Chapter 6: Acellular Pathogens

1. Viruses

2. The Viral Life Cycle

4. Viroids, Virusoids, and Prions

3. Animal Viruses

1. Viruses

What exactly is a Virus?Viruses are extremely small entities that are obligateintracellular parasites with no metabolic capacity of their own.

• have none of the characteristics of living cells

• do NOT reproduce or metabolize on their own

• do NOT respond to their environment or maintain homeostasis

• depend on host cells for reproduction

**It’s hard to “kill” something that’s not really alive, so antibiotics that kill bacteria, fungi, etc, generally do NOT harm viruses**

The Size of Viruses

• almost all viruses aresmaller thanthe smallestprokaryoticcells

• pass throughfilters that trap cells(non-filterable)

What’s a Virus made of?All viruses consist of at least 2 components:

Genetic Material• DNA or RNA

A Protein Capsid

Some also contain:

An Envelope• membrane from host cell with viral proteins that surrounds the capsid

• made of capsomere proteins and houses the genetic material

Bacteriophages contain other

complex structures as shown here.

Viral Capsid ShapesHelical Viruses

• capsomeres arranged in a spiral or helical shape forming a tube shape

Icosahedral Viruses• capsomeres

arranged in a 20-sided hollow structure

• aka “polyhedral”

The Viral EnvelopeThe capsid of some viruses is enclosed in a phospholipid membrane called an envelope containing viral proteins called “spikes”:

• membrane comes from host cell

• spike proteins are involved in attachment and entry into host cell

Viral Genetic Material

DNA or RNA

+ strand = sense or coding strand

– strand = antisense or template strand

Viral genomes range from ~4000 to 250,000 bp and can be:

Double- (ds) or single-stranded (ss)

• if single-stranded, it is referred to as “+” or “–”

2. The Viral Life Cycle

Stages of the Bacteriophage “Life” Cycle

Step 1 – AttachmentVirus attaches to the surface of a host cell:

• involves molecular complementarity between viral & host cell molecules (usually proteins)

• viral molecules in “outer layer” (i.e., capsid or envelope)

• molecules in host cell wall (bacteria) or plasma membrane (animal cells)

**These molecular interactions determine the host range of a virus, and thus limit infection to

very specific species and cell types**

Step 2 – Penetration or EntryMethod of getting viral DNA or RNA into the host cell depends on the type of virus & host cell:

• bacteriophages puncture the cell wall with lysozyme & inject viral genetic material

• animal viruses typically enter cells by endocytosis or fusion with host membrane

Step 3 – Biosynthesis1. The expression of viral genes…

• transcription & translation of viral genes to produce viral proteins

• requires host cell gene expression machinery

• enzymes, ribosomes, tRNA, nucleotides, AA’s…

2. Copying the viral genetic material• involves host enzymes,

nucleotides, etc…Viruses may provide some of their own

enzymes and transcription factors.

Step 4 – Maturation or AssemblyMaturation refers to the self-assembly of viral proteins and genetic material (DNA or RNA) into intact viral particles:

• capsid & other structural proteins self-assemble

• genetic material ispackaged into capsid

• enveloped viruses do notacquire envelope untilexiting the host cell

Step 5 – Lysis or ReleaseWhen complete viral particles are assembled they can be released from the host cell in 2 basic ways:

1) cell lysis

• specific viral proteins cause disruption of the plasma membrane (& cell wall in bacteria)

• destroys host cell while releasing new viruses

2) budding, exocytosis• many enveloped viruses “bud” from host cell, acquiring the viral

envelope in the process

• other animal viruses are released by exocytosis

Lysogenic CycleInvolves dormant

prophageinserted into host

chromosome. A cell with a prophage is

called a lysogen.

Specialized TransductionExcision of prophage from host DNA can be imprecise resulting in a piece of host DNA, along with phage DNA, transferred to another host:

3. Animal Viruses

Life Cycles of Animal VirusesCertain steps in the life cycle of animal viruses differ from bacteriophages due to lack if cell walls in animal cells:

Penetration• by direct penetration, endocytosis or fusion of the envelope with the

cytoplasmic membrane

• may require uncoating of the virus (release of DNA, RNA) inside host cell

Release• by lysis, exocytosis or budding

• host cell is not necessarily killed

Penetration and Release in Animal Host CellsDirect Penetration

FusionEndocytosisBudding

Life Cycle of a DNA VirusOnce viral DNA enters nucleus, gene expression and viral DNA replication occur, followed by maturation and release.

Viral DNA may also be integrated into a

chromosome as a provirus.

Examples of DNA Viruses

Types of RNA VirusesThe genetic material of RNA viruses comes in 3 basic forms:

• double-stranded RNA

• + strand RNA (single coding or sense strand)

• - strand RNA (single noncoding or template strand)

A 4th type are the retroviruses which copy RNA into DNA using the enzyme reverse transcriptase

Unique Features of RNA VirusesCopying of viral RNA poses a unique problem:

1) viral RNA must be converted to DNA which can then be transcribed to produce more RNA

2) viral RNA must somehow serve as a template to produce more RNAOR

In reality, RNA viruses use both approaches:

• reverse transcriptase to make DNA from an RNA template

• RNA-dependent RNA polymerase to make RNA from an RNA template

Life Cycle of an RNA Virus• RNA-dependent RNA polymerase (RdRP) uses + strand as template to make –strand and vice versa

Retrovirus Life CycleUncoating releases viral RNA, and the viral enzymes integraseand reverse transcriptase (RT):

• RT uses RNA as template to make complementary DNA

• RT new DNA strand as template to make complementary DNA

• integrase inserts viral double-stranded DNA into chromosome

• viral RNA is transcribed from provirus DNA for making proteins and assembly of new viruses

Examples of RNA Viruses

Persistent Viral InfectionsSome viruses are persistent in causing repeated infections (e.g., herpes viruses) which is due to 2 general reasons:

1. Chronic Infection

• the virus is never fully cleared and viral reproduction is an ongoing process (e.g., HIV, hepatitis C virus)

• after acute infection, the virus goes into a dormant or latent state in the form of a provirus or episome (circular DNA outside of chromosome)

2. Latency

Conditions of stress may reactivate the virus at any time, such as shingles, which is due to the same virus that causes chicken pox.

4. Viroids, Virusoids and Prions

What are Viroids, Virusoids, and Prions?Prions are infectious proteins causing spongiform encephalopathies:

• e.g., “mad cow” disease, kuru, Creutzfelt-Jacob disease (CJD), scrapie

• involves NO nucleic acid (DNA or RNA)

• discovered by Stanley Prusiner in 1980’s

Viroids are infectious RNA molecules found in plants.

Virusoids are infectious RNA molecules in plants that require a “helper virus” for their reproduction.

• essentially RNA viruses without capsids

Model of Prion based Illness

PrPSC is extremely hard to destroy or denature.

• contact between PrPC

& PrPSC

induces PrPSC

• insoluble PrPSC

can’t be broken down, accumulates, kills cells

Prion Pathology

PrpC PrpSC

Normal vsPathogenic Prp

Spongiform Encephalopathy

Key Terms for Chapter 6

• capsid, capsomere, envelope, spikes

• attachment, penetration, biosynthesis, maturation, release

• icosahedral/polyhedral, helical, enveloped, complex viruses

• prophage, lysogen, lysozyme, specialized transduction• bacteriophage, lytic vs lysogenic

• uncoating, latent, provirus, episome

• reverse transcriptase, retrovirus

• RNA-dependent RNA polymerase

• prion, PrpC vs PrpSC, viroid, virusoid