1. Lesson Overview
20.1 Viruses

2. THINK ABOUT IT Imagine that farmers have begun to
lose their tobacco crop to a plant disease
that first appears as a yellowing of the
leaves.
To determine what is causing the disease,
you take leaves from a diseased plant and
crush them to produce a liquid extract. You place a few drops of that liquid
on the leaves of healthy plants. A few days later,the leaves turn yellow
where you placed the drops.
The liquid contains disease-causing agents so small that they are not
visible under a microscope and can pass right through a filter.
What would you do next? How would you deal with the invisible?

3. Discovery of Viruses
In 1892, Dmitri Ivanovski, a Russion biologist demonstrated that the cause of this disease was found in the liquid extracted from infected plants. This plant disease is called tobacco mosaic disease.
In 1897, Dutch scientist Martinus Beijerinck suggested that tiny particles in the juice caused the disease, and he named these particles viruses, after the Latin word for “poison.”

4. Discovery of Viruses
In 1935, the American biochemist, Wendell Stanley isolated crystals of tobacco mosaic virus. Since living organisms do not crystallize, Stanley inferred that viruses were not truly alive. This is a conclusion that biologists still recognize as being valid today.

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

6. 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.

7. Structure and Composition
The protein coat surrounding a virus is called a capsid.
Some viruses, such as the influenza virus, have an additional membrane that surrounds the capsid. It is called an envelope.
The simplest viruses contain only a few genes, whereas the most complex may have more than a hundred genes.

8. Structure and Composition
To enter a host cell, 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.

9. Structure and Composition
Because viruses must bind precisely to proteins on the host cell surface and then use the host’s genetic system, 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 which literally means “bacteria eaters.”

10. Chicken pox virus HIV virus

11. Viral Infections
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.
These two patterns of infections are called lytic and lysogenic infection.

12. 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.
It has a DNA core inside a protein
capsid that binds to the surface
of a host cell.

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

14. Lytic Infections
The cell then begins to make messenger RNA (mRNA) from the viral genes.

15. Lytic Infections
The viral mRNA is translated into viral proteins that chop up the cell’s DNA.
Controlled by viral genes, the host cell’s metabolic system makes copies of viral nucleic acid and capsid proteins.

16. Lytic Infections
The viral nucleic acid and capsid proteins are then assembled into new virus particles.

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

18. Lysogenic Infection
Some bacterial viruses cause a lysogenic infection, in which a host cell is not immediately taken over.

19. Lysogenic Infection
Instead, the viral nucleic acid is inserted into the host cell’s DNA, where it is copied along with the host DNA without damaging the host.

20. Lysogenic Infection
Viral DNA multiplies as the host cells multiply. In this way, each generation of daughter cells derived from the original host cell is infected.

21. Lysogenic Infection
Bacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage. The prophage may remain part of the DNA of the host cell for many generations.

22. Lysogenic Infection
Influences from the environment—radiation, heat, etc—trigger the
prophage to become active.
It then removes itself from the host cell DNA, directs the synthesis of new
virus particles, and now becomes an active lytic infection.

23. Lytic vs. Lysogenic Cycle

24. 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.

25. 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.

26. 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.

27. 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.

28. HIV When a retrovirus infects a cell, it makes a DNA copy of its RNA.
The copy inserts itself into the DNA of the host cell.

29. 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. Once activated, it begins to destroy the very system of the body that would normally fight infection.

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

31. Viruses and Cells
Despite the fact that they are not alive, viruses have many of the
characteristics of living things.
After infecting living cells,
viruses can reproduce,
regulate gene expression, and
even evolve.

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

33. 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.