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Viruses

Chapter 20

I. Origin

A. Evolution of viruses is difficult to prove due to lack of fossils.

II. Virus--Latin for poison

III. Nonliving--can reproduce only when they invade a host cell.

IV. Comparative Size25-300 nanometers

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V. Shapes

A. Bacteriophage

B.Polyhedron

C. Rod

VI. Structure

A. Nucleic acid--either DNA or RNA (retrovirus)

B. Protein coat called a capsid

C. Often have membrane envelopes derived from the host cell exceptionadenoviruses have no membrane envelope, infect animals.

D. Some have enzymes, especially retroviruses

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VII. Life Cycles

A. Lytic

1. Attachment to cell; often specificity caused by cell surface markers.

2. Penetration, invasion

3. Synthesis of protein coats- transcription, translation, and then replication by host DNA.

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4. Assembly of the virusDNA core is wrapped in the protein coat

5. Release from the host cell either by - -lysis (fever blisters) or budding

(HIV)

6. Virionsnewly released viruses existing outside the cell ready to

invade other cells

B. Lysogenic

1. Delayed lysis

2. Attaches to host chromosome and gets duplicated every time the host cell divides.

a. Some become retroelements or genes with a viral origin.

b. Some of human DNA is composed of old viral DNA and is inactive.

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3. Eventually something triggers the viral DNA to break off from the host DNAcould be stress, time similar to an incubation period, or an environmental trigger.

4. Enters the lytic phase

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VIII. Effect on host cell

A. Cell and tissue destruction

B. Some may trigger cancer

C. Destroys cell function

D. Some are harmless

Destructive Plant Virus

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Breeders of tulips in

Holland several

hundreds years ago

discovered this color

variation. It became

one of the most

expensive tulip

colors. The color is

produced by a viral

infection.

IX. Why are antibiotics or drugs so difficult to develop that are effective for viruses?

A. Viral proteins mutate rapidly.

B. Virus is inside the cell.

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X. Diseases

A. Measles (R) (child with 7 day rash)

B. AIDS

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D. Smallpox (D)

C. Chicken pox and shingles (D)

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E. Herpes Simplex 1--fever blisters (D)

F. Herpes Simplex 2--genital herpes- incurable (D)

Warts and sores can develop on the labia, penis, and anal area.

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G. Influenza (R)

H. Cold (R)

I. Mumps (R)

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J. Poliomyelitis (R)

K. Rabies (R)

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L. Infectious Mononucleosis (D)

M. Viral Pneumonia (R)

N. Hepatitis (D)

O. Warts (D)

P. West Nile (R)