Viruses

General Information for Viruses

• non-living

• smaller than bacteria

• only perform one life function;

reproduction. But they require

the help of living cells to

replicate.

– These living cells where

viruses replicate are called

host cells.

• named for diseases they cause or the

tissue they infect

• viruses that infect bacteria are called

bacteriophages

Structure of a Virus

• inner core of nucleic acid: RNA or DNA

• outer protein coat called a capsid

– Composed of protein complexes called

capsomers Ex: TMV and adenoviruses

• some larger viruses have an envelope

outside of the capsid with

glycoproteins used to attach

to host cells Ex: herpes & HIV

• viruses are grouped based

on their shape and structure

• outside a host a virus is called

a virion

Virus Classification • By the presence/absence of an envelope

• Type of nucleic acid

– DNA or RNA Retrovirus- RNA viruses with complex replication cycles;

RNA cannot be integrated into DNA, so a virus that has RNA as

its nucleic acid must first make its RNA into DNA before

meshing with the DNA. To make DNA from RNA an enzyme

must be present: reverse transcriptase. Ex: AIDS

Retroviruses

Virus Classification • Type of nucleic acid

– DNA or RNA (on previous slide)

– Single stranded or double stranded

– Positive (+) sense or negative (-) sense

• Positive sense- genome is the same sequence as

the mRNAs

• Negative sense- mRNAs have complementary

sequences to the DNA

Virus Classification • Viral shape

– Shape of virus & composition of the protein coat

enables viral penetration into the host cell

• Type of host cell

– Bacteriophages

– Rhinoviruses- infect epithelial

cells of the respiratory system

– Different kinds of organisms a virus can infect

constitute a host range

• Zoonosis- multiple species Ex: influenza, rabies

• Reservoirs- species that harbor the virus but show no

symptoms Ex: ducks for influenza

– These species can act like vectors to transmit disease

Viral Infection • Epidemic- a disease that afflicts a large

population at once

• Pandemic- a worldwide epidemic

Lytic Cycle

1. Viral Recognition

2. Attachment and Entry into cell

3. The virus takes over the cell to

make more viruses.

4. The virus destroys the cell and

releases tons of new viruses

(progeny virions) to invade

nearby cells.

View Lytic Cycle

Lytic Cycle

Lytic Cycle

Lysogenic Cycle Also called "temperate viruses“

1. Entry

2. Viral nucleic acid (prophage in bacteria) is

integrated into cell’s DNA

All cells produced by this cell will have the

virus- now call a provirus

3. Provirus is activated (days to years after

infection – latent infection)

4. Provirus takes over cell processes (lytic cycle

entered)

5. Cell is destroyed thereby releasing viruses to

infect nearby cells

Lysogenic Cycle

Viroids

• smallest known

particles that

are able to

replicate

• short, single

strand, highly

wound RNA

Prions Proteinaceous Infectious Particles

• Abnormal forms of

misfolded proteins

that clump together

inside cells

• Can cause other

proteins to misfold

• Ex: mad cow

disease

Prions Proteinaceous Infectious Particles

• Abnormal forms of

misfolded proteins

that clump together

inside cells

• Can cause other

proteins to misfold

• Ex: mad cow

disease

Vaccines Inactivated- do not replicate in a

host system

Attenuated- genetically altered virus

that cannot cause

disease – provides

greater protection

and lasts longer