1. Lesson Overview
20.1 Viruses

2. Discovery of Viruses
A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids.
Viruses can reproduce only by infecting living cells.

3. Structure and Composition
Viruses differ widely in terms of size and structure.
Most viruses are so small they can be seen only with the aid of a powerful electron microscope.

4. Structure and Composition
Most viruses have proteins on their surface membrane or capsid that bind to receptor proteins on the host cell.
The proteins “trick” the cell to take the virus, or in some cases just its genetic material, into the cell.
Once inside, the viral genes are eventually expressed and may destroy the cell.

5. Structure and Composition
Most viruses infect only a very specific kind of cell.
Plant viruses infect plant cells.
Most animal viruses infect only certain related species of animals.
viruses that infect bacteria are called bacteriophages.

6. Viral Infections What happens after a virus infects a cell?
Inside living cells, viruses use their genetic information to make
multiple copies of themselves.
Some viruses replicate immediately, while others initially
persist in an inactive state within the host.

7. Types of Viral Infections
Lytic infection
Lysogenic Infection

8. Lytic Infections
In a lytic infection, a virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse.
Bacteriophage T4 is an example of a bacteriophage that causes such an infection.

9. Lytic Infections
Bacteriophage T4 has a DNA core inside a protein capsid that binds to the surface of a host cell.

10. Lytic Infections
The virus injects its DNA into the cell.

11. Lytic Infections
Controlled by viral genes, the host cell’s metabolic system makes copies of viral nucleic acid.

12. Lytic Infections
The host cell lyses, releasing hundreds of virus particles that go on to infect other cells.

13. Lytic Infections
A lytic virus is similar to an outlaw in the Wild West of the American frontier in the demands the virus makes on its host.
First, the outlaw eliminates the town’s existing authority.
In a lytic infection, the host cell’s DNA is chopped up.

14. Lytic Infections
Next, the outlaw demands to be outfitted with new equipment from the local townspeople.
In a lytic infection, the viruses use the host cell to make viral DNA and viral proteins.

15. Lytic Infections
Finally, the outlaw forms a gang that leaves the town to attack new communities.
In a lytic infection, the host cell bursts, releasing hundreds of virus particles.

16. Lysogenic Infection
Some bacterial viruses cause a lysogenic infection.

17. Lysogenic Infection
In a lysogenic infection a host cell is not immediately taken over.

18. Lysogenic Infection
The viral nucleic acid is inserted into the host cell’s DNA.

19. Lysogenic Infection
The viral DNA is then copied along with the host DNA without damaging the host.

20. Lysogenic Infection
Viral DNA multiplies as the host cells multiply.

21. Lysogenic Infection
In this way, each generation of daughter cells derived from the original host cell is infected.

22. Lysogenic Infection
Bacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage.

23. Lysogenic Infection
The prophage may remain part of the DNA of the host cell for many generations.

24. Lysogenic Infection
Influences from the environment—radiation, heat, etc—trigger the prophage to become active.

25. Lysogenic Infection
It then removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.

26. A Closer Look at Two RNA Viruses
About 70 percent of viruses contain RNA rather than DNA.
In humans, RNA viruses cause a wide range of infections, from relatively mild colds to severe cases of HIV.
Certain kinds of cancer also begin with an infection by viral RNA.

27. The Common Cold
Cold viruses attack with a very simple, fast-acting infection.
A capsid settles on a cell, typically in the host’s nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.

28. The Common Cold
The host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it into capsids and other viral proteins.
The new capsids assemble around the viral RNA copies, and within 8 hours, the host cell releases hundreds of new virus particles to infect other cells.

29. Common Cold Video- How the Flu Virus infects
A virus infects a host cell and then uses it to make more viruses
The viruses then infect more cells, until the host is weakened or killed
Here is a video that explains how a common cold virus infects cells

30. HIV
The deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV).
HIV belongs to a group of RNA viruses that are called retroviruses.
The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.

31. HIV
When a retrovirus infects a cell, it makes a DNA copy of its RNA.

32. HIV
The copy inserts itself into the DNA of the host cell.

33. How HIV Infects the Cell Video
HIV virus attacks a cell and then uses it to make more viruses

34. HIV
Retroviral infections are similar to lysogenic infections of bacteria. Much like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.

35. Viruses and Cells
All viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process.
Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts.

36. Viruses and Cells
Viruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.

37. Viruses and Cells
Some of the main differences between cells and viruses are summarized in this chart.

38. Viruses and Cells
Although viruses are smaller and simpler than the smallest cells, it is unlikely that they were the first living organisms.
Because viruses are dependent upon living organisms, it seems more likely that viruses developed after living cells.
The first viruses may have evolved from the genetic material of living cells. Viruses have continued to evolve, along with the cells they infect, for billions of years.