Chapter 8 Recognition of host cells How do viruses recognize appropriate cells Each virus has a specific range type of cells that it can infect A cold virus can only bind to cell in our nasal passage and infect those cells It will never cause food poisoning Receptors on host cells have a huge affect on which cells can be affected by host viruses The range is dictated by interaction between viral attachment proteins and host cell receptor molecules Host cell receptors are often molecules critical for cellular function and therefore not easily eliminated to prevent infections You cannot eliminate receptors because they are needed for almost everything Recognition of host cells Examples of receptor virus pairings Viral entry and uncoating How do viruses enter host cells and uncoat After attachment mechanisms of viral entry vary depending on the type of virus and the host cell organism Entry of plant viruses tomato yellow leaf virus often requires damage induced by insects to infect the plant cells Could also be wind hail or people damage Each virus may use a different method but the viral genome must be inserted into the Replication strategies vary widely depending on genome type Baltimore classification of viruses based on genome places them into seven groups Viral uncoating host cell Viral replication How do viruses replicate 3 DNA virus groups 4 RNA virus groups Group I Double stranded DNA Uses its own or host DNA polymerase for replication Herpes virus group cold sores venereal disease chicken pox remain latent in the body for years becoming active during stressful conditions Group II single stranded DNA parvovirus uses host DNA polymerase to make complimentary DNA then dsDNA is transcribed by host Group III double stranded RNA Rotavirus member of reovirus family most common cause of infant diarrhea Use viral RNA dependent RNA polymerase to make mRNA directly from RNA genome Usually package viral RNA polymerase with genome before exiting host cell Group IV sense coding strand single stranded RNA RNA genome acts directly as mRNA for making proteins Replication of genome requires synthesis of complementary strand RNA dependent RNA polymerase West Nile virus rhinovirus colds Group V sense single stranded RNA genomic RNA does not serve directly as mRNA but is transcribed into a complement that functions as mRNA Must package viral RNA dependent RNA polymerase for transcribing RNA to RNA Often have segmented genomes Influenza rhabdovirus rabies Group VI retroviruses strand RNA which is transcribed into double stranded DNA by enzyme reverse transcriptase Must package reverse transcriptase with genome dsDNA is then integrated into host genome Lentivirus such as HIV feline leukemia virus FeLV Some retroviruses FeLV carry oncogenes can transform host cells into cancer cells Group VII Pararetroviruses Double stranded DNA genome which is copied into RNA then reverse transcribed into progeny DNA using reverse transcriptase Hepatitis B cauliflower mosaic virus Replication of bacteriophages Lysogenic cycle Lambda Lysogenic Temperate phage enter state called lysogeny viral genome integrates is replicated with host chromosome Don t immediately make phage virus is called a prophage when its genome exists as part of host genome Bacterial host cells that harbor prophages are called lysogens Eventually new phage are made lytic cycle occurs cells lyse Introduction of prophage lytic cycle host cell lysis Viral assembly and egress How do virus particles form and exit cells Egress exit from the cell Mechanism depends on virus type and host cell type Enveloped viruses plant viral proteins in the host cell plasma membrane attach to them bud out of the cell taking a portion of the plasma membrane with them Viral assembly and egress How do virus particles form and exit cells Egress exit from the cell Naked viruses almost always exit a cell by lysis destruction or cracking open of the host cell Plant viruses may move from cell to cell within one plant by specific cytoplasmic connections Plant viruses may move from one plant to another by disruption of cell wall structures often by insects Antiviral drugs How do antiviral drugs work Antiviral drugs How do antiviral drugs work from the infected cell An example of a good antiviral is a nucleoside analog e g AZT for HIV treatment These compounds look like nucleosides that would be used by viral polymerases to make viral genomes but prevent them from lengthening Another example of a good antiviral is tamiflu This drug works by preventing the activity of the enzyme that helps a new virus detach