9) Viruses Summary 9744 2018

Genetics and Inheritance (9744) Viruses 2018 Prepared by: Mrs Selvamani Nair, Mdm Sharon Cross and Mrs Wong Seok Hui Raffles Institution 1  Are viruses are considered living or non-living? Living as they contain genetic material. However, non-living because they have no cellular organization and only show characteristics of living things when in host cell. Characteristics of living things include1) metabolic activity 2) cellular organization 3) ability to reproduce and grow in numbers 4) ability to respond to stimuli and adapt to environment  Why are viruses obligate parasites? This is because viruses, like obligate parasites, depend on host cells to complete their life cycle. Structure of Viruses Size: 10-300nm Bacteriophages Animal Viruses Enveloped T4 phage Lambda phage Influenza Human Immunodeficiency Virus (HIV) Genome  Nucleic acid that codes for synthesis of viral components and enzymes for viral replication & assembly  Can be either DNA/RNA, single/double-stranded  Double- stranded DNA  (-) strand RNA viral genome is complementary to viral mRNA  8 different segments of single stranded RNA associated with nucleoproteins  Each RNA segment is packed with 3 polymerase proteins which come together to form an RNA-dependent RNA polymerase enzyme complex which replicates and transcribes the viral genome in the host cell  (+) strand RNA viral genome has the same sequence as viral mRNA  2 identical copies of single stranded RNA bound to nucleocapsid proteins Capsid  Protein coat that surrounds and protects viral genome  Comprise subunits called capsomeres  Icosahedral capsid head  Present.  Present, conical shaped Enzymes reverse transcriptase, integrase and protease found in capsid Envelope  Phospholipid bilayer surrounding the nucleocapsid  Derived from host cell membrane  Embedded with viral glycoproteins involved in host cell recognition  Absent  Glycoproteins embedded in envelope: haemagglutinin (80%) & neuraminidase (20%)  Glycoprotein embedded in envelope: gp41 gp120 is attached to gp41 T4 phage Lambda phage Influenza Virus Antigenic Drift and Antigenic Shift Antigenic Drift : When the influenza virus replicates in its host cell, mutations frequently occur due to the poor proofreading mechanism of the viral RNA-dependent RNA polymerase and the fast replication rate of the virus. Over time, there is an accumutation of mutations in the viral genome. Sometimes, these mutations produce viruses with modified** surface antigens (e.g. glycoproteins such as haemagglutinin or neuraminidase) with different conformation. If these viruses infect a host that does not have the antibodies that recognise these modified surface antigens, the host becomes susceptibleto the virus. Antigenic Shift: When a bird strain of influenza A and human strain of influenza A infect a single cell of an intermediate host (e.g.a pig), genetic reassortment can occur. Thus when new viruses are assembled in the host cell, a new combinations of RNA segments can come together. Sometimes, genetic reassortment produces viruses with new** surface antigens (e.g. glycoproteins such as haemagglutinin or neuraminidase). If these viruses infect a human host the host becomes susceptibleto the virus, as the host will not have the antibodies that recognise these new** surface antigens, Haemagglutinin Neuraminidase Envelope Matrix proteins Reverse transcriptase Capsid 8 RNA segments, each associated with 1) an RNA dependent RNA polymerase 2) nucleoproteins 2 copies of single stranded RNA genome, each associated with nucleocapsid proteins Protease Integrase Envelope Capsid gp41 Icosahedral capsid head containing double stranded DNA genome Protease Tail surrounded by contractile sheath Tail fibre Collar gp120 Base plate Tail pin Human Immunodeficiency Virus (HIV)

Genetics and Inheritance (9744) Viruses 2018 Prepared by: Mrs Selvamani Nair, Mdm Sharon Cross and Mrs Wong Seok Hui Raffles Institution 2 Virus Life Cycle Stages Bacteriophage Enveloped animal viruses T4 phage (Lytic phage) Lambda phage (Temperate phage) Influenza HIV 1. Attachment Virus recognises and attaches to host cell  Attachment sites on tail fibres adsorbs to complementary receptor sites on bacterial surface (e.g. E.coli) Enveloped viruses use viral glycoproteins to bind to specific receptor molecules on host cell.  Hemagglutinin binds to complementary sialic acid receptor on host cell (e.g. epithelial cells in respiratory tract) membrane  gp120 binds to complementary CD4 receptors on T helper cells or (macrophages) with the help of a co- receptor. 2. Penetration Viral genome introduced into host cell  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 cell wall  When the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell  The empty capsid remains outside Release of capsid into host cell cytosol  Virus enters host cell by endocytosis (the process involves invagination of membrane)  Endocytic vesicle fuses with lysosome  which lowers the pH  causes viral envelope to fuse with lipid bilayer of vesicle  nucleocapsid is released into cytosol  With the help of gp41, the viral envelope fuses with host cell membrane  nucleocapsid is released into cytosol (NB: HIV can also enter by endocytosis) Degradation of capsid to release viral genome (uncoating) Capsid degraded by cellular enzymes and the 8 viral RNA segments that are released into cytosol enter the nucleus Capsid degraded by cellular enzymes the 2 viral RNA strands and enzymes are released into the cytosol 3. Replication Synthesis of viral components & viral genome replication  Host cell macromolecular synthesizing machinery is used to synthesise phage proteins  Early phage proteins: degrade host DNA  Phage DNA synthesized using host cell nucleotides and early proteins  Late phage proteins: are phage enzymes and structural components  Linear phage DNA circularizes and inserted into host cell genome by enzyme integrase  The integrated phage DNA is known as a prophage  Expression of phage genes is repressed by phage repressor proteins. Hence new phages are not synthesized  Prophage replicates along with bacterial chromosome  During spontaneous induction, cellular proteases are activated. They destroy the repressor proteins  The prophage is then excised from the bacterial genome  The replication phase of lytic cycle then occurs.(see left)  Viral RNA-dependent RNA polymerase uses viral genome as a template to synthesise mRNA  mRNA 1. enters cytosol translated into viral structural components (Capsid proteins are made in the cytosol. Envelope glyc