Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
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