The Viruses
January 14th, 2010
Virus Basics
• Viruses are nucleic acid and protein
structures
• Very small; typically between 20-200 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
http://biology.kenyon.edu/slonc/gene-web/Lentiviral/Lentivi2.html
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
http://www.jci.org/articles/view/22139/figure/1
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