1. Chapter 19
Viruses

2. Overview: A Borrowed Life
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.
A virus consists of a nucleic acid surrounded by a protein coat.

3. Tobacco Mosaic Virus Causes Disease
RESULTS
Tobacco Mosaic Virus Causes Disease 1
Extracted sap
from tobacco
plant with
tobacco
mosaic
disease 2
Passed sap
through a
porcelain
filter known
to trap
bacteria 3
Rubbed filtered
sap on healthy
tobacco plants
Figure 19.2 What causes tobacco mosaic disease? 4
Healthy plants
became infected

4. Structure of Viruses : Viruses are not cells.
Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and often a membranous envelope.
A capsid is the protein shell that encloses the viral genome.
Viral envelopes surround the capsids of viruses and enable the viruses to evade detection by the host likely because viral envelopes are derived from the host cell’s membrane.

5. Viruses / Structures RNA DNA Membranous envelope Head RNA Capsomere
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

6. Bacteriophages, also called phages, are viruses that infect bacteria.
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.

7. Viral Reproductive Cycle
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

8. Phages have two reproductive cycles: the lytic cycle and the lysogenic cycle
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.
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA.

9. Lytic Cycle Attachment Entry of phage DNA and degradation of host DNA1
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

10. 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.
This integrated viral DNA in the host genome is known as a prophage = dormant. Every time the host divides, it copies the dormant phage DNA and passes the copies to new cells.
Stress or immunosuppression triggers the virus genome to exit the bacterial chromosome and switch to the lytic mode.(Cold sores)

11. Viral Reproductive Cycles: Lytic and Lysogenic
Daughter cell
with prophage
Phage
DNA
The phage injects its DNA.
Binary Fission
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 dormant prophage &
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 dormant prophage.

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

13. Animal Viruses
Table 1

14. Viral Envelopes - Help Viruses Attach to Host and Evade Host Detection
Viral glycoproteins on the membranous 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.

15. Virus and Host Cell Capsid and viral genome enter the cell Capsid RNA
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

16. RNA as Viral Genetic Material
Retroviruses use reverse transcriptase to copy their RNA genome into DNA. (reverse of transcription)
HIV (human immunodeficiency virus) is a retrovirus.

17. RetroVirus: Reverse Transcriptase
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

18. 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.

19. Evolution of Viruses
Viruses do not fit our definition of living organisms - exceptions to Cell Theory.
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.

20. 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.
Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory.

21. Flu epidemics are caused by new strains of influenza virus to which people have little immunity.
Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity.
These strains can cause pandemics, global epidemics.
The “avian flu” is a virus that recently appeared in humans and originated in wild birds.

22. Viroids & Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules that infect plants and disrupt their growth. (No protein)
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, CWD and Creutzfeldt-Jakob disease in humans are all caused by prions.

23. Prions = Infectious Proteins
Original
prion
Prion
Aggregates
of prions
New
prion
Normal
protein
Figure Model for how prions propagate

24. Prions
Prions are scary because they can be infectious between species and are difficult to sanitize against
Can be the result of natural-occurring genetic mutation or infection
Eat your brains

25. 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.