BIOL2010 Week 6 - Viruses PDF

www.udst.edu.qa BIOL2010 Week 6 - Viruses What is a Virus? A Virus is an acellular infectious agent that cannot self replicate Must infect a cell in order to make copies of itself Examples of viruses include Influenza, HIV & Ebola Viral Structure Although there is variation in viral structure,...

www.udst.edu.qa BIOL2010 Week 6 - Viruses What is a Virus? A Virus is an acellular infectious agent that cannot self replicate Must infect a cell in order to make copies of itself Examples of viruses include Influenza, HIV & Ebola Viral Structure Although there is variation in viral structure, there are some common features 1) RNA or DNA genome 2) Capsid of Proteins 3) Envelope Lipid Bi-layer 4) Surface receptors – Spikes of Protein or Glycoprotein Viral Replication Cycle Replicative cycle by a Virus depends upon its Genome For example, replicative cycle of a ssRNA virus is different than that of a dsDNA Virus Image at right shows a general mechanism Viral Replication Cycle 1. Attachment: Proteins on the Viral envelope or Capsid recognize & bind with Target host cell receptors Viral Replication Cycle 2. Penetration: Entry is by one of 3 possible mechanisms: 1) Direct penetration (naked viruses) – Only viral genome enters host (typical of bacteriophages) 2) Fusion – Viral envelope fuses with host membrane, capsid containing the genome enters cell (e.g. HIV) 3) Receptor mediated endocytosis – Attachment stimulates endocytosis of entire virus (e.g. Influenza) Viral Replication Cycle 3. Uncoating: Virus removes Capsid (uncoats) & nucleic acid of Genome is exposed to cytoplasm Viral Replication Cycle 4. Transport to Nucleus: Viral genome enters nucleus Viral Replication Cycle 5. Synthesis (Transcription & Translation): Reproduction of Viral genome (transcription in nucleus) & Viral proteins (translation in cytoplasm) Viral Replication Cycle 6. Assembly: Assembly of New virons (complete virus). Genome & Capsid are put together to form New virus Viral Replication Cycle 7. Release: Release of Virus from Host cell (viral shedding) by one of 3 mechanisms… Viral Shedding 1. Apoptosis: Host cell lyses & releases mature viral particles (naked viruses). Host Cell dies. 2. Budding: Through nuclear or plasma membrane, creating an envelope. Does not kill host. 3. Exocytosis: Viruses leave the host cell using vesicles but does not kill host. Influenza Virus Influenza virus is an enveloped RNA Virus RNA Viruses make more “mistakes” than DNA viruses They don’t have self-regulation & do not correct mistakes RNA Viruses adapt readily to environmental changes because of high mutation rates Envelope is covered with 2 proteins (Antigens) required for the infection process 1. Hemagglutinin (H) spike 2. Neuraminidase (N) spike Influenza Virus Hemagglutinin (ha) Glycoprotein on the surface of Influenza virus binds Virus to cells with sialic acid on host cell membranes, such as cells in upper respiratory tract or erythrocytes – helps VIRUS enter cell Sialic Acid is a receptor found on most vertebrate cells Neuraminidase (na) Enzyme helps virus to penetrate mucus of respiratory tract & also aids in viral shedding of influenza (budding). It does this by cutting Sialic Acid from Host Glycoproteins as virus is being Released MUTATIONS in GENES that code for these 2 PROTEINS are responsible for development of new strains of FLU Antigentic Variation Antigenic variation occurs in 2 ways with Flu Virus 1. Antigenic drift results from point mutation of genes coding for Hemagglutinin & Neurominidase 2. Antigenic shift caused by Reassortment of Viral Genes Antigenic Drift Mutations in Genes that code for Hemagglutinin & neuraminidase These 2 proteins are the antigens that cause formation of host antibodies. Produces new strains of flu virus that host antibodies won’t recognize. If enough of an antigenic drift – this will cause a new flu epidemic (localized flu). Antigenic drift causes new formulations of flu vaccine every year. Antigenic Shift Results from Gene re-assortment from 2 different Viruses that infect same cell Viruses exchange a large part of genome Virus that emerges is Antigenically different from either of the 2 viruses Antibodies formed from the 2 viruses are ineffective against new combined virus genome H5N1 = bird flu H1N1 = swine flu Can lead to PANDEMICS Swine Flu Viroid Re-assortment Bacteriophage Bacteriophages, (or just phages) are viruses that only infect bacterial cells. Phages (Viruses) cannot be grown on agar plates like bacteria, they have to be grown using host cells, in this case bacteria. For animal viruses, hens eggs, or live animals can be used. Bacteriophage Lambda One of the earliest model system for studying the physical nature of DNA and genes. Isolated from an E. coli cells in 1951. Has contributed enormously to our understanding of molecular biology and protein synthesis mechanisms. Often used a vector for the cloning of recombinant DNA. Bacteriophage Replication Can have a Lytic or Lysogenic mode of replication. Lytic cycle – Results to destruction of the hosts cells membranes, phages that use this cycle are called virulent. Lysogenic cycle – Viral DNA is integrated into the host bacteria’s genome, the bacteria continues to live and function normally, these are referred to as temperate phages. Bacteriophage Lambda