Chapter 19:

Viruses

4. Viroids & Prions

3. Animal Viruses

2. Bacteriophages

1. Viral Structure & Reproduction

1. Viral Structure & Reproduction

Chapter Reading – pp. 393-396

What exactly is a Virus?

Viruses are extremely small entities that

are obligate intracellular 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 in any way

• depend on host cells for their reproduction(which are typically destroyed in the process)

**It’s hard to “kill” something that’s not really alive, so antibiotics

that kill bacteria, fungi, etc, do NOT harm viruses**

What’s a Virus made of?

All viruses consist of at least 2 components:

Genetic Material

• usually a single DNA or RNA molecule

• can be single or double stranded, linear or circular

A Capsid• a hollow protein capsule which houses the

genetic material

Some viruses also contain:

An Envelope

• membrane from host cell with viral proteins (spikes)

that surrounds the capsid

• contains the viral genes

The Viral CapsidCapsids are hollow, protein “shells” that:

• are an array of

protein subunits

called capsomeres

• consist of >1 type

of protein

• house the genetic

material (DNA/RNA)

• are frequently

involved in host

recognition & entry

• vary in shape, size

among viruses

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

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 (or nt) and can be:

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

• if single-stranded, it is referred to

as “+” or “–”

Viral MorphologyViruses come in 4 basic morphological types:

1. Icosahedral Viruses

• capsomeres in capsid have a

“polyhedral” arrangement

2. Helical Viruses

• capsomeres in capsid have a

“helical” arrangement

4. Complex Viruses

• consist of multiple

types of structures

3. Enveloped Viruses

Capsomereof capsid

RNA Capsomere

DNA

Glycoprotein Glycoproteins

Membranousenvelope RNA

Capsid

Head

DNA

Tailsheath

Tailfiber

18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)

20 nm 50 nm 50 nm 50 nm

(a) Tobaccomosaic virus

(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Variety of Viral Structure

The Viral Reproductive CycleVIRUS

2

1

3

4

Attachment& Entry

Replication ofViral Gemone

Expression ofViral Genes

Self-assembly ofnew virus particles

DNA

Capsid

HOSTCELL

Viral DNA

ViralDNA

mRNA

Capsidproteins

5 Release from the host cell

• attachment requires

highly specific

interactions between

viral capsid or envelope

proteins and host cell

surface molecules

• attachment determines

the host range of virus

2. Bacteriophages

Chapter Reading – pp. 396-398

What’s a Bacteriophage?

A bacteriophage is a virus that infects and

destroys bacterial cells.

Bacteriophages are of many different types

(some w/DNA or RNA, etc), however 2 types are of

particular interest due to decades of study:

“T-even” bacteriophages (T2, T4, T6)

bacteriophage lambda (l)

***More is known about the

biology of these viruses

than any other type of virus!***

0.5 mm

Attachment

2

1

5

43

Entry of phageDNA anddegradation of host DNA

Release

Synthesis ofviral genomesand proteins

Assembly

Phage assembly

Head Tail Tailfibers

Bacteriophage Reproductive Cycle

T-even bacteriophages

have an exclusively

lytic reproductive

cycle such as this.

Lytic vs Lysogenic CycleBacteriophage l has 2 options: lytic vs lysogenic

New phage DNA and proteins

are synthesized and assembled

into phages.

The cell lyses, releasing phages.

Phage

Phage

DNAThe phage

injects its DNA.

Bacterial

chromosome

Lytic cycle

lytic cycle

is inducedor

Phage DNA

circularizes.

Certain factors

determine whether

lysogenic cycle

is entered

Lysogenic cycle

Prophage

Daughter cell

with prophage

Occasionally, a prophage

exits the bacterial chromosome,

initiating a lytic cycle.

Cell divisions

produce a

population of

bacteria infected

with the prophage.

The bacterium reproduces,

copying the prophage and

transmitting it to daughter

cells.

Phage DNA integrates into

the bacterial chromosome,

becoming a prophage.

3. Animal Viruses

Chapter Reading – pp. 398-401

Life Cycle of Animal Viruses

The basic life cycle stages of animal viruses

differ from bacteriophages in some key ways:

1) attachment and entry

• requires specific interactions between host cell

membrane proteins & viral “spike” proteins (enveloped)

or capsid proteins (non-enveloped)

• entry by direct penetration, endocytosis or fusion of

the envelope with the cytoplasmic membrane

• uncoating of the virus (release of DNA, RNA)

2) replication of viral genome

• in nucleus (DNA or RNA) or cytoplasm (RNA)

4) assembly

• RNA viruses typically assemble in cytoplasm

• DNA viruses typically assemble in nucleus

5) release

• via lysis (rupture of plasma membrane), budding or

exocytosis

• host cell is not necessarily killed

3) expression of viral genes

• can occur entirely in the cytoplasm for some RNA

viruses

Reproductive

Cycle of a

DNA Virus

Unique Features of RNA Viruses

Copying 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 RNA

OR

In reality, RNA viruses use both approaches:

• retroviruses use reverse transcriptase to make

DNA from an RNA template

• all other RNA viruses use RNA-dependent RNA

polymerase to transcribe from an RNA template

Capsid

RNA

Envelope (withglycoproteins)

Capsid and viral genomeenter the cell

HOST CELL

Viral genome(RNA)Template

mRNA

ERCapsidproteins

Copy ofgenome(RNA)

New virus

Glyco-proteins

Reproductive Cycle of RNA Viruses

• copying of the viral

RNA genome requires

RNA-dependent RNA

polymerase

• is commonly packaged

in viral capsid (essential

for RNA – strand viruses)

Glycoprotein

Reverse

transcriptaseHIV

Viral envelope

Capsid

RNA (two

identical

strands)HOST

CELL

Viral RNA

Reverse

transcriptase

RNA-DNA

hybrid

DNA

NUCLEUS

ProvirusChromosomal

DNA

RNA genome

for the

next viral

generationmRNA

New virus

HIV

Membrane

of white

blood cell

0.25 m

HIV entering a cell

New HIV leaving a cell

Retroviral

Cycle

Latent Viral Infections

Some DNA viruses and retroviruses integrate

viral DNA into host cell chromosomal DNA :

• analogous to the lysogeny of bacteriophage l

• the inserted viral DNA is considered a provirus

which can remain dormant indefinitely

• such an infection is considered to be latent

• the provirus can become active due to various

conditions of stress in the cell and re-enter the

standard viral life cycle

Important Animal Viruses

4. Viroids & Prions

Chapter Reading – pp. 402-406

What are Viroids?

Viroids are very small, circular RNA molecules

(hundreds of nucleotides) that infect plants:

• viroid RNA does NOT encode any proteins

• the viroid RNA is copied in the host cell

• RNA silencing via miRNA is thought to perturb

host plant gene expression causing the

pathology of viroid infection

• transmission appears to be mechanical:

• grazing animals

• cutting tools

What are Prions?

Prions are unique, infectious proteins that

cause spongiform encephalopathies:

• e.g., “mad cow” disease, kuru, Creutzfelt-Jacob

disease (CJD), scrapie

• involves NO nucleic acid (DNA or RNA)

• involves aberrant folding of a normal protein

(PrP) expressed in neural tissue

• normal = PrPC; aberrant & infectious = PrPSC

• PrPSC is extremely stable, forms insoluble aggregates

• consumed PrPSC induces host PrPC to become PrPSC

Model of Prion based Illness

• contact between PrPC & PrPSC induces PrPSC

• insoluble PrPSC accumulates, kills cells

Prion Pathology

PrpC PrpSC

Normal vs

Abberrant Prp

Spongiform

Encephalopathy

Key Terms for Chapter 19

• capsid, envelope, spikes

• attachment, entry, release, budding

• icosahedral, helical, enveloped, complex viruses

• prophage, lysogen

• bacteriophage, lytic vs lysogenic

• uncoating, latent, provirus

• reverse transcriptase, retrovirus

• RNA-dependent RNA polymerase

• viroid, prion, spongiform encephalopathy

Relevant

Chapter

Questions 1, 4-6, 8