1. T4 bacteriophage infecting an E. coli cell
0.5 m

2. Science as a Process
Research into TMV led to the conclusion that the pathogen was smaller than a bacterial cell
The pathogen was named virus

3. Comparing the size of a virus, a bacterium, and an animal cell
Animal cell nucleus
Comparing the size of a virus, a bacterium, and an animal cell

4. Infection by Tobacco Mosaic Virus

5. Figure 18.4 Viral Structure
18  250 mm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
20 nm
50 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
RNA
Capsomere of capsid
DNA
Capsomere
Glycoprotein
Membranous envelope
Capsid
Head
Tail fiber
Tail sheath

6. Capsids and Envelopes
Capsid = protein coat that surrounds the viral genome
viral envelope = derived from host cell membranes; it helps the virus invade the host cell

7. Figure 18.02x2 Phages

8. Figure 18.9 Viral infection of plants

9. Viral Genome
Double stranded DNA
Single Stranded DNA
Double stranded RNA
Single stranded RNA
A virus has only one of these types of nucleic acids

10. Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid

11. Figure 18.6 The reproductive cycle of an enveloped virus

12. Viral Replication What are the possible patterns of viral replication?
DNA --> DNA
RNA --> RNA, where viral genes code for RNA replicase
RNA --> DNA --> RNA; where viral gene uses reverse transcriptase

13. Bacterial Viruses
Which scientists used bacteriophages to prove that DNA was the hereditary material?
Hershey and Chase
What are the two mechanisms of phage infection?
Lytic and Lysogenic cycles

14. Figure 18.4 The lytic cycle of phage T4

15. Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage

16. Bacterial Defense
What defense do bacteria have against phage infection?
Restriction enzymes
What do restriction enzymes do?
They cut up DNA The bacterial DNA is modified to protect it from the restriction endonucleases.

17. Animal Viruses What is the viral envelope?
An outer membrane that helps the virus to invade the animal cell.
The invasion of the virus has the following stages ...

18. 1. Attachment
2. Entry
3. Uncoating
4. RNA and protein synthesis
5. Assembly and release

19. Herpesvirus Consists of double stranded DNA
Envelope derived from host cell nuclear envelope not from plasma membrane
It, therefore, reproduces within the nucleus
May integrate its DNA as a provirus
Tends to recur throughout lifetime of infected individual.

20. Figure 18.x6 Herpes

21. The structure of HIV, the retrovirus that causes AIDS
Reverse transcriptase
Viral envelope
Capsid
Glycoprotein
RNA (two identical strands)

22. Figure 18.7x1 HIV infection

23. The reproductive cycle of HIV, a retrovirus
Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane. 7
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 6
The double-stranded DNA is incorporated
as a provirus into the cell’s DNA. 4
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 5
Reverse transcriptase
catalyzes the synthesis of a second DNA strand
complementary to the first. 3
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA. 2
New viruses bud
off from the host cell. 9
Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules. 8
mRNA
RNA genome for the next viral generation
Viral RNA
RNA-DNA hybrid
DNA
Chromosomal DNA
NUCLEUS
Provirus
HOST CELL
Reverse transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV
Membrane of white blood cell
The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the viral proteins and RNA. 1

24. What is the current hypothesis concerning how viruses evolved?
Viral Evolution
What is the current hypothesis
concerning how viruses evolved?

25. Viral Disease
Some viruses have toxic components and cause infected cells to release enzymes from lysosomes
Recovery involves ability to repair damaged region of the body. Ex: polio may permanently damage nerve cells.

26. Figure 18.x3 Polio

27. Vaccines / Drugs What are vaccines and how do they work?
Introduce body to harmless or weakened strain of the virus, so that your immune system learns to recognize the virus prior to invasion
Few drugs around to fight viruses, most interfere with DNA, RNA, or protein synthesis

28. Figure 18.x1 Smallpox

29. Emerging Viruses HIV Ebola Influenza
From where do these viruses emerge?
From mutated versions of current viruses
Jump from current host to new host
Move from a previously isolated region of the world

30. Figure 18.8x Deer Mouse

31. SARS (severe acute respiratory syndrome), a recently emerging viral disease
(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

32. Viruses and Cancer Hepatitus B virus can cause liver cancer
Some viral genes can trigger cancerous genetic conditions
Oncogenes = viral genes that trigger cancerous characteristics
proto-oncogenes = genes already found in normal cells, usually regulate growth factors

33. Viroids and Prions Viroids are naked circular RNA that infect plants
Prions are proteins that infect cells
Examples of prions seen in Scrapies in sheep, mad-cow disease, and Creutzfeldt-Jakob disease in humans
How can a prion spread infection?
Altered versions of proteins that can alter other proteins

34. Figure 18.13 Model for how prions propagate
Normal protein
Original prion
New prion
Many prions

35. Figure 18.11 Replication of the bacterial chromosome

36. Figure 18.x7 E. coli

37. Figure 18.x8 E. coli dividing

38. Figure 16.1 Transformation of bacteria

39. Figure 18.x9 Bacterium releasing DNA with plasmids

40. Figure 18.x10 Plasmids

41. Figure 18.13 Transduction (Layer 4)

42. Figure 18.14 Bacterial mating

43. Figure 18.15 Conjugation and recombination in E. coli (Layer 4)