virus Flashcards
why are viruses considered living and non living?
living: because they contain genetic material. the genetic material can undergo mutations when viruses are in a living host cell. hence viruses can evolve.
non-living as they are acellular and lack cellular organelles; do not carry out metabolism; are unable to reproduce independently and are unable to respond to stimuli
why are viruses obligate parasites?
they depend on host cells to complete their reproductive cycles
what is the structure of viruses?
Genome; capsid and envelope
structure of influenza
Genome: negative strand RNA; viral genome is complementary to the viral mRNA
8 different segments of SS RNA are associated with proteins, each RNA segment is packed with 3 polymerase proteins which together form the enzyme RNA-dependent RNA polymerase which uses the viral genome as a template to replicate the viral genome and synthesise viral mRNA in host cell.
capsid: present
Envelope: glycoproteins embedded in envelope; haemagglutinin and neuraminidase
structure of HIV
Genome: positive strand RNA; viral genome has the same sequence as the viral mRNA
2 identical copies of SS RNA are bound to nucleocapsid proteins
capsid: present; conical shaped and contains reverse transcriptase, integrase and protease
Envelope: GP120 is attached to Gp41
structure of bacteriophages
genome: DS DNA
capsid: icosahedral capsid head
envelope: absent
define antigenic shift
mutations frequently occur due to poor proofreading mechanism of viral RNA-dependent RNA polymerase and fast replication rate of the virus. over time there is an accumulation of mutations in the viral genome. sometimes the mutations produce viruses with modified surface antigens with a different conformation. IS cant recognise the new viral antigens and will be susceptible to infection.
stages of a virus life cycle
attachment –> penetration –> replication –> maturation –> release
define antigenic shift
when two strains from separate species infect a single cell of an intermediate host, random genetic reassortment may occur. thus when new viruses are assembled in the host cell, new combinations of RNA segments can come tgt. when viruses with new surface antigens are produced; IS cant recognise the new viral antigens and will be susceptible to infection.
t4 phage life cycle
1: attachment sites on tail fibres adsorbs to complementary receptor sites on bacterial surface
2: bacteriophage releases lysozyme which digests bacterial cell wall; this allows the release of molecules from the bacterium which triggers a change in shape of the proteins in the base plate which causes the contraction of tail sheath which will drive the hollow core tube through the cell wall. when the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell
3: Host cell macromolecular synthesizing machinery is used to synthesise phage proteins; early phage proteins degrade host DNA; phage DNA is then synthesised using host cell nucleotides and phage proteins. next, phage enzymes and structural components are made
4: phage DNA and capsid assemble into a DNA-filled head; head tail and tail fibres assembled independently and join in a specific sequence
5:phage lysozyme synthesised within the cell breaks down the bacterial cell wall. bacterial cell membrane lyses and release the newly formed virions.
lambda phage life cycle
1: attachment sites on tail fibres adsorbs to complementary receptor sites on bacterial surface
2: bacteriophage releases lysozyme which digests bacterial cell wall; this allows the release of molecules from the bacterium which triggers a change in shape of the proteins in the base plate which causes the contraction of tail sheath which will drive the hollow core tube through the cell wall. when the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell
3: linear phage DNA circularises and inserted into host cell genome by integrase; the integrated phage DNA is known as a prophage. expression of phage genes is repressed by phage repressor proteins. prophage remains latent replicates along with bacterial chromosome. during spontaneous induction, cellular proteases are activated that destroy the repressor proteins. the prophage is then excised from the bacterial genome, then lytic cycle occurs.
4: phage DNA and capsid assemble into a DNA-filled head; head tail and tail fibres assembled independently and join in a specific sequence
5:phage lysozyme synthesised within the cell breaks down the bacterial cell wall. bacterial cell membrane lyses and release the newly formed virions.
influenza life cycle
1.haemagglutinin recognises and binds to complementary sialic acid receptor on host cell membrane
2. enters host cell by endocytosis
endocytic vesicle fuses with lysosome which lowers the pH , causing viral envelope to fuse with lipid bilayer of vesicle, nucleocapasid is released into cytosol
capsid degraded by cellular enzymes and the 8 viral RNA segments enter the nucleus
3. viral RDRP uses viral genome as a template to synthesise mRNA.
mRNA enters cytosol and is translated into viral structural components; or act as a template for synthesis of new viral RNA genome in the nucleus.
4. capsid proteins associate with host cell membrane where viral glycoproteins are inserted. The RNA genome then interacts with capsid proteins that have associated with the glycoproteins embedded on the plasma membrane, initiating the budding process.
5. newly formed virii bud off by evagination, acquiring host cell membrane with embedded viral glycoproteins. neuraminidase facilitates the release of the new virions from the host cell membrane by cleaving sialic acid from the host cell receptor
HIV life cycle
- gp120 recognises and binds to complementary CD4 receptors on T helper cells with the help of a co-receptor
- with the help of gp41, the viral envelope fuses with host cell membrane, releasing the nucleocapsid into cytosol.
capsid is degraded by cellular enzymes, releasing the 2 viral RNA strands and enzymes into the cytosol - reverse transcriptase makes DNA strand using viral RNA as a template to form a DNA-RNA hybrid; the rna is then degraded and theh2nd DNA strand is made; producing a DS DNA molecule
viral DNA enters nucleus and is inserted into host cell genome by integrase, viral DNA is known as provirus and can remain latent for long periods of time. upon activation, the viral DNA is transcribed to viral RNA which enters cytosol. viral RNA can either act as an mRNA template and be translated into proteins or become part of the genome of the new virions.
the mRNA undergoes translation at the ribosomes in the cytosol forming viral polyproteins; or hthe RER where the proteins are glycosylated into glycoproteins gp120 and gp41 in the host CSM. - for HIV, maturation is completed only after the release of virus. components assemble at CSM with gp120 and gp41
- newly formed viruses bud off by evagination, acquiring host cell membrane with embedded viral glycoproteins. virla protease cleaves polyproteins, forming functional viral enzymes and structural proteins. the genome and enzymes are then encapsulated by a protein coat to form a capsid.
what is the pathogenicity of influenza (how does it make u sick)
infects epithelial cells of respiratory tract; infected epithelial cells lyse and build up results in inflammation and symptoms.
what is the pathogenicity of HIV (how does it make u sick)
binds to CD4 receptor on t helper cells, depresses the immune system and susceptibility to infections increases.
