1. Chapter 19
Viruses

2. Overview: A Borrowed Life
Viruses lead “a kind of borrowed life” between life-forms and chemicals
They straddle the existence between living and nonliving.
Bacteriophages infecting E. coli.

3. Structure of Viruses
The word “Virus” is Latin for “poison”.
Viruses are not cells
They are small infectious particles made of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
Many are smaller than ribosomes and must be viewed with an electron microscope.

4. Viral genomes consist of either
Double- or single-stranded DNA, or
Double- or single-stranded RNA
Thus they are classified as either a DNA virus or an RNA virus

5. Capsids and Envelopes
A capsid is the protein coat
Capsids are built from protein subunits called capsomeres
A capsid can have two shapes:
Rod (helical-shape)
Icosohedral

6. 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 are derived from the host cell’s membrane and contain a combination of viral and host cell molecules

7. Bacteriophages, also called phages, are viruses that infect bacteria
Phages are robot-shaped with two parts:
an elongated capsid head that encloses their DNA
A protein tail piece

8. RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath
Fig. 19-3
RNA
DNA
Membranous
envelope
Head
RNA
Capsomere
DNA
Capsid
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
Figure 19.3 Viral structure
20 nm
50 nm
50 nm
50 nm
(a) Tobacco mosaic
virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

9. Concept 19.2: Viruses reproduce only in host cells
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
Identify host’s by a “lock and key” fit between viral surface proteins and cell receptors

10. General Features of Viral Reproductive Cycles
Once a viral genome has entered a cell, the cell begins to manufacture viral proteins and copy viral DNA or RNA
The virus uses the host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Animation: Simplified Viral Reproductive Cycle

11. VIRUS Entry and uncoating DNA Capsid Transcription and manufacture
Fig. 19-4
VIRUS
Entry and
uncoating 1
1. Virus enters and is
uncoated, releasing
viral DNA and capsid
proteins
DNA
Capsid
Transcription
and manufacture
of capsid proteins 3 2
Replication
Host enzymes replicate
the viral genome.
3. Host enzymes transcribe the
viral genome into viral mRNA, which
host ribosomes use to make more
capsid proteins.
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Figure 19.4 A simplified viral reproductive cycle
Viral genomes and capsid proteins
self-assemble into new viruses which
exit the cell.
Self-assembly of
new virus particles
and their exit from
the cell 4

12. Reproductive Cycles of Phages
Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

13. Animation: Phage T4 Lytic Cycle
The Lytic Cycle
The lytic cycle results in the quick death of the host cell
The lytic cycle produces new phages that digests the host’s cell wall, and burst out of the cell.
A phage that reproduces only by the lytic cycle is called a virulent phage
Restriction enzymes serve as a defense against phages by recognizing and cutting up phage DNA
Animation: Phage T4 Lytic Cycle

14. Attachment 1 Fig. 19-5-1 The T4 bacteriophage is a very virulent phage
and is often used to illustrate the lytic cycle.
“Lytic” refers to the last stage of infection in which
The bacterium lyses (breaks open)
Figure 19.5 The lytic cycle of phage T4, a virulent phage

15. Attachment Entry of phage DNA and degradation of host DNA 1 2
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage

16. Attachment Entry of phage DNA and degradation of host DNA
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage 3
Synthesis of viral
genomes and
proteins

17. Attachment Entry of phage DNA and degradation of host DNA
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage 4
Assembly 3
Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers

18. Attachment Entry of phage DNA and degradation of host DNA Release
Fig 1
Attachment 2
Entry of phage
DNA and
degradation of
host DNA 5
Release
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage 4
Assembly 3
Synthesis of viral
genomes and
proteins
Head
Tail
Tail fibers

19. Animation: Phage Lambda Lysogenic and Lytic Cycles
The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated into the host cell’s chromosome and becomes known as a prophage
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
Animation: Phage Lambda Lysogenic and Lytic Cycles

20. An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
Phages that use both the lytic and lysogenic cycles are called temperate phages
The lambda bacteriophage is a temperate phage that is commonly used to illustrate the lysogenic cycle (“lysogenic” refers to their ability to generate the active phages that can lyse the bacterium)

21. The phage injects its DNA.
Fig. 19-6
Phage λ (lambda) has a single, short tail fiber.
Daughter cell
with prophage
Phage
DNA
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
Lysogenic cycle
is entered
Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage
Prophage
New phage DNA and proteins
are synthesized and
assembled into phages.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.

22. Reproductive Cycles of Animal Viruses
There are two key variables used to classify viruses that infect animals:
DNA or RNA?
Single-stranded or double-stranded?

23. Table 19-1a
Table 1

24. Table 19-1b
Table 1

25. Viral Envelopes
Many viruses that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

26. Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane
The reproductive cycle of many enveloped viruses does not necessarily kill the host cell, in contrast to the lytic cycles of phages.

27. Capsid and viral genome enter the cell Capsid
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
2. Capsid and RNA enter cell:
Capsid is digested by cellular
enzymes.
RNA
3. The viral genome (red)
is a template for synthesis
of complementary RNA
strands (pink) by a viral
enzyme which is packaged
inside the viral capsid
HOST CELL
Envelope (with
glycoproteins)
Glycoproteins on virus bind to
receptors on cell (not shown)
Viral genome (RNA)
Template
Complementary RNA strands
also act as mRNA, which is
translated into both capsid proteins
(in cytosol) and glycoproteins
(in ER)
mRNA
New copies of viral
genome RNA are made
using complementary
RNA strands as templates
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
Capsid assembles around
viral genome
Vesicles transport envelope
glycoproteins to the plasma membrane
Virus buds from the cell, its envelope
studded with viral glycoproteins embedded in
membrane derived from the host cell.
New virus

28. RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals
Retroviruses use reverse transcriptase to copy their RNA genome into DNA. This flow of information (RNA DNA) is backwards: hence the name “retro” virus.
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

29. Animation: HIV Reproductive Cycle
The viral DNA that is integrated into the host genome is called a provirus
Unlike a prophage, a provirus remains a permanent resident of the host cell
The host’s RNA polymerase transcribes the proviral DNA into RNA molecules
The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell
Animation: HIV Reproductive Cycle

30. Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse
Fig. 19-8a
Glycoprotein
Viral envelope
2. Virus fuses w/ plasma membrane. Capsid proteins
are removed, releasing viral proteins and RNA
HIV is an enveloped virus
w/ 2 identical strands of RNA
and 2 molecules of reverse
transcriptase.
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST CELL
HIV
Reverse
transcriptase
Viral RNA
3. Reverse transcriptase synthesizes a DNA
strand complementary to the viral RNA
RNA-DNA
hybrid
**Note: The provirus never leaves
the cell (unlike prophages)
Reverse transcriptase synthesizes a
second DNA strand complementary to the first.
DNA
5. Dbl stranded DNA is
incorporated as a provirus into
cell’s DNA.
NUCLEUS
Provirus
Chromosomal
DNA
6. Proviral genes
transcribed into RNA
RNA genome
for the
next viral
generation
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
mRNA
Capsids assemble around viral
genomes and reverse transcriptase.
7. Viral proteins include capsid
proteins and reverse transcriptase
(made in cytosol) and envelope
glycoproteins (made in ER)
10. New viruses bud from cell
8. Vesicles transport glycoproteins
to plasma membrane
New virus

31. Membrane of white blood cell HIV HIV entering a cell
Fig. 19-8b
Membrane of
white blood cell
HIV
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
0.25 µm
HIV entering a cell
New HIV leaving a cell

32. Evolution of Viruses
Viruses do not fit our definition of living organisms
Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid
Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments
Plasmids, transposons, and viruses are all mobile genetic elements

33. Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants
Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide
Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

34. Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version
Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

35. Original Prion prion Aggregates of prions New prion Normal protein
Fig
Original
prion
Prion
Aggregates
of prions
New
prion
Normal
protein
Figure Model for how prions propagate

36. Viral Diseases in Animals
A viral infection can produce symptoms by several different methods.
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
Some viruses cause infected cells to produce toxins that lead to disease symptoms
Others have envelope proteins that are toxic
The level of damage done often depends on the ability of the infected cells to regenerate
We recover from colds because respiratory cells regenerate.
Damage from polio virus is permanent because nerve cells do not regenerate.

37. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
Vaccines can prevent certain viral illnesses
Viral infections cannot be treated by antibiotics
Antiviral drugs can help to treat, though not cure, viral infections.
Most antiviral drugs are designed to interfere with the enzymes that synthesize viral DNA or those that are required for the assembly of the virus.

38. Emerging Viruses
Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists
Ex: HIV, Ebola, West Nile and most recently SARS
Severe acute respiratory syndrome (SARS) recently appeared in China (2002)
Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

39. Flu epidemics are caused by new strains of influenza virus to which people have little immunity
Viral diseases in a small isolated population can emerge and become global
New viral diseases can emerge when viruses spread from animals to humans
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

40. These strains can cause pandemics, global epidemics
The “avian flu” is a virus that recently appeared in humans and originated in wild birds

41. (Spanish Flu: H1N1) (a) The 1918 flu pandemic Fig. 19-9a
Figure 19.9 Influenza in humans and other animals
(a) The 1918 flu pandemic
(Spanish Flu: H1N1)

42. (b) Influenza A H5N1 (Hong Kong Flu: 1997) virus (green) 0.5 µm
Fig. 19-9b
0.5 µm
Figure 19.9 Influenza in humans and other animals
(b) Influenza A H5N1
virus (green)
(Hong Kong Flu: 1997)

43. (c) Vaccinating ducks: to prevent spread of H5N1
Fig. 19-9c
Figure 19.9 Influenza in humans and other animals
(c) Vaccinating ducks: to prevent spread of H5N1

44. Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots
Most plant viruses have an RNA genome

45. Fig
Figure Viral infection of plants
Viral infections cause the irregular
brown patches on tomatoes, black blotching on
squash, and streaking in tulips due to redistribution
of pigment granules.

46. Plant viruses spread disease in two major modes:
Horizontal transmission, entering through damaged cell walls
Vertical transmission, inheriting the virus from a parent

47. host cell and injects its DNA Phage DNA
Fig. 19-UN1
The phage attaches to a
host cell and injects its DNA
Phage
DNA
Bacterial
chromosome
Prophage
Lytic cycle
Lysogenic cycle
Virulent or temperate phage
Destruction of host DNA
Production of new phages
Lysis of host cell causes release
of progeny phages
Temperate phage only
Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the
chromosome and initiate a lytic cycle

48. Fig. 19-UN2 A B
Number of bacteria
Number of viruses
Time
Time

49. Fig. 19-UN3

50. You should now be able to:
Explain how capsids and envelopes are formed
Distinguish between the lytic and lysogenic reproductive cycles
Explain why viruses are obligate intracellular parasites
Describe the reproductive cycle of an HIV retrovirus
Describe three processes that lead to the emergence of new diseases
Describe viroids and prions