1. 18.1 Section Objectives – page 475
Identify the different kinds of viruses and their structures.
Compare and contrast the replication cycles of viruses.
18.1 Section Objectives – page 475

2. Section 18.1 Summary – pages 475-483
What is a virus?
You’ve probably had the flu—influenza—at some time during your life.
Nonliving particles called viruses cause influenza.
Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest bacterium.
Section 18.1 Summary – pages

3. Section 18.1 Summary – pages 475-483
What is a virus?
Most biologists consider viruses to be nonliving because they don’t exhibit all the criteria for life.
They don’t carry out respiration, grow, or develop. All viruses can do is replicate—make copies of themselves—and they can’t even do that without the help of living cells.
A cell in which a virus replicates is called the host cell.
Section 18.1 Summary – pages

4. Section 18.1 Summary – pages 475-483
What is a virus?
Viruses, such as rabies viruses and polioviruses, were named after the diseases they cause.
Other viruses were named for the organ or tissue they infect.
Section 18.1 Summary – pages

5. Section 18.1 Summary – pages 475-483
What is a virus?
Today, most viruses are given a genus name ending in the word “virus” and a species name.
However, sometimes scientists use code numbers to distinguish among similar viruses that infect the same host.
A virus that infects a bacterium is called a bacteriophage (bak TIHR ee uh fayj), or phage for short.
Section 18.1 Summary – pages

6. Section 18.1 Summary – pages 475-483
Viral Structure
A virus has an inner core of nucleic acid, either RNA or DNA, and an outer protein coat called a capsid.
Capsid
Nucleic
acid
Envelope
Section 18.1 Summary – pages

7. Section 18.1 Summary – pages 475-483
Viral Structure
Some relatively large viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.
Capsid
Nucleic
acid
Envelope
Section 18.1 Summary – pages

8. Section 18.1 Summary – pages 475-483
Viral Structure
Envelopes are composed primarily of the same materials found in the plasma membranes of all cells.
Capsid
Nucleic
acid
Envelope
Section 18.1 Summary – pages

9. Section 18.1 Summary – pages 475-483
Viral Structure
Nucleic acid
Viral nucleic acid is either DNA or RNA and contains instructions for making copies of the virus.
Capsid
Some viruses have only four genes, while others have hundreds.
Section 18.1 Summary – pages

10. Section 18.1 Summary – pages 475-483
Viral Structure
Nucleic acid
The tobacco mosaic virus has a long, narrow helical shape.
Capsid
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11. Section 18.1 Summary – pages 475-483
Viral Structure
The arrangement of proteins in the capsid of a virus determines the virus’s shape.
Capsid
Nucleic acid
Polyhedral viruses resemble small crystals.
Section 18.1 Summary – pages

12. Section 18.1 Summary – pages 475-483
Viral Structure
The protein arrangement also plays a role in determining what cell can be infected and how the virus infects the cell.
Capsid
Nucleic acid
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13. Section 18.1 Summary – pages 475-483
Attachment to a host cell
Before a virus can replicate, it must enter a host cell.
A virus recognizes and attaches to a host cell when one of its proteins interlocks with a molecular shape that is the receptor site on the host cell’s plasma membrane.
Section 18.1 Summary – pages

14. Section 18.1 Summary – pages 475-483
Attachment to a host cell
A protein in the tail fibers of the bacteriophage T4 recognizes and attaches the T4 to its bacterial host cell.
Capsid
Nucleic
acid
Tail
Tail fiber
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15. Section 18.1 Summary – pages 475-483
Attachment to a host cell
Capsid
In other viruses, the attachment protein is in the capsid or in the envelope.
Nucleic
acid
Tail
Tail fiber
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16. Section 18.1 Summary – pages 475-483
Attachment is a specific process
Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.
In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.
Section 18.1 Summary – pages

17. Section 18.1 Summary – pages 475-483
Viral Replication Cycles
Once attached to the plasma membrane of the host cell, the virus enters the cell and takes over its metabolism.
Only then can the virus replicate.
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18. Section 18.1 Summary – pages 475-483
Disease symptoms of proviruses
Having chicken pox, which usually occurs before age ten, gives lifelong protection from another infection by the virus. However, some chicken pox viruses may remain as proviruses in some of your body’s nerve cells.
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19. Section 18.1 Summary – pages 475-483
Disease symptoms of proviruses
Later in your life, these proviruses may enter a lytic cycle and cause a disease called shingles—a painful infection of some nerve cells.
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20. Section 18.1 Summary – pages 475-483
Release of viruses
Either lysis, the bursting of a cell, or exocytosis, the active transport process by which materials are expelled from a cell, release new viruses from the host cell.
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21. Section 18.1 Summary – pages 475-483
Release of viruses
In exocytosis, a newly produced virus approaches the inner surface of the host cell’s plasma membrane.
The plasma membrane surrounds the virus, enclosing it in a vacuole that then fuses with the host cell’s plasma membrane.
Then, the viruses are released to the outside.
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22. Section 18.1 Summary – pages 475-483
Retroviruses
Many viruses, such as the human immunodeficiency virus (HIV) that causes the disease AIDS, are RNA viruses—RNA being their only nucleic acid.
HIV virus
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23. Section 18.1 Summary – pages 475-483
Retroviruses
The RNA virus with the most complex replication cycle is the retrovirus
HIV virus
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24. Section 18.1 Summary – pages 475-483
Retroviruses
Once inside a host cell, the retrovirus makes DNA from its RNA.
To do this, it uses reverse transcriptase, an enzyme it carries inside its capsid.
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25. Section 18.1 Summary – pages 475-483
HIV: An infection of white blood cells
Once inside a human host, HIV infects white blood cells.
Normal white blood cells
Newly made viruses are released into the blood stream by exocytosis and infect other white blood cells.
Section 18.1 Summary – pages

26. Section 18.1 Summary – pages 475-483
HIV: An infection of white blood cells
Most people with an HIV infection eventually get AIDS because, over time, more white blood cells are infected and produce new viruses.
Because white blood cells are part of a body’s disease-fighting system, their destruction interferes with the body’s ability to protect itself from organisms that cause disease, a symptom of AIDS.
Section 18.1 Summary – pages

27. Section 18.1 Summary – pages 475-483
Plant viruses
The first virus to be identified was a plant virus, called tobacco mosaic virus, that causes disease in tobacco plants.
Tobacco mosaic virus causes yellow spots on tobacco leaves, making them unmarketable.
Section 18.1 Summary – pages

28. Question 1
Which of the following is NOT a reason that viruses are considered to be nonliving?
A. Viruses don’t replicate.
B. Viruses don’t respire.
C. Viruses don’t grow.
D. Viruses don’t develop.
The answer is A.
Section 1 Check

29. Question 2 Which is NOT a component of a virus? A. RNA B. capsid
C. DNA
D. phage
The answer is D.
Section 1 Check

30. Question 4
What two ways do viruses have of getting into host cells?
Answer
The virus can inject its nucleic acid into the host cell, or attach to the host cell’s membrane and become surrounded by the membrane and placed in a vacuole. The virus then bursts out of the vacuole and releases its nucleic acid into the cell.
IN: 1.20
Section 1 Check

31. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
There are three types of archaebacteria that live mainly in extreme habitats where there is usually no free oxygen available.
One type of archaebacterium lives in oxygen-free environments and produces methane gas.
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32. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
These methane-producing archaebacteria live in marshes, lake sediments, and the digestive tracts of some mammals, such as cows.
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33. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
They also are found at sewage disposal plants, where they play a role in the breakdown of sewage.
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34. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
A second type of archaebacterium lives only in water with high concentrations of salt.
Dead Sea
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35. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
A third type lives in the hot, acidic waters of sulfur springs.
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36. Section 18.2 Summary – pages 484-495
Archaebacteria: The extremists
This type of anaerobic archaebacterium also thrives near cracks deep in the ocean floor, where it is the autotrophic producer for a unique animal community’s food chain.
Section 18.2 Summary – pages

37. Section 18.2 Summary – pages 484-495
What is bacterium?
A bacterium consists of a very small cell.
Although tiny, a bacterial cell has all the structures necessary to carry out its life functions.
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38. Section 18.2 Summary – pages 484-495
The structure of bacteria
Prokaryotic cells have ribosomes, but their ribosomes are smaller than those of eukaryotes.
They also have genes that are located for the most part in a single circular chromosome, rather than in paired chromosomes.
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39. Section 18.2 Summary – pages 484-495
The structure of bacteria
Ribosome
Cytoplasm
Chromosome
Flagellum
Cell Membrane
Gelatinlike
capsule
Cell Wall
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40. Section 18.2 Summary – pages 484-495
A Typical Bacterial Cell
A typical bacterium, such as Escherichia coli would have some or all of the structures shown in this diagram of a bacterial cell.
Capsule
Cell Wall
Chromosome
Plasma membrane
Flagellum
Pilus
Plasmid
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41. Section 18.2 Summary – pages 484-495
The structure of bacteria
A bacterial cell remains intact as long as its cell wall is intact.
If the cell wall is damaged, water will enter the cell by osmosis, causing the cell to burst.
Scientists used a bacterium’s need for an intact cell wall to develop a weapon against bacteria that cause disease.
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42. Section 18.2 Summary – pages 484-495
The structure of bacteria
In 1928, Sir Alexander Fleming accidentally discovered penicillin, the first antibiotic—a substance that destroys bacteria—used in humans.
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43. Section 18.2 Summary – pages 484-495
The structure of bacteria
Later, biologists discovered that penicillin can interfere with the ability of some bacteria to make cell walls.
When such bacteria grow in penicillin, holes develop in their cell walls, water enters their cells, and they rupture and die.
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44. Section 18.2 Summary – pages 484-495
Identifying bacteria
Bacterial cell walls also give bacteria different shapes.
Shape is another way to categorize bacteria.
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45. Section 18.2 Summary – pages 484-495
Identifying bacteria
The three most common shapes are spheres, called coccus; rods, called bacillus; and spirals, called spirillum.
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46. Section 18.2 Summary – pages 484-495
Reproduction by binary fission
Bacteria reproduce asexually by a process known as binary fission.
To reproduce in this way, a bacterium first copies its chromosome. Then the original chromosome and the copy become attached to the cell’s plasma membrane for a while.
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47. Section 18.2 Summary – pages 484-495
Reproduction by binary fission
The cell grows larger, and eventually the two chromosomes separate and move to opposite ends of the cell.
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48. Section 18.2 Summary – pages 484-495
Reproduction by binary fission
Then, a partition forms between the chromosomes. This partition separates the cell into two similar cells.
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49. Section 18.2 Summary – pages 484-495
Reproduction by binary fission
Because each new cell has either the original or the copy of the chromosome, the resulting cells are genetically identical.
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50. Section 18.2 Summary – pages 484-495
Reproduction by binary fission
Under ideal conditions, some bacteria can reproduce every 20 minutes, producing enormous numbers of bacteria quickly.
But bacteria don’t always have ideal growing conditions. They run out of nutrients and water, they poison themselves with their own wastes, and predators eat them.
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51. Section 18.2 Summary – pages 484-495
The Importance of Bacteria
Disease-causing bacteria are few compared with the number of harmless and beneficial bacteria on Earth.
Bacteria help to fertilize fields, to recycle nutrients on Earth, and to produce foods and medicines.
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52. Section 18.2 Summary – pages 484-495
Nitrogen fixation
All organisms need nitrogen because the element is a component of their proteins, DNA, RNA, and ATP.
Yet few organisms, including most plants, can directly use nitrogen from the air.
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53. Section 18.2 Summary – pages 484-495
Nitrogen fixation
Several species of bacteria have enzymes that convert N2 into ammonia (NH3) in a process known as nitrogen fixation.
Other bacteria then convert the ammonia into nitrite (NO2–) and nitrate (NO3–),which plants can use.
Bacteria are the only organisms that can perform these chemical changes.
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54. Section 18.2 Summary – pages 484-495
Nitrogen fixation
Some nitrogen- fixing bacteria live symbiotically within the roots of some trees and legumes.
Farmers grow legume crops after the harvesting of crops such as corn, which depletes the soil of nitrogen.
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55. Section 18.2 Summary – pages 484-495
Recycling of nutrients
Autotrophic bacteria and also plants and algae, which are at the bottom of the food chains, use the nutrients in the food they make.
This food is passed from one heterotroph to the next in food chains and webs.
In the process of making food, many autotrophs replenish the supply of oxygen in the atmosphere.
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56. Section 18.2 Summary – pages 484-495
Food and medicines
Some foods that you eat—mellow Swiss cheese, crispy pickles, tangy yogurt— would not exist without bacteria.
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57. Section 18.2 Summary – pages 484-495
Food and medicines
Specific bacteria are used to make different foods, such as vinegar, cheeses, and sauerkraut.
Bacteria also inhabit your intestines and produce vitamins and enzymes that help digest food.
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58. Section 18.2 Summary – pages 484-495
Food and medicines
In addition to food, some bacteria produce important antibiotics that destroy other types of bacteria.
Streptomycin, erythromycin, bacitracin, and neomycin are some of these antibiotics.
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59. Section 18.2 Summary – pages 484-495
Bacteria cause disease
Diseases Caused by Bacteria
Disease
Transmission
Symptoms
Treatment
Strep throat
(Streptococcus)
Inhale or
ingest through
mouth
Fever, sore throat,
swollen neck glands
Antibiotic
Tuberculosis
Inhale
Fatigue, fever, night
sweats, cough, weight
loss, chest pain
Antibiotic
Tetanus
Puncture
wound
Stiff jaw, muscle
spasms, paralysis
Open and clean wound,
antibiotic; give antitoxin
Lyme disease
Bite of
infected tick
Rash at site of bite,
chills, body aches,
joint swelling
Antibiotic
Dental
cavities (caries)
Bacteria
in mouth
Destruction of tooth
enamel, toothache
Remove and fill the
destroyed area of tooth
Inhale or
close contact
Sore throat, fever,
heart or breathing
failure
Vaccination to prevent, antibiotics
Diptheria
Section 18.2 Summary – pages

60. Question 2
What part of a bacterial cell is most affected by penicillin?
A. pilus
B. plasmid
C. flagellum
D. cell wall
IN: 1.20
Section 2 Check

61. The answer is D, cell wall.
IN: 1.20
Section 2 Check

62. Question 5 What is a pilus used for in a bacterium? IN: 1.12
Section 2 Check

63. Question 8
What causes anthrax?
Chapter Assessment

64. Question 9
Describe the process in which bacteria make nitrogen in the air accessible for use by plants.
Chapter Assessment

65. Answer
Several species of bacteria have enzymes that convert nitrogen gas into ammonia. Other bacteria then convert the ammonia into nitrite and nitrate that plants can use.
Chapter Assessment