Download Viruses - Napa Valley College

11/15/2011
Chapter 19 - Viruses
Outline
I. Viruses
A.
B.
C.
D.
Structure of viruses
Common Characteristics of Viruses
Viral replication
HIV
II. Prions
Copyright © 2009 Pearson Education, Inc.
The Good the Bad and the Ugly
Structural Features and Characteristics
 Non cellular, viral particles = virions
 Viruses – fit into the bad category
 Virions are very small
 Viruses are not like other living organisms.
 They are acellular, which means they don’t
have their own cells – they hijack other cells
and use them to reproduce themselves.
 Viruses may remain inactive or latent in the
host for many years.
 Viruses lack ribosomes
Copyright © 2009 Pearson Education, Inc.
Structural Features and Characteristics
Copyright © 2009 Pearson Education, Inc.
Viral Genomes
 Viruses usually have:
 Viral genomes may consist of either
1. Genetic material (DNA or RNA)
2. Protein coat = capsid (helix or icosahedral)
3. Some may have a fatty membrane = envelope
Copyright © 2009 Pearson Education, Inc.
 Double- or single-stranded DNA, or
 Double- or single-stranded RNA
 Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
1
11/15/2011
Fig. 27.1
Figure 19.3
capsid
RNA
DNA
Membranous
RNA
envelope
Capsid
DNA
Head
Tail
sheath
capsid
Tail
fiber
Glycoprotein
18  250 nm
(a) Tobacco
mosaic virus
Copyright © 2009 Pearson Education, Inc.
Viruses vary in size, as well as in shape
20 nm
Glycoproteins
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
Copyright © 2009 Pearson Education, Inc.
Bacteriophage Virus
Copyright © 2009 Pearson Education, Inc.
Figure 19.1
0.5 mm
Copyright © 2009 Pearson Education, Inc.
2
11/15/2011
Corona
Virus
Responsible
for SARS
epidemic in
2003
Characteristics of Living Organisms
Are Viruses Alive?
1. Contain biological molecules including:
Proteins, nucleic acids, carbohydrates and
lipids
2. Cellular
3. Reproduce
4. Acquire and use energy - Metabolism
5. Growth and Development
6. Respond to environment
7. Maintain Homeostasis
8. Populations of living organisms evolve and
have adaptive traits
 They are acellular
Copyright © 2009 Pearson Education, Inc.
Types of viruses - Retrovirus
 Retroviruses – contain RNA and an
enzyme reverse transcriptase.
 They can’t perform metabolic activity and
reproduction without a host
 Contains either DNA or RNA
 Do not contain ribosomes, lack the enzymes
needed to transcribe DNA
Copyright © 2009 Pearson Education, Inc.
Types of viruses
 Bacteriophages – viruses that infect
bacteria
 Reverse transcriptase turns RNA into DNA
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
3
11/15/2011
DNA VIRUS
Figure 19.4
1 Entry and
uncoating
3 Transcription
and manufacture of
capsid proteins
Capsid
2 Replication
HOST
CELL
Viral DNA
mRNA
Viral
DNA
4 Self-assembly of
new virus particles
and their exit from
the cell
Animation: Simplified Viral Reproductive Cycle
Right-click slide / select “Play”
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Capsid
proteins
Copyright © 2009 Pearson Education, Inc.
Replicative Cycles of Phages
 Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
The Lytic Cycle
 The lytic cycle is a phage replicative cycle that
culminates in the death of the host cell
 The lytic cycle produces new phages and lyses
(breaks open) the host’s cell wall, releasing the
progeny viruses
 A phage that reproduces only by the lytic cycle is
called a virulent phage
 Bacteria have defenses against phages, including
restriction enzymes that recognize and cut up
certain phage DNA
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Figure 19.5-1
1 Attachment
Animation: Phage T4 Lytic Cycle
Right-click slide / select “Play”
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Copyright © 2009 Pearson Education, Inc.
4
11/15/2011
Figure 19.5-2
Figure 19.5-3
1 Attachment
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
2 Entry of phage
DNA and
degradation
of host DNA
3 Synthesis of
viral genomes
and proteins
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
Figure 19.5-4
Figure 19.5-5
1 Attachment
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Phage assembly
Tail
Release
Phage assembly
4 Assembly
Head
5
2 Entry of phage
DNA and
degradation
of host DNA
Tail
fibers
3 Synthesis of
viral genomes
and proteins
Copyright © 2009 Pearson Education, Inc.
4 Assembly
Head
Tail
Tail
fibers
3 Synthesis of
viral genomes
and proteins
Copyright © 2009 Pearson Education, Inc.
Steps in lytic cycle
1. Attach to a host cell
2. Penetrate host cell – the genetic material must
enter the host cell, capsid may remain on
outside
3. Virus cuts host DNA into pieces
4. Uses host cell for protein synthesis
5. Assemble new viral particles
6. Lysis - Release by rupturing host cell.

Some virus do not lyse host cell, instead they bud
off.
Copyright © 2009 Pearson Education, Inc.
0.25 µm
Copyright © 2009 Pearson Education, Inc.
5
11/15/2011
Lysogenic cycle
The Lysogenic Cycle
 Some viruses don’t immediately kill their hosts
 They integrate their DNA into the host DNA
 When the viral DNA is integrated into the host
DNA, the genetic material it is called a
prophage, the infected cell is the lysogen
 Induction: The virus will switch to the lytic
phase
 The lysogenic cycle replicates the phage genome
without destroying the host
 The viral DNA molecule is incorporated into the host
cell’s chromosome
 This integrated viral DNA is known as a prophage
 Every time the host divides, it copies the phage DNA
and passes the copies to daughter cells
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Copyright © 2009 Pearson Education, Inc.
Daughter cell
with prophage
Figure 19.6b
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
lysogenic cycle
lytic cycle
or
Prophage
is entered
is induced
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Animation: Phage Lambda Lysogenic and Lytic Cycles
Right-click slide / select “Play”
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Figure 19.6a
Phage
DNA
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Copyright © 2009 Pearson Education, Inc.
Replicative Cycles of Animal Viruses
The phage
injects its DNA.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
 There are two key variables used to classify
viruses that infect animals
 DNA or RNA?
 Single-stranded or double-stranded?
Lytic cycle
The cell lyses, releasing phages.
Certain factors
determine whether
lysogenic cycle
lytic cycle
or
is entered
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
Copyright © 2009 Pearson Education, Inc.
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
6
11/15/2011
Figure 19.7
Retroviruses
Capsid
Capsid and viral genome
enter the cell
RNA
Envelope (with
glycoproteins)
 Retroviruses have RNA which gets
changed to a a double stranded DNA
HOST CELL
Template
Viral genome
(RNA)
mRNA
 Retroviruses have an enzyme called
reverse transcriptase to do this
ER
 HIV is a retrovirus, HIV uses the lysogenic
cycle then switches to the lytic cycle
Capsid
proteins
Copy of
genome
(RNA)
Glycoproteins
New virus
Copyright © 2009 Pearson Education, Inc.
Figure 19.8a
Copyright © 2009 Pearson Education, Inc.
Glycoprotein
Figure 19.8b
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST
CELL
HIV
Viral RNA
HIV
Membrane
of white
blood cell
Reverse
transcriptase
RNA-DNA
hybrid
DNA
Chromosomal
DNA
RNA genome
for the
next viral
generation
NUCLEUS
Provirus
mRNA
0.25 m
HIV entering a cell
New virus
Copyright © 2009 Pearson Education, Inc.
New HIV leaving a cell
Copyright © 2009 Pearson Education, Inc.
Steps in HIV Virus Replication
1. Attach to a host cell = helper T Cell (CD4
cells)


HIV envelope has gp120 fits CD4 receptor on T
cell
Then binds with coreceptor ie CCR5
2. Penetrate host cell using endocytosis
3. Viral RNA is turned into viral DNA

Using the enzyme reverse transcriptase
4. Integrate the viral DNA into the host
Animation: HIV Reproductive Cycle

Using the enzyme integrase
Right-click slide / select “Play”
© 2011©Pearson
Education,
Inc.Inc.
Copyright
2009 Pearson
Education,
Copyright © 2009 Pearson Education, Inc.
7
11/15/2011
Steps in HIV Virus Replication
Fig. 27.6
5. Transcribe the integrated viral DNA to make
viral mRNA
6. Protein synthesis
7. Assemble new viral particles

Using the enzyme protease
8. Release, using exocytosis
Copyright © 2009 Pearson Education, Inc.
HIV
 Human Immunodeficiency Virus: Cause AIDS
 HIV is a particularly devastating virus because
it attacks the hosts immune system
 The human immune system has T cells to
protect us against foreign bodies like viruses
and bacteria.
 T cells have receptors on their surface that
recognize foreign vs our own cells
Copyright © 2009 Pearson Education, Inc.
HIV Infection
 Reverse transcriptase does the opposite of
RNA polymerase: It turns RNA into a double
stranded DNA molecule
 Integrase puts this viral DNA into the T cell’s
DNA
 For a period of time the DNA may not
produce any protein but if the host cell
replicates then the viral DNA is also
replicated
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
HIV Infection
 HIV receptors on the surface of the envelope
bind with the receptors CD4 and coreceptor
CCR5) on the T cells, penetrated host cell
 Protein coat (capsid) dissolves
 HIV has three main enzymes:
 Integrase, reverse transcriptase and
protease
Copyright © 2009 Pearson Education, Inc.
HIV Infection
 Eventually the viral DNA in the host cell will
begin to make proteins needed to make new
HIV
 Proteases help package the new virus
components into a new envelope using the
host cell membrane
 The new viruses are released from the host
to infect other cells
Copyright © 2009 Pearson Education, Inc.
8
11/15/2011
HIV Treatments
 There is no cure. These treatments can slow
the spread of the virus in the body but not
completely get rid of it
 There are three main targets to fight HIV:
1. Reverse Transcriptase Inhibitors
AZT anti-viral drug
 AZT is a reverse transcriptase inhibitor.
 It also inhibits DNA polymerase but has 100 –
300 X greater affinity for reverse
transcriptase
 AZT is a thymidine analog
 Bind to and disable reverse transcriptase
2. Protease Inhibitors
3. Fusion Inhibitors - prevent HIV entry into cells.
Copyright © 2009 Pearson Education, Inc.
HIV in the US
 In 2009 there were an estimated 48,100 new
HIV infections in the US
 1.2 million people in the United States are
living with HIV infection and 1 in 5 are
unaware of their infection.
 More than 16,000 people with AIDS were
estimated to have died in 2008, and nearly
594,500 people with AIDS in the US have
died since the epidemic began.
Influenza Virus
 In 1918 – 1919 influenza killed 20 - 50 million
people, more than the numbers of soldiers in
WWI
 The type of influenza that causes the most
problem infects both bird, swine and human
hosts
 Influenza virus capsid covered with envelope
with protein spikes. Different strains have
different types of proteins.
Copyright © 2009 Pearson Education, Inc.
 In 2007, an estimated 33 million people
worldwide ( 2.5 million children) were living
with HIV/AIDS.
 Approximately two-thirds of these people live
in Sub-Saharan Africa; another 20 percent
live in Asia and the Pacific.
 More than 25 million people with HIV/AIDS
have died since the first AIDS cases were
identified in 1981.
 In 2007 alone, HIV/AIDS-associated illnesses
caused the deaths of approximately 2.1
million people worldwide (330,000 children)
Hantavirus
 The hantavirus causes pneumonia
 Hosts include deer mice (peromyscus) and
humans
 Transmitted through respiratory exposure
to rodent fecal droppings and urine
Copyright © 2009 Pearson Education, Inc.
9
11/15/2011
Ebola virus
Viruses can cause cancer
 Ebola virus causes severe hemorrhagic fever
 Natural host unknown
 Death rates 50 – 90% of those infected
Copyright © 2009 Pearson Education, Inc.
 Hepatitis B infection – liver cancer
 Papilloma virus – cervical cancer
Copyright © 2009 Pearson Education, Inc.
Prions
 A similar disease is Scabies in sheep
 Mad Cow Disease, Kuru and Creutzfelt-Jakob
diseases are all caused by prions
 Prions are abnormal proteins, they are not as
soluble as the normal proteins. The normal
protein is needed for nerve cells to operate
properly
 Prions are able to convert normal proteins into
more abnormal proteins
 The abnormal proteins coagulate in the brain
causing transmissible spongiform
encephalopathies (TSEs)
Copyright © 2009 Pearson Education, Inc.
 In England infected sheep were fed to cows,
leading to an outbreak of mad cow disease
 Humans ate the infected cow meat and some
people have contracted mad cow disease –
the estimates of how many people vary
widely
 To contract mad cow disease you need to eat
the part of the cow that is contaminated: brain
or spinal tissue.
Copyright © 2009 Pearson Education, Inc.
Figure 19.11
Important Concepts
 Know the vocabulary in the lecture
Prion
Normal
protein
 Structural features and characteristics of viruses,
what are common shapes of the caspids
Original
prion
New
prion
Aggregates
of prions
 Steps in a virus replication both lysogenic cycle
and lytic cycle
 Be able to describe in detail the steps of HIV
infection of T Cells
 What are retroviruses and what enzyme do they
have
Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
10
11/15/2011
Important Concepts
 What three enzymes does HIV have, what are their
functions
 What cells are the host cell of HIV
 What are the main targets of HIV drugs
 Examples of viruses given in class and their hosts
 Know the examples of viruses that cause cancer,
and what cancers they cause.
 Prions – what are they, what diseases are
associated with them
Copyright © 2009 Pearson Education, Inc.
11