virus

• virus are considered living things as

  they contain genetic material
• virus are considered non-living things as

  1. they have no cellular organisation
  2. only show character of living things when in host cell
• virus are thus considered as obligate parasites, which are organisms that:

  1. cannot live independently of host cell
  2. depends on host cell to complete its life cycle
• 3 characteristics of virus

  1. depends on host cell to complete life cycle
  2. contains only 1 type of nucleic acid, dna or rna
  3. viral components must assemble into complete viruses before it can go from one cell to another
• features of viral rep

  1. can only replicate within host cell: viruses lack biosynthetic machinery, nuclear environment of host cell provides compartmentalisation for suitable environment for rep
  2. host specific: can only infect a certain range of host cells
• lytic cycle:

  • attachment sites on tail fibre adsorb to complementary receptor sites on bacterial surface
  • bacteriophage releases lysozymes that digests bacterial cell wall, allowing release of molecules from bacterium. this causes a conformational change in base plate, causing contractile sheath to contract, driving hollow core tube into cell wall
  • once tip of hct reached plasma membrane, phage dna is injected into bacteria
  • empty caspid heard remains outside
• lytic cycle:

  • bacteria's macromolecular synthesising machinery is used to syntheise phage proteins. early page proteins: degrade host dna, use host nucleotides to synthesise phage dna. late phage proteins: synthesise phage enzymes and structural components
  • phage dna and caspid assemble into a dna filled head
  • head, tail and tail fibres assemble independently, then join in a specific sequence
  • phage lysozyme breaks down bacterial cell wall, bacterial cell memb lyses and releases newly formed virions
• lysogenic cycle: additional step, phage dna either expressed or not expressed

  • linear phage dna circularised and inserted into host cell genome via integrase
  • integrated phage dna is known as prophage
  • expression of phage genes repressed by phage repressor protein, new phages not synthesised
  • prophage replicated along bacterial chromosome
  • during spontaneous induction, cellular protease are activated and breaks down repressor proteins
  • phage dna then excised from host cell genome
• prophage: phage dna integrated in hc genome

  provirus: animal virus dna integrated in hc genome

  virion: enveloped virus that derived its lipid bilayer from hc's plasma membrane
• influenza:

  • haemagglutinin binds to complementary sialic acid receptors on host cell
  • virus enters cell via endocytosis. endocytic vesicle fuses with. lysosome, lowering pH, causing viral envelope to fuse with lipid bilayer of vesicle, nucleocaspud released into cytosol
  • caspid degraded by cellular enzymes, 8 viral rna degments are released into cytosol and enter nucleus
• • rdrp uses viral genome as T to syns mrna. mrna can remain in nucleus and be used as T for syn of new viral RNA genome which eventually exits nucleus OR exit nucleus enter cytosol to be translated into viral structural components
  • nucleoproteins associate with rna genome then interact with caspid protein that has already associated with hc memb (embedded with viral gp), initiating budding proccess
  • newly formed viruses bud off via evagination, acquiring hc memb with embedded viral gp. neuraminidase facilitates release of new virions from hc memb by cleaving sialic acid receptor from hc receptor
• hiv:

  • gp 120 binds to complementary cd4 receptors on t helper cells w the help of a co-receptor
  • with the help of gp 41, viral envelop fuses with hcm, nucleocaspid released into cytosol
  • caspid degraded by cellular enzymes, 2 viral rna strands and enzymes released into cytosol
  • reverse transcriptase synthesises dna by using viral rna as T to synthesise dna-rna hybrid, rna is degraded, forming 2nd strand of dna, double stranded dna mol produced
• • viral dna enters nucleus, inserted into hc genome via integrase, viral dna now known as provirus and can remain latent for long time
  • upon activation, viral dna transcribed into viral rna which enters cytosol
  • viral rna can either act as mrna and be translated into viral polyproteins & viral gp OR be part of the genome of new virions
• • viral rna genome and polyprotein associate as hc memb with embedded viral gp
  • newly formed viruses bud off via evagination, acquring hc memb embedded w viral gp
  • viral protease cleaves polyprotein into viral enzymes and proteins, viral rna and genome then encapsulated by a protein coat, forming a caspid (maturation after release)
  • mature virion now able to infect neighbouring cells
• antigenic drift: eg modified variants of influenza

  • poor proofreading abilities of rdrp & fast rep rate of virus: accumu of mutations in viral genome over time, producing viruses with modified surface antigens with different conformation.
  • if viruses infect a HC that does not have antibodies that recog these modified surface antigens, host is now susceptible to virus
• antigenic shift:

  • bird strain of influenza A & human strain of influenza A infect same host cell of an intermediate host, genetic reassortment of diff rna segments occurs in a virion
  • when new viruses assemble in HC, new combinations of RNA segments, producing viruses with NEW combination of surface antigens, recog & bind to complementary human HCR
  • host does not have antibodies that recognise these new surface antigens, susceptible to virus