1. Chapter 19
Viruses

2. Overview: A Borrowed Life
Viruses called bacteriophages infect genetically takeover bacteria, like E. coli
Viruses have borrowed life- between life-forms and chemicals
Molecular biology began w/ study of bacteriophages

3. Fig. 19-1
Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?
0.5 µm

4. Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat
Wendel Stanley – 1935 confirmed hypothesis that tobacco mosaic virus caused by submicroscopic particles
by crystallizing tobacco mosaic virus (TMV) from infected tobacco leaves
TMV- causes stunted growth in tobacco plants

5. Structure of Viruses Viruses are not cells
Viruses- very small infectious particles consisting of nucleic acid enclosed in a protein coat and, possibly, a membranous envelope

6. Viral Genomes Viral genomes may be made of
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

7. Capsids and Envelopes
A capsid is the protein shell that encloses the viral genome
Capsids are built from protein subunits called capsomeres
Capsid structures vary
Viral envelopes help some viruses infect hosts
Seen in influenza viruses and many animal viruses
derived from the host cell’s membrane
contains a combination of viral and host cell molecules

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. Bacteriophages- viruses that infect bacteria
have an elongated capsid head that encloses DNA
protein tail attaches virus to the host & injects phage DNA inside

10. Concept 19.2: Viruses reproduce only in host cells
Viruses are obligate intracellular parasites- can reproduce only inside a host cell
Each virus has a host range- a specific kind of cell or organism that it can infect

11. General Features of Viral Reproductive Cycles
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

12. VIRUS Entry and uncoating DNA Capsid Transcription and manufacture
Fig. 19-4
VIRUS
Entry and
uncoating 1
DNA
Capsid
Transcription
and manufacture
of capsid proteins 3 2
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Figure 19.4 A simplified viral reproductive cycle
Self-assembly of
new virus particles
and their exit from
the cell 4

13. Reproductive Cycles of Phages
Phages – the most studied viruse
Phages- two reproductive mechanisms: the lytic cycle and the lysogenic cycle

14. The Lytic Cycle The lytic cycle- ends in the death of host cell
produces new phages, digests host cell wall, releasing progeny viruses
virulent phage - phage that reproduces only by lytic cycle
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

15. Fig 1
Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage

16. 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

17. 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

18. 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

19. 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

20. The Lysogenic Cycle
lysogenic cycle- copies phage genome without destroying the host
viral DNA molecule is incorporated into host cell’s chromosome
prophage – integrated viral DNA
Every time host divides, it copies phage DNA and passes copies to daughter cells
environmental signal triggers virus genome to exit bacterial chromosome & switch to lytic mode
Phages that use both the lytic and lysogenic cycles are called temperate phages

21. The phage injects its DNA.
Fig. 19-6
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
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 infecting animals have membranous envelope
Viral glycoproteins on envelope surface bind to specific receptor molecules on host cell surface
Some are formed from the host cell’s plasma membrane as the viral capsids exit
Other from the host’s nuclear envelope (Later replaced by envelope from Golgi apparatus)

26. Capsid and viral genome enter the cell Capsid
Fig. 19-7
Capsid and viral genome
enter the cell
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
Capsid
proteins ER
Copy of
genome (RNA)
Glyco-
proteins
Figure 19.7 The reproductive cycle of an enveloped RNA virus
New virus

27. 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
HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

28. Fig. 19-8
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HIV
Membrane of
white blood cell
HIV
HOST CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal
DNA
Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell

29. 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

30. The viral DNA integrated into host genome is called provirus
Unlike prophage, a provirus remains a permanent resident of the host cell
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

31. Evolution of Viruses
Viruses don’t fit our definition of living organisms
Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid
Possible sources of viral genomes-
plasmids, circular DNA in bacteria and yeasts
transposons, small mobile DNA segments
Plasmids, transposons, and viruses are all mobile genetic elements
Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered
There is controversy about whether this virus evolved before or after cells

32. 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

33. Viral Diseases in Animals
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

34. 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

35. Emerging Viruses
Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists
Severe acute respiratory syndrome (SARS) recently appeared in China
Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

36. 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

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

38. (a) The 1918 flu pandemic (b) Influenza A H5N1 virus
Fig. 19-9
(a) The 1918 flu pandemic
0.5 µm
Figure 19.9 Influenza in humans and other animals
For the Discovery Video Emerging Diseases, go to Animation and Video Files.
(b) Influenza A
H5N1 virus
(c) Vaccinating ducks

39. 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
Plant viruses spread disease in two major modes:
Horizontal transmission, entering through damaged cell walls
Vertical transmission, inheriting the virus from a parent

40. Fig
Figure Viral infection of plants

41. 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

42. 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

43. 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