Download Chapter 19: Viruses 1. Viral Structure & Reproduction What exactly is a Virus?

11/7/2015
Chapter 19:
Viruses
1. Viral Structure & Reproduction
2. Bacteriophages
3. Animal Viruses
4. Viroids & Prions
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
**It’s hard to “kill” something that’s not really alive, so antibiotics
that kill bacteria, fungi, etc, do NOT harm viruses**
• depend on host cells for their reproduction
(which are typically destroyed in the process)
1
11/7/2015
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
• contains the viral genes
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
The Viral Capsid
Capsids 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 Envelope
The 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
2
11/7/2015
Viral Genetic Material
Viral genomes range from ~4000 to
250,000 bp (or nt) and can be:
DNA or RNA
Double- (ds) or single-stranded (ss)
• if single-stranded, it is referred to
as “+” or “–”
+ strand = sense or coding strand
– strand = antisense or template strand
Viral Morphology
Viruses 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
3. Enveloped Viruses
4. Complex Viruses
• consist of multiple
types of structures
Variety of Viral Structure
RNA
Capsomere
DNA
Membranous
RNA
envelope
Capsid
DNA
Head
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
18  250 nm
70–90 nm (diameter)
50 nm
20 nm
(a) Tobacco
(b) Adenoviruses
mosaic virus
Glycoproteins
80–200 nm (diameter)
80  225 nm
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
3
11/7/2015
The Viral Reproductive Cycle
1 Attachment
& Entry
DNA
VIRUS
Capsid
2 Replication of
Viral Gemone
3 Expression of
Viral Genes
HOST
CELL
Viral DNA
mRNA
Viral
DNA
Capsid
proteins
• attachment requires
highly specific
interactions between
viral capsid or envelope
proteins and host cell
surface molecules
• attachment determines
the host range of virus
4 Self-assembly of
new virus particles
5 Release from
the host cell
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
4
11/7/2015
Bacteriophage Reproductive Cycle
1 Attachment
T-even bacteriophages
have an exclusively
lytic reproductive
cycle such as this.
2 Entry of phage
DNA and
degradation
of host DNA
5 Release
Phage assembly
4 Assembly
Head Tail
3 Synthesis of
viral genomes
and proteins
Tail
fibers
Lytic vs Lysogenic Cycle
Bacteriophage l has 2 options: lytic vs lysogenic
Phage
DNA
Daughter cell
with prophage
The phage
injects its DNA.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lytic cycle
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
lytic cycle
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
or
lysogenic cycle
is entered
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
5
11/7/2015
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)
3) expression of viral genes
• can occur entirely in the cytoplasm for some RNA
viruses
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
Reproductive
Cycle of a
DNA Virus
6
11/7/2015
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
OR
2) viral RNA must somehow serve as a template to
produce more RNA
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
Reproductive Cycle of RNA Viruses
Capsid
Capsid and viral genome
enter the cell
RNA
Envelope (with
glycoproteins)
• copying of the viral
RNA genome requires
RNA-dependent RNA
polymerase
HOST CELL
Template
Viral genome
(RNA)
mRNA
Capsid
proteins
ER
• is commonly packaged
in viral capsid (essential
for RNA – strand viruses)
Copy of
genome
(RNA)
Glycoproteins
New virus
Glycoprotein
Retroviral
Cycle
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HIV
Membrane
of white
blood cell
HOST
CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 m
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell
7
11/7/2015
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
8
11/7/2015
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
9
11/7/2015
Prion Pathology
Normal vs
Abberrant Prp
PrpC
Spongiform
Encephalopathy
PrpSC
Key Terms for Chapter 19
• capsid, envelope, spikes
• icosahedral, helical, enveloped, complex viruses
• attachment, entry, release, budding
• bacteriophage, lytic vs lysogenic
• prophage, lysogen
• uncoating, latent, provirus
• RNA-dependent RNA polymerase
Relevant
Chapter
Questions
1, 4-6, 8
• reverse transcriptase, retrovirus
• viroid, prion, spongiform encephalopathy
10