1. Viral Life Cycles & Viruses
Chapter 10 part 2
Viral Life Cycles & Viruses
Lecture by Mary C. Colavito

2. MICROBIAL GENETICS
Copyright © 2009 Pearson Education, Inc.

3. 10.17 Viral DNA may become part of the host chromosome
Viruses have two types of reproductive cycles
Lytic cycle
Viral particles are produced using host cell components
The host cell lyses, and viruses are released
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. Students (and instructors) might enjoy thinking of a prophage as a smudge mark on the master copy of a class handout. The smudge is replicated every time the original is copied!
Copyright © 2009 Pearson Education, Inc.

4. Phage attaches to bacterial cell. Phage injects DNA.
Phage attaches
to bacterial cell.
Phage injects DNA.
Phage DNA directs host
cell to make more phage
DNA and protein parts.
New phages assemble.
Cell lyses and
releases new phages.
Figure 10.1C A phage reproductive cycle.

5. 10.17 Viral DNA may become part of the host chromosome
Viruses have two types of reproductive cycles
Lysogenic cycle
Viral DNA is inserted into the host chromosome by recombination
Viral DNA is duplicated along with the host chromosome during each cell division
The inserted phage DNA is called a prophage
Most prophage genes are inactive
Environmental signals can cause a switch to the lytic cycle
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. Students (and instructors) might enjoy thinking of a prophage as a smudge mark on the master copy of a class handout. The smudge is replicated every time the original is copied!
Animation: Phage Lambda Lysogenic and Lytic Cycles
Animation: Phage T4 Lytic Cycle
Copyright © 2009 Pearson Education, Inc.

6. Figure 10.17 Two types of phage reproductive cycles.
Phage 1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA 2 4
Lytic cycle
Phages assemble
Phage DNA
circularizes
Figure Two types of phage reproductive cycles.
The transfer of phage DNA via transduction was shown to convert a nontoxigenic strain of the botulism-causing bacterium Clostridium to a toxigenic strain. 3
New phage DNA and
proteins are synthesized

7. Figure 10.17 Two types of phage reproductive cycles.
Phage 1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA 7 2
Many cell
divisions 4
Lytic cycle
Lysogenic cycle
Phages assemble
Lysogenic bacterium reproduces
normally, replicating the
prophage at each cell division
Phage DNA
circularizes
Figure Two types of phage reproductive cycles.
The transfer of phage DNA via transduction was shown to convert a nontoxigenic strain of the botulism-causing bacterium Clostridium to a toxigenic strain.
Prophage 3 5 6 OR
New phage DNA and
proteins are synthesized
Phage DNA inserts into the bacterial
chromosome by recombination

8. Phage DNA circularizes1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Cell lyses,
releasing phages
Phage injects DNA 2 4
Lytic cycle
Figure Two types of phage reproductive cycles.
The transfer of phage DNA via transduction was shown to convert a nontoxigenic strain of the botulism-causing bacterium Clostridium to a toxigenic strain.
Phages assemble
Phage DNA
circularizes 3
New phage DNA and
proteins are synthesized

9. Phage DNA circularizes1
Attaches
to cell
Bacterial
chromosome
Phage DNA
Phage injects DNA 7 2
Many cell
divisions
Lysogenic cycle
Figure Two types of phage reproductive cycles.
The transfer of phage DNA via transduction was shown to convert a nontoxigenic strain of the botulism-causing bacterium Clostridium to a toxigenic strain.
Lysogenic bacterium reproduces
normally, replicating the
prophage at each cell division
Phage DNA
circularizes
Prophage 5 6
Phage DNA inserts into the bacterial
chromosome by recombination

10. 10.18 CONNECTION: Many viruses cause disease in animals and plants
Both DNA viruses and RNA viruses cause disease in animals
Reproductive cycle of an RNA virus
Entry
Glycoprotein spikes contact host cell receptors
Viral envelope fuses with host plasma membrane
Uncoating of viral particle to release the RNA genome
mRNA synthesis using a viral enzyme
Protein synthesis
RNA synthesis of new viral genome
Assembly of viral particles
Mumps is an RNA virus that does not have a DNA phase in its reproductive cycle. The RNA genome is complementary to mRNA, so the virion particles contain an RNA-dependent RNA polymerase (from a viral gene) to produce the mRNA once the genome is inside the cell. This enzyme will also produce copies of the genome from the mRNA (step 5).
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. As noted in Module 10.18, viruses can spread throughout a plant by moving through plasmodesmata. This is like smoke spreading throughout a building by moving through air ducts.
Copyright © 2009 Pearson Education, Inc.

11. 10.18 CONNECTION: Many viruses cause disease in animals and plants
Some animal viruses reproduce in the cell nucleus
Most plant viruses are RNA viruses
They breach the outer protective layer of the plant
They spread from cell to cell through plasmodesmata
Infection can spread to other plants by animals, humans, or farming practices
While the mumps virus is produced in the cell cytoplasm, copies of the herpes virus are produced in the nucleus of a host cell.
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. As noted in Module 10.18, viruses can spread throughout a plant by moving through plasmodesmata. This is like smoke spreading throughout a building by moving through air ducts.
Animation: Simplified Viral Reproductive Cycle
Copyright © 2009 Pearson Education, Inc.

12. Figure 10.18 The reproductive cycle of an enveloped virus.
Glycoprotein spike
Protein coat
Viral RNA
(genome)
Membranous
envelope
Plasma membrane
of host cell 1
Entry 2
Uncoating
Viral RNA
(genome) 3
RNA synthesis
by viral enzyme
Protein
synthesis
RNA synthesis
(other strand) 4 5
Template
mRNA
New viral
genome
New
viral proteins 6
Assembly
Figure The reproductive cycle of an enveloped virus.
Exit 7

13. VIRUS Glycoprotein spike Protein coat Viral RNA (genome) Membranous
VIRUS
Glycoprotein spike
Protein coat
Viral RNA
(genome)
Membranous
envelope
Plasma membrane
of host cell 1
Entry
Uncoating 2
Viral RNA
(genome)
Figure The reproductive cycle of an enveloped virus.
RNA synthesis
by viral enzyme 3

14. Protein RNA synthesis synthesis (other strand) Template mRNA New viral
Protein
synthesis
RNA synthesis
(other strand) 4 5
Template
mRNA
New viral
genome
Assembly
New
viral proteins 6
Figure The reproductive cycle of an enveloped virus.
Exit 7

15. 10.19 EVOLUTION CONNECTION: Emerging viruses threaten human health
How do emerging viruses cause human diseases?
Mutation
RNA viruses mutate rapidly
Contact between species
Viruses from other animals spread to humans
Spread from isolated populations
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. There is an interesting relationship between the speed at which a virus kills or debilitates a host and the extent to which it spreads from one organism to another. This is something to consider for a class discussion. Compare two viral infections. Infection A multiplies within the host, is spread by the host to other people through casual contact, but does not cause its lethal symptoms until 5–10 years after infection. Virus B kills the host within 1–2 days of infection, is easily transmitted, and causes severe symptoms within hours of contact. Which virus is likely to spread the fastest through the human population on Earth? Which might be considered the most dangerous to humans?
2. The annual mutations and variations in flu viruses require the production of a new flu vaccine every year. The Centers for Disease Control and Prevention monitors patterns of flu outbreaks, especially in Asia (where many variations of flu viruses originate). They must predict which strains are most likely to be dangerous in the coming year and then synthesize an appropriate vaccine.
Copyright © 2009 Pearson Education, Inc.

16. 10.19 EVOLUTION CONNECTION: Emerging viruses threaten human health
Examples of emerging viruses
HIV
Ebola virus
West Nile virus
RNA coronavirus causing severe acute respiratory syndrome (SARS)
Avian flu virus
For the Discovery Video Emerging Diseases, go to Animation and Video Files.
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. There is an interesting relationship between the speed at which a virus kills or debilitates a host and the extent to which it spreads from one organism to another. This is something to consider for a class discussion. Compare two viral infections. Infection A multiplies within the host, is spread by the host to other people through casual contact, but does not cause its lethal symptoms until 5–10 years after infection. Virus B kills the host within 1–2 days of infection, is easily transmitted, and causes severe symptoms within hours of contact. Which virus is likely to spread the fastest through the human population on Earth? Which might be considered the most dangerous to humans?
2. The annual mutations and variations in flu viruses require the production of a new flu vaccine every year. The Centers for Disease Control and Prevention monitors patterns of flu outbreaks, especially in Asia (where many variations of flu viruses originate). They must predict which strains are most likely to be dangerous in the coming year and then synthesize an appropriate vaccine.
Copyright © 2009 Pearson Education, Inc.

17. Figure Ducks in Vietnam being checked for infection by the Avian flu virus.

18. Figure Ducks in Vietnam being checked for infection by the Avian flu virus.

19. Figure Ducks in Vietnam being checked for infection by the Avian flu virus.

20. 10.20 The AIDS virus makes DNA on an RNA template
AIDS is caused by HIV, human immunodeficiency virus
HIV is a retrovirus, containing
Two copies of its RNA genome
Reverse transcriptase, an enzyme that produces DNA from an RNA template
One of the approaches for treating HIV-positive patients involves inhibiting reverse transcriptase to prevent the formation of the DNA copy of the RNA genome. AZT (azidothymidine, zidovudine) is a thymidine analog that acts as a chain terminator, since its 3′OH group has been replaced with an azido group. When reverse transcriptase incorporates AZT into the chain, DNA synthesis stops. Since humans do not produce reverse transcriptase, AZT should affect only the viral enzyme. However, a DNA polymerase within mitochondria has a greater sensitivity to AZT than other host cell DNA polymerases, which may account for some of the side effects of the drug.
Another unique aspect of HIV duplication is that the initial product of translation needs to be cleaved into individual proteins. A viral-encoded protease carries out this function. Protease inhibitors are used to prevent the formation of active viral proteins from the translated product. Therapies combining a reverse transcriptase inhibitor and a protease inhibitor have been highly effective in reducing the amount of viral production in HIV-positive patients.
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
3. Many misconceptions about AIDS exist. A list of 18 common misconceptions is located at
Teaching Tips
1. The Centers for Disease Control and Prevention has extensive information about AIDS at
2. Students often do not understand the disproportionate distribution of HIV infections and AIDS in our world. Consider an Internet assignment, asking students to identify the regions of the world most affected by HIV-AIDS.
Copyright © 2009 Pearson Education, Inc.

21. 10.20 The AIDS virus makes DNA on an RNA template
HIV duplication
Reverse transcriptase uses RNA to produce one DNA strand
Reverse transcriptase produces the complementary DNA strand
Viral DNA enters the nucleus and integrates into the chromosome, becoming a provirus
Provirus DNA is used to produce mRNA
mRNA is translated to produce viral proteins
Viral particles are assembled and leave the host cell
For the BLAST Animation AIDS Treatment Strategies, go to Animation and Video Files.
For the BLAST Animation HIV Structure, go to Animation and Video Files.
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
3. Many misconceptions about AIDS exist. A list of 18 common misconceptions is located at
Teaching Tips
1. The Centers for Disease Control and Prevention has extensive information about AIDS at
2. Students often do not understand the disproportionate distribution of HIV infections and AIDS in our world. Consider an Internet assignment, asking students to identify the regions of the world most affected by HIV-AIDS.
Animation: HIV Reproductive Cycle
Copyright © 2009 Pearson Education, Inc.

22. Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse
Envelope
Glycoprotein
Protein
coat
RNA
(two identical
strands)
Figure 10.20A A model of HIV structure.
Reverse
transcriptase

23. Viral RNA CYTOPLASM NUCLEUS DNA Chromosomal strand DNA Double-
Viral RNA
CYTOPLASM 1
NUCLEUS
DNA
strand
Chromosomal
DNA 2
Double-
stranded
DNA
Provirus
DNA 3 4 5
Viral
RNA
and
proteins
Figure 10.20B The behavior of HIV nucleic acid in a host cell.
RNA 6

24. 10.21 Viroids and prions are formidable pathogens in plants and animals
Some infectious agents are made only of RNA or protein
Viroids: circular RNA molecules that infect plants
Replicate within host cells without producing proteins
Interfere with plant growth
Prions: infectious proteins that cause brain diseases in animals
Misfolded forms of normal brain proteins
Convert normal protein to misfolded form
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. Viroids can cause significant damage to plants. The authors note elsewhere that over 10 million coconut palms in the Philippines have been killed by viroids.
Copyright © 2009 Pearson Education, Inc.

25. 10.22 Bacteria can transfer DNA in three ways
Three mechanisms allow transfer of bacterial DNA
Transformation is the uptake of DNA from the surrounding environment
Transduction is gene transfer through bacteriophages
Conjugation is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplasmic bridge
Recombination of the transferred DNA with the host bacterial chromosome leads to new combinations of genes
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. The authors note that the figures in Module represent the size of the bacterial chromosome as much smaller than they actually are. They note that a bacterial chromosome is hundreds of times longer than the cell. These chromosomes use extensive folding to fit inside the cell.
2. You might challenge students to explain why conjugation is sometimes called bacterial sex. Students might note that two organisms cooperate to produce a new, genetically unique bacterium.
Copyright © 2009 Pearson Education, Inc.

26. DNA enters cell Fragment of DNA from another bacterial cell
DNA enters
cell
Fragment of
DNA from
another
bacterial cell
Figure 10.22A Transformation.
Transformation has been proposed as a method for transferring antibiotic resistance for the ulcer-causing bacteria Helicobacter pylori, both for members within this species and between species in the Helicobacter genus. This could undermine the currently successful use of antibiotics to treat ulcers.
Bacterial chromosome
(DNA)

27. Phage Fragment of DNA from another bacterial cell (former phage host)
Phage
Fragment of
DNA from
another
bacterial cell
(former phage
host)
Figure 10.22B Transduction.
During phage infection, the bacterial chromosome becomes fragmented. Bacterial DNA molecules of a size similar to the phage DNA can be packaged into phage particles. These phage particles inject bacterial DNA into another cell, leading to a possible change in genotype for the host. Phage particles carrying bacterial DNA do not continue the infection.

28. Mating bridge Sex pili Donor cell (“male”) Recipient cell (“female”)
Mating bridge
Sex pili
Figure 10.22C Conjugation.
As described in Module 10.23, conjugation depends on a plasmid called the F factor (fertility factor) that is either separate from the chromosome in an F+ cell or integrated into the chromosome in an Hfr cell (High frequency of recombination). The F plasmid can integrate at random locations, so different Hfr cells will have the factor at a unique position on the chromosome. This figure shows conjugation for an Hfr cell. During conjugation, one strand of DNA containing a portion of the F factor and its adjacent bacterial genes is transferred to the donor. The number of bacterial genes transferred will depend on the length of time that the cytoplasmic bridge is maintained. It is unlikely that the cells would remain attached long enough to transfer the entire bacterial chromosome and all portions of the F factor. This process uses “rolling circle” replication, whereby the donor replaces the donated strand using the strand remaining in the cell as a template. Recombination is required to integrate transferred genes into the recipient cell’s chromosome.
Donor cell
(“male”)
Recipient cell
(“female”)

29. Donated DNA Crossovers Degraded DNA Recipient cell’s chromosome
Donated DNA
Crossovers
Degraded DNA
Figure 10.22D Integration of donated DNA into the recipient cell’s chromosome.
Since the bacterial chromosome is a circle, two crossovers are needed to integrate transferred genes. If there were only one crossover event, the chromosome would be opened up into a linear structure, which would be subject to nuclease digestion within the cell.
Recipient cell’s
chromosome
Recombinant
chromosome

30. 10.23 Bacterial plasmids can serve as carriers for gene transfer
Plasmids are small circular DNA molecules that are separate from the bacterial chromosome
F factor is involved in conjugation
When integrated into the chromosome, transfers bacterial genes from donor to recipient
When separate, transfers F-factor plasmid
R plasmids transfer genes for antibiotic resistance by conjugation
The R plasmid has genes for sex pilus formation along with genes for antibiotic resistance.
For the BLAST Animation Plasmid, go to Animation and Video Files.
Student Misconceptions and Concerns
1. Students and many parents with young children expect antibiotics to be used to treat many respiratory infections, even though such infections may result from a virus. Students will benefit from a thorough explanation of why antibiotics are inappropriate for viral infections as well as the rising numbers of antibiotic-resistant bacteria that have evolved as a result of the overprescription of antibiotics.
2. The success of modern medicine has perhaps led to overconfidence in our ability to treat disease. Students often do not understand that there are few successful treatments for viral infections. Instead, the best defense against viruses is prevention, by reducing the chances of contacting the virus and the use of vaccines.
Teaching Tips
1. The figures in Module provide essential imagery for a detailed discussion of bacterial conjugation. The abstract details presented in Module are likely new to most of your students.
2. Module notes the possible consequences of widespread use of antibiotics. Consider asking your students to consider the value of widespread use of antibacterial soaps throughout their homes.
Copyright © 2009 Pearson Education, Inc.

31. transfer of chromosome
Male (donor)
cell
F factor
(integrated)
Origin of F
replication
Bacterial
chromosome
F factor starts
replication and
transfer of chromosome
Recipient
cell
Figure 10.23A Transfer of chromosomal DNA by an integrated F factor.
This shows the transfer of a portion of the F factor and adjacent bacterial genes from a cell where the F factor has integrated into the bacterial chromosome.
Only part of the
chromosome
transfers
Recombination
can occur

32. F factor (plasmid) Male (donor) cell Bacterial chromosome
Male (donor)
cell
Bacterial
chromosome
F factor starts
replication
and transfer
Figure 10.23B Transfer of an F-factor plasmid.
This figure shows that the F-factor plasmid can be transferred from donor to recipient. At the end of the process the recipient is now a donor.
Plasmid completes
transfer and
circularizes
Cell now male

33. Plasmids
Figure 10.23C Plasmids and part of a bacterial chromosome released from a ruptured E. coli cell.