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 ENVELOPE DNA SPIKES

Virus vs Cell CharacteristicVirusCell Structure DNA or RNA core, capsid Cell membrane, cytoplasm; eukaryotes also contain nucleus & organelles Reproduction Only within hostIndependent cell division either asexually or sexually Genetic Code DNA or RNADNA Growth & Development NoYes, in multicellular organisms, cells increase in number & differenetiate Obtain & Use Energy NoYes Response to Environment NoYes Change Over Time 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