Transcript Viruses
Viruses What are Viruses? • Virus – particles of nucleic acid, protein, and in some cases lipids – – – – Not alive Do NOT exhibit characteristics of life Can only reproduce when inside a host cell Cannot maintain homeostasis • Very simply a virus is genetic information enclosed in a protein coat Viral Structure • Viruses contain DNA or RNA but never both • DNA or RNA is surrounded by a protein coat call a capsid – Proteins in the capsid give the virus different shapes – Shape of the virus causes the virus to be cell specific (can only infect a certain type of cell) Viral Structure • Some viruses are enclosed by a protective protein envelope • Some viruses have spikes to help attach to the host cell CAPSID DNA ENVELOPE SPIKES Virus vs Cell Characteristic Virus Cell Structure DNA or RNA core, capsid Cell membrane, cytoplasm; eukaryotes also contain nucleus & organelles Reproduction Only within host Independent cell division either asexually or sexually Genetic Code DNA or RNA DNA Growth & Development No Yes, in multicellular organisms, cells increase in number & differenetiate Obtain & Use Energy No Yes Response to Environment No Yes Change Over Time Yes Yes Viruses & Disease **Remember ALL viruses are parasitic which means they require a living host and they cause harm to that host** Bacteriophages • A virus that infects bacteria is known as a bacteriophage or phage • Bacteriophages are used for research tools in genetics – Ex: to “program” bacteria to produce insulin for the treatment of diabetes Retroviruses • Group of viruses that contain RNA as their genetic material • Retroviruses have a complicated reproductive cycle because they must carry out reverse transcription • Contain a special enzyme call reverse transcriptase – This enzyme helps a cell transcribe DNA from RNA • Examples: HIV and feline leukemia ENZYME Fighting Viral Infections • Antiviral drugs work by interfering with the genetic material or the formation of the capsid – This will prevent the virus from replicating in the host cell • Examples: tamiflu, acyclovir Benefits of Viruses • Not all viruses are bad! • Example: – Vaccines for viral diseases are made from weaker strains of the virus – A weaker virus is capable of stimulating an immune response and creating immunity, but not causing illness Viral Hosts • Remember that a host is necessary for viral replication • Broad host range = infect several species – Ex: Swine flu and rabies • Narrow host range = infect only one species • Very narrow host range = infect specific tissue types of one species – Ex: Human cold virus attack cells of the respiratory system Viral Cycles There are three initial steps that are common to all viral infections: 1. Virus attach to the cell membrane of the host cell 2. Trick cell into allowing it inside 3. Virus releases its genetic information (DNA or RNA) into the host cell - Replication – Viral DNA is copied Transcription – Viral DNA is converted into RNA Translated – Viral proteins are made from the RNA Viral Cycles Once inside the host cell, there are two ways that a virus can take over and reproduce: 1. Lytic Infection 2. Lysogenic Infection Lytic Infection 1. Attachment to the cell 2. Penetration (injection) of viral DNA or RNA 3. Virus reprograms host to copy viral DNA and make viral proteins 4. New viruses assemble and mature 5. Cell lyses (bursts) and releases the new viruses to attack other cells 6. Results in death of the host cell Lysogenic Infection – – – – – Virus attaches to host and injects DNA Viral DNA joins host DNA forming a prophage Viral DNA replicates along with host DNA Remain inactive for a period (generations) Eventually viral DNA exits host cell and enters lytic cycle