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
Compare antigenic drift and antigenic shift
Change in antigen New strain or subtype No. of types of virus involved Host species Change in genome Process leading to change in genome Freq of occurrence Consequences
