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