Chapter 19 - Viruses
Ms. Whipple – Brethren Christian High School
1. What is a virus?
A virus is a genome enclosed in a
protective coat. It is not alive!!!
Viruses lead “a kind of borrowed life”
between life-forms and chemicals
The origins of molecular biology lie in early
studies of viruses that infect bacteria
2. Describe the discovery of the Tobacco
Mosaic Virus?
 The story of how viruses were discovered begins in 1883
with research on the cause of tobacco mosaic disease
by Adolf Mayer.
 This disease stunts tobacco plant growth and mottles plant
leaves.
 Mayer concluded that the disease was infectious when he
found that he could transmit the disease by rubbing sap from
diseased leaves onto healthy plants.
 He concluded that the disease must be caused by an extremely
small bacterium.
 Ten years later, Dimitri Ivanovsky demonstrated that the sap
was still infectious even after passing through a filter designed
to remove bacteria.
2. Describe the discovery of the Tobacco
Mosaic Virus?
 In 1897, Martinus Beijerinck ruled out the possibility that
the disease was due to a filterable toxin produced by a
bacterium by demonstrating that the infectious agent
could reproduce.
 The sap from one generation of infected plants could be used to
infect a second generation of plants that could infect
subsequent generations.
 Beijerinck also determined that the pathogen could reproduce
only within the host, could not be cultivated on nutrient media,
and was not inactivated by alcohol, generally lethal to bacteria.
 In 1935, Wendell Stanley crystallized the pathogen, the
tobacco mosaic virus (TMV).
Fig. 19-2
RESULTS
1 Extracted sap 2 Passed sap
from tobacco
plant with
tobacco
mosaic
disease
3 Rubbed filtered
through a
porcelain
filter known
to trap
bacteria
4 Healthy plants
became infected
sap on healthy
tobacco plants
3. What is the structural components of a
virus?
 Stanley’s discovery that some viruses could be
crystallized was puzzling because not even the simplest
cells can aggregate into regular crystals.
 So viruses are not cells.
 Viruses are infectious particles consisting of nucleic acid
encased in a protein coat called a Capsid and, in some
cases, a membranous envelope.
 The tiniest viruses are only 20 nm in diameter—smaller than a
ribosome.
 The genome of viruses may consist of double-stranded
DNA, single-stranded DNA, double-stranded RNA, or
single-stranded RNA, depending on the kind of virus.
3. What is the structural components of a
virus?
 A capsid is the protein shell that encloses the viral
genome
Capsids are built from protein subunits called
capsomeres
A capsid can have various structures
 Some viruses have membranous envelopes that
help them infect hosts
These viral envelopes surround the capsids of
influenza viruses and many other viruses found in
animals
Viral envelopes, which are derived from the host cell’s
membrane, contain a combination of viral and host
cell molecules
Fig. 19-3
RNA
DNA
Capsomere
Membranous
envelope
RNA
Head
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Glycoproteins
Glycoprotein
18  250 nm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
virus
50 nm
Tail
fiber
80  225 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
4. Viruses that infect Bacteria are called
Bacteriophages!!!
5. How do Viruses replicate themselves?
What is the Host Range of a Virus?
 Viruses are obligate intracellular parasites, which
means they can reproduce only within a host cell
 Each virus has a host range, a limited number of
host cells that it can infect
 Once a viral genome has entered a cell, the cell
begins to manufacture viral proteins
 The virus makes use of host enzymes, ribosomes,
tRNAs, amino acids, ATP, and other molecules
 Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
Fig. 19-4
VIRUS
1 Entry and
DNA
uncoating
Capsid
3 Transcription
and manufacture
of capsid proteins
2 Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
4 Self-assembly of
new virus particles
and their exit from
the cell
6. Describe the Lytic Cycle of a Viruses life.
Use the terms Virulent Phage & Restriction
Enzymes in your answer.
The lytic cycle is a phage reproductive cycle
that culminates in the death of the host cell
The lytic cycle produces new phages and
digests 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
Fig. 19-5-1
1 Attachment
Fig. 19-5-2
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Fig. 19-5-3
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
3 Synthesis of viral
genomes and
proteins
Fig. 19-5-4
1 Attachment
2 Entry of phage
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
Fig. 19-5-5
1 Attachment
2 Entry of phage
5 Release
DNA and
degradation of
host DNA
Phage assembly
4 Assembly
3 Synthesis of viral
genomes and
proteins
Head
Tail Tail fibers
7. Describe the Lysogenic Cycle of a Viruses
life. Use the terms Temperate Phage &
Prophage in your answer.
 The lysogenic cycle replicates the phage genome
without destroying the host
 A virus that can use both the Lytic & Lysogenic cycles is
called a Temperate Phage.
 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
8. What can cause a prophages to initiate a
Lytic cycle?
An environmental signal such as stress or a
weakened immune system can trigger the
virus genome to exit the bacterial
chromosome and switch to the lytic mode
Fig. 19-6
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
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
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and proteins
are synthesized and
assembled into phages.
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
9. What is a RetroVirus? How does it infect
cells? What type of Virus is HIV?
The broadest variety of RNA genomes is found in
viruses that infect animals
Retroviruses use reverse transcriptase to copy
their RNA genome into DNA
HIV (human immunodeficiency virus) is the
retrovirus that causes AIDS (acquired
immunodeficiency syndrome)
Fig. 19-8a
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA (two
identical
strands)
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
mRNA
Fig. 19-8b
HIV
Membrane of
white blood cell
0.25 µm
HIV entering a cell
New HIV leaving a cell
10. Briefly describe the theorized evolution of
viruses. Do Viruses attack all life forms? Is there
some debate as to when viruses evolved? Why?
 Viruses do not fit our definition of living organisms
 Most molecular biologists favor the hypothesis that viruses originated from
fragments of cellular nucleic acids that could move from one cell to another.
 A viral genome usually has more in common with the genome of its host than with
those of viruses infecting other hosts.
 However, some viruses have genetic sequences that are quite similar to seemingly
distantly related viruses.
 This genetic similarity may reflect the persistence of groups of viral genes that were
evolutionarily successful during the early evolution of viruses and their eukaryotic host cells.
 Mimivirus, a double-stranded DNA virus, is the largest virus yet
discovered
 There is controversy about whether this virus evolved before
or after cells