The Viruses January 14 th, 2010

Virus Basics Viruses are nucleic acid and protein structures Very small; typically between nm No cellular structures –No ribosomes –No metabolic pathways (Glycolosis, Kreb’s cycle, electron transport chain, etc.) –Few or no enzymes

Virus Basics Viruses carry out NO growth or metabolism on their own They are dependent on living cells for their replication Can exist in the environment, but do not replicate To replicate, they must come in contact with a host organism

Virus Basics Viruses replicate by infecting a host cell and hijacking the host cell’s replication machinery to produce more viruses –Host cell DNA replication –Host cell RNA transcription –Host cell RNA translation –Host cell protein and membrane building capacity

Virus Basics They are generally host-specific Infect only certain cell types –Influenza and lung tissue –Norovirus and intestines –Hepatitis B and liver cells

Structure

Virus structure Viruses are mostly nucleic acid and protein Protein shell May be surrounded by a lipid envelope Nucleic acid inside

Virus structure

The viral capsid Made of protein subunits Repeating patterns Symmetrical structure Can have proteins, lipids, and sugars on the surface These outer structures interact with host cells

Example: poliovirus

Virus envelopes Viruses can be enveloped or non- enveloped Enveloped – Lipid bilayer surrounds the capsid –Similar to a cell membrane Non-enveloped –Protein shell only

Internal structures Capsid forms a shell around the nucleic acid Some viruses carry their own enzymes inside the capsid

Nucleic Acids Typically a single nucleic acid Can be DNA or RNA Single or double stranded Size: 32-kbp Encode proteins the virus needs to take over a cell and reproduce itself –Replication proteins –Structural proteins

Example: the poliovirus genome

Viral Life Cycle

Virus attachment Viruses attach to cells via a receptor Molecule on the surface of a cell that the virus can recognize and attach to Like a lock and key These cell receptors often serve a useful function for the host cell; viruses have simply evolved to exploit them

Virus entry Once attached, virus goes through the cell membrane into the cytoplasm Entry happens through different mechanisms –Penetration –Membrane fusion

Virus attachment

Production of new viruses Take over the cell’s replication machinery Stop the cell’s own nucleic acid replication, transcription, and translation processes All the cell’s energy goes into creating more viruses

Production of viral protein

Viral replication: DNA viruses Virus DNA serves as the template Host cell enzymes replicate more pieces of viral DNA Host cell enzymes transcribe viral DNA into mRNA Host cell ribosomes translate viral mRNA into proteins

Viral replication: RNA viruses Viral RNA serves as the template Translated directly into proteins by the host cell ribosomes However, the viral RNA still needs to be replicated for making more viruses –Human and animal cells do not have enzymes for replicating RNA –These enzymes are encoded on the viral genome –The host cell ribosomes produce these enzymes, allowing the virus to replicate its own nucleic acid

Replication

Virus assembly Nucleic acids associate with capsid proteins Proteins assemble into capsid structures Complete virus particles assemble in the cytoplasm of the cell

Virus assembly

Release from the cell Viruses can pass through the cell membrane May rupture the cell and escape Enveloped viruses: may “bud” out, taking part of the cell membrane to form their envelope Infect adjacent cells Release into the environment Release into host cell body fluids that can spread infection

Release

Viruses in the environment Can remain viable in a variety of environments Water Air Soil Food Can remain infectious for long periods until they encounter a host cell