Biology Slide 1 of 34

(1)

Slide 1 of 34

Copyright Pearson Prentice Hall

Biology

(2)

Slide 2 of 34

19–2 Viruses

(3)

19–2 Viruses

Slide 3 of 34

What Is a Virus?

What Is a Virus?

Viruses are particles of nucleic acid, protein, and in some cases, lipids.

Viruses can reproduce only by infecting living cells.

(4)

19–2 Viruses

Slide 4 of 34

Viruses differ widely in terms of size and structure.

All viruses enter living cells and use the infected cell to produce more viruses.

(5)

19–2 Viruses

Slide 5 of 34

What is the structure of a virus?

(6)

19–2 Viruses

Slide 6 of 34

Head Tail sheath

DNA

T4 Bacteriophage Tobacco Mosaic Virus

Influenza Virus

RNA

Membrane envelope Tail

fiber

RNA

Capsid proteins

Capsid

Surface proteins

(7)

19–2 Viruses

Slide 7 of 34

A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.

A capsid is the virus’s protein coat.

(8)

19–2 Viruses

Slide 8 of 34

Capsid proteins bind to receptors on the cell surface and “trick” the cell into allowing it inside.

Once inside, viral genes are expressed and the cell transcribes and translates them into viral capsid

proteins.

The host cell may makes copies of the virus, and be destroyed.

(9)

19–2 Viruses

Slide 9 of 34

Most viruses are highly specific to the cells they infect.

Viruses that infect bacteria are called bacteriophages.

(10)

19–2 Viruses

Slide 10 of 34

Viral Infection

How do viruses cause infection?

(11)

19–2 Viruses

Slide 11 of 34

Viral Infection

Viral Infection

Once the virus is inside the host cell, two different processes may occur.

•

Some viruses replicate immediately, killing the host cell.

•

Others replicate, but do not kill the host cell immediately.

(12)

19-2 Viruses

Slide 12 of 34

Viral Infection

Bacteriophage injects DNA into bacterium

Bacteriophage DNA forms a circle

Lytic Infection Lysogenic Infection

(13)

19–2 Viruses

Slide 13 of 34

Viral Infection

Lytic Infection

In a lytic infection, a virus enters a cell,

makes copies of itself, and causes the cell to burst.

(14)

19–2 Viruses

Slide 14 of 34

Viral Infection

First, the bacteriophage injects DNA into a bacterium.

The bacteriophage DNA forms a circle.

(15)

19–2 Viruses

Slide 15 of 34

Viral Infection

(16)

19–2 Viruses

Slide 16 of 34

Viral Infection

(17)

19–2 Viruses

Slide 17 of 34

Viral Infection

(18)

19–2 Viruses

Slide 18 of 34

Viral Infection

Lysogenic Infection

Other viruses cause lysogenic infections in which a host cell makes copies of the virus indefinitely.

In a lysogenic infection, a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates

along with the host cell's DNA.

(19)

19–2 Viruses

Slide 19 of 34

Viral Infection

A lysogenic infection begins the same way as a lytic infection.

The bacteriophage injects DNA into a bacterium.

The bacteriophage DNA forms a circle.

The viral DNA embedded in the host's DNA is called a prophage.

(20)

19–2 Viruses

Slide 20 of 34

Viral Infection

(21)

19–2 Viruses

Slide 21 of 34

Viral Infection

(22)

19–2 Viruses

Slide 22 of 34

Viral Infection

(23)

19–2 Viruses

Slide 23 of 34

Viral Infection

(24)

19–2 Viruses

Slide 24 of 34

Retroviruses

Retroviruses

Retroviruses contain RNA as their genetic information.

When retroviruses infect cells, they make a DNA copy of their RNA.

This DNA is inserted into the DNA of the host cell.

(25)

19–2 Viruses

Slide 25 of 34

Retroviruses

A retrovirus’ genetic information is copied backward—from RNA to DNA.

The virus that causes AIDS is a retrovirus.

(26)

19–2 Viruses

Slide 26 of 34

Viruses and Living Cells

Viruses and Living Cells

Viruses must infect a living cell in order to grow and reproduce.

They take advantage of the host’s respiration, nutrition, and all other functions of living things.

(27)

19–2 Viruses

Slide 27 of 34

Viruses have many of the characteristics of living things.

After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.

(28)

19–2 Viruses

Slide 28 of 34

Because viruses are dependent on living things, it seems likely that viruses developed after living cells.

The first viruses may have evolved from genetic material of living cells.

Viruses have continued to evolve over billions of years.

(29)

- or -

Continue to: Click to Launch:

Slide 29 of 34

19–2

(30)

Slide 30 of 34

19–2

Viruses that contain RNA as their genetic information are known as

a. prions.

b. oncoviruses.

c. retroviruses.

d. bacteriophage.

(31)

Slide 31 of 34

19–2

The first type of virus to be studied was the a. bacteriophage.

b. tobacco mosaic virus.

c. influenza virus.

d. AIDS virus.

(32)

Slide 32 of 34

19–2

Which of the following statements about viruses is true?

a. Viruses appear similar to bacteria when studied with a light microscope.

b. Viruses display the essential characteristics of living things.

c. Viruses can reproduce independently if they contain DNA.

d. Viruses cannot reproduce unless they infect a living cell.

(33)

Slide 33 of 34

19–2

A virus integrates its DNA into the DNA of the host cell but remains inactive for a while in

a. a lytic infection.

b. a lysogenic infection.

c. neither a lytic nor a lysogenic infection.

d. retroviral infection.

(34)

Slide 34 of 34

19–2

Retroviruses are considered unique because a. they have RNA in their capsid and not DNA.

b. they have DNA in their capsid and not RNA.

c. after infection of a host cell, their RNA makes DNA.

d. after infection of a host cell, their DNA makes RNA.

(35)

Biology Slide 1 of 34

19–2 Viruses

What Is a Virus?

Viral Infection

•

•

Retroviruses

Viruses and Living Cells

END OF SECTION