsid
- protein coat that surrounds and protects viral genome
- subunits: capsomeres
Envelope
- p.lipid bilayer surrounding nucleocapsid
- Derived from host cell membrane via budding
- Embedded w viral glycoprotein involved in host cell recognition
Describe the structure of T4 phage/lambda phage
General+
dsDNA, icosahedral capsid head, no envelope
Describe the structure of influenza virus
General+
Genome:
- 8 diff segments of (-)ssRNA associated w nucleoproteins→ complementary to viral mRNA
- Each segment packed w 3 polymerase proteins which come tgt to form RNA-dependent RNA polymerase enzyme complex/ RNA replicase, which replicates and transcribes viral genome in host
Capsid present
Envelope
- Present
- Glycoproteins embedded in envelope: haemagglutinin & neuraminidase
Describe the structure of HIV
General+
Genome
- 2 identical (+) ssRNA bound to nucleocapsid proteins→ nucleocapsid
Capsid: conical shaped, enzymes reverse transcriptase, integrase and protease found in capsid
Envelope
- Present
- Glycoprotein embedded in envelope: gp41 and gp120
Why is HIV described as a retrovirus
RNA virus that duplicate via reverse transcription in cell, using RNA genome as template to produce DNA via cbp
Describe the life cycle of a T4 phage
Attachment: Attachment sites on tail fibres (recognise and) adsorb to complementary receptor sites on bacterial cell wall
Penetration
- Phage tail releases lysozyme (enzyme), which digests bacterial cell wall→ release of mlcs that trigger changes in conformation of base plate proteins→ tail sheath contracts→ drive hollow core tube through cell wall
- Core tip reaches plasma membrane, viral DNA genome injected into cell. Empty capsid remains outside cell
Replication via lytic cycle:
- Host cell macromolecular synthesising machinery taken over to synthesise phage proteins: phage DNA transcribed to mRNA using host’s RNA polymerase.
- Early phage proteins degrade host DNA, nt reused
- Phage DNA synthesised using host machinery, nt, DNA polymerase and phage proteins
- Host’s metabolic machinery used to synthesise late phage proteins: phage enzymes & structural components.
Maturation:
- Phage DNA and capsid assembled into a DNA-filled head.
- Head, tail and tail fibres assembled independently & joined in a seq
Release: Lysozyme, coded for by phage gene, synthesised within cell and breaks down bac cell wall→ (H2O enter by osmosis) bac cell membrane lyses→ release new virions
Describe the life cycle of a lambda phage
Attachment: Attachment sites on tail fibres (recognise and) adsorb to complementary receptor sites on bacterial cell wall
Penetration
- Phage tail releases lysozyme (enzyme), which digests bacterial cell wall→ release of mlcs that trigger changes in conformation of base plate proteins→ tail sheath contracts→ drive hollow core tube through cell wall
- Core tip reaches plasma membrane, viral DNA genome injected into cell. Empty capsid remains outside cell
Replication via lysogenic cycle:
- Linear phage DNA circularises & integrated into host cell genome by integrase→ prophage
- Phage uses host cell to produce repressor proteins, which repress expression of prophage genes→ remains in latent state
- Prophage replicates along w bac chromosome
- Spontaneous induction (rare): cellular proteases activates and destroy repressor proteins→ prophage excised, no longer repressed→ enter lytic cycle
Maturation:
- Phage DNA and capsid assembled into a DNA-filled head.
- Head, tail and tail fibres assembled independently & joined in a seq
Release: Lysozyme, coded for by phage gene, synthesised within cell and breaks down bac cell wall→ (H2O enter by osmosis) bac cell membrane lyses→ release new virions
Describe the life cycle of influenza virus
Attachment: Glycoprotein haemagglutinin recognises & binds to complementary & specific receptor mlcs, sialic acid receptor, on host cell membrane
Penetration: Virus enters by endocytosis. Host plasma membrane invaginates & pinches off→ endosome w virus, which fuse w lysosome→ pH drop, becomes low→ cause viral envelope to fuse w lipid bilayer of vesicle membrane→ nucleocapsid released into cytosol, which is degraded by cellular enzymes→ releasing viral RNA into cytosol, which enter nucleus
Replication:
- Transcription: Viral RNA-dependent RNA polymerase use (-)RNA genome as templates to synthesise a complementary (+)mRNA strand, which…
acts as template for synthesis of new viral RNA genome, catalysed by RNA dependent RNA polymerase. Viral RNA genome then exits nucleus
exit nucleus & enters cytosol/ RER → translated into viral structural components
- free ribosomes→ capsid proteins
- RER→ glycoproteins (H/N)
Maturation:
- Viral glycoproteins transported by vesicles from ER, embedded into plasma membrane
- capsid proteins associate w host cell membrane where glycoproteins are inserted
- viral genome associates w nucleoprotein→ Helical nucleoprotein, which interacts w capsid protein that hv associated w glycoproteins embedded on plasma membrane→ initiates budding
Release:
- New virions buds from cell via evagination, acquiring host cell membrane w embedded glycoproteins, haemagglutinin and neuraminidase
- Neuraminidase facilitates release, by cleaving sialic acid from host cell receptor
Describe the life cycle of HIV
Attachment: Glycoprotein gp120 recognises and binds to complementary CD4 receptor on T helper cells, w help of co-receptor
Penetration:
- With help of gp41, viral envelope fuse w host cell membrane→ nucleocapsid released into cytosol, leaving envelope behind
- Capsid degraded by cellular enzymes→ release viral enzymes & 2 (+) ssRNA into cytosol
Replication:
- Reverse transcriptase uses viral RNA as template to synthesise a complementary DNA strand→ DNA-RNA hybrid→ RNA strand degraded, 2nd DNA strand complementary to the first is synthesised→ viral dsDNA mlc
- Viral dsDNA enters nucleus→ incorporated into host DNA by integrase→ provirus, may remain latent for long time
- Upon activation, provirus (DNA) transcribed into viral RNA, by RNA polymerase, which enters cytosol and…
–> becomes RNA genome for new virions
–> act as mRNA to be translated into…
> RER: envelope glycoproteins gp 120 and gp 41, transported to plasma membrane via vesicles, where they are embedded
> cytoplasm: viral polyproteins
Release:
- Polyproteins and viral RNA genome assemble at plasma membrane where viral glycoproteins have been inserted
- New virions bud off by evagination, acquiring host cell membrane embedded w viral glycoproteins gp41 and gp120.
Maturation:
- Viral HIV protease cleaves polyproteins→ viral enzymes and functional proteins
- Viral RNA genome and enzymes encapsulated by protein coat to form a capsid
- Virion is now mature and ready to infect another cell
Compare the lytic and lysogenic cycle
- Control of host cell’s protein synthesising machinery
- Integration of phage DNA
- Repressor protein
- Latent stage
- Propagation of virus
Compare the replication of bacteriophages and animal viruses.
Adsorption Penetration Uncoating Genome replication Release
Describe antigenic drift
Antigenic drift: minor change in surface antigens
- Accumulation of mutations in genes encoding surface glycoproteins of virus→ change ribonucleotide seq
→ sf antigens/glycoproteins w different conformation→ cannot be recognised and bound by antibodies against prev strains
Why does antigenic drift occur frequently?
- lack of proofreading ability of RNA-dependent RNA polymerase
- fast/high rate of viral replication
- viral RNA is ss, don’t have backup copy to carry out repair mechanism
Describe antigenic shift
Antigenic shift: sudden & major change in viral sf antigens
- 2 or more diff strains of a virus infect the same cell of an intermediate host simultaneously→ during maturation, genetic reassortment of diff RNA segments→ recombination of genetic material in virion→ antigenic shift
→ new virus subtypes with new sf antigens & new combination of hemagglutinin and neuraminidase at the viral envelope
