Viruses

What are they?How do they work?

Where do they come from?And… What good are they?

Non-Living Infectious Agents

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Non-viral Infectious Agents

• Viroids- The smallest known particle that can replicate.– Single Strand of RNA– Disrupt plant cell metabolism– Can destroy entire crops

• Prions – Abnormal forms of proteins that clump together inside cells.– Clumping eventually

kills the cell– Example

• Mad Cow Disease

Overview of Viruses• Nonliving• Composed of Nucleic acid and protein• Cause many diseases• Virology – Study of Viruses• Comparison of Viruses and Cells below

Char. Of Life Virus Cell

Growth No Yes

Homeostasis No Yes

Metabolism No Yes

Mutation Yes Yes

Nucleic acid DNA or RNA DNA

Reproduction Only within host cell Independently by cell division

Structure Nucleic acid core, protein covering, some have envelope

Cytoplasm, cell membrane, etc..

Characteristics of Viruses• 2 essential features• 1. Nucleic Acid

– May be DNA or RNA– Helical, closed loop, or long strand

• 2. Protein Coat – called CAPSID• Some have ENVELOPE

– Ex. Influenza, chickepox, herpes simplex, HIV• VIRAL SHAPE

– Icosahedron – 20 triangular faces• Ex.) herpes, chickenpox, polio

– Helix – Coiled spring• EX.) Rabies, measles, tobacco mosaic

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• Bacteriophages: like tiny little syringes that inject DNA from the virus into the cell

Lytic Cycle

BacteriophageHost cell DNA

1. Attachment- Virus finds host cell

2. Entry- Viral DNA injected into host

3. Replication- Host makes viral DNA & parts

4. Assembly- New viruses made

5. Lysis & Release- viruses burst out of host (dies); Find new host

Here is a non-enveloped bacteria virus inserting it’s DNA into a bacterial cell.

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Some virus are pushed out by the cell, taking some of the cell membrane with

them.

Lysogenic Cycle• After Step #2 of the Lytic Cycle, the

Virus may become Dormant and enter into the Lysogenic cycle

• Dormant- non-active time of virus– Host is infected, but doesn’t know it– Ex: Herpes (cold sores) Person doesn’t

have them all the time, but has “outbreaks” throughout year

– Ex: HIV Mono

Provirus

From Step #2 Lytic cycle, enters Lysogenic cycle here

2. Entry- Viral DNA injected into host

3. Integration - Viral DNA becomes part of host DNA

4. Cell Multiplication – Host replicates with viral DNA

Many cell divisions

5. Cell enters back into Lytic Cycle- Assemble, Lysis & Release

Lysogenic Cycle

Classification of Viruses 1. By Shape 2. Host type 3. function

Retroviruses attack a certain way.

DNA viruses attack another way.Animal viruses

Plant viruses

Bacteria viruses

Viruses and Human Disease• Control and Prevention of spread.

– Vaccination & Antiviral drugs• Ex.) chickenpox vaccine, AZT, Acyclovir, protease

inhibitors.

• Emerging Viruses – exist in isolated habitats– Do not usually infect humans unless

environmental conditions favor contact.

• Several viruses are now linked to cancers such as leukemia, liver cancer, Burkitt’s lymphoma, cervical cancer.

English physician and scientist from Berkeley, Gloucestershire, who was the pioneer of smallpox vaccine

• Cowpox and smallpox are closely related, but cowpox is a much less serious disease than smallpox.

• Jenner noticed milkmaids were immune to smallpox.

• He used puss from a cowpox blister to inocculate an 8 year old boy

• He called his method “vaccination,” from the Latin word vacca, or cow, and today we use vaccination to refer to immunization against any disease.

• A vaccine contains a killed/weakened part of a germ.

• When a person receives a vaccine, the body reacts by making protective antibodies.

Wendell M. Stanley• Wendell M. Stanley

crystallized and described the molecular structure of the tobacco mosaic virus.

PAPOVAVIRUSES: Tumor causing viruses

Most common: Human papillomavirus (HPV) (warts)

ADENOVIRUSES: causes respiratory issues and pink eye

HERPESVIRUSES: easily transmitted by direct contact with a lesion or the body fluid of an

infected individual

POXVIRUSES

PICORNAVIRUSESThe name is derived from pico,

meaning small, and RNA, referring to the ribonucleic acid genome, so "pico-rna-virus" literally means small RNA

virus.Ranges from the common cold to

polio!

MYXOVIRUSES

RHABDOVIRUSES

RETROVIRUSES: genes are encoded in RNA instead of DNA

Here is a classic picture of HIV viral progeny being released from the surface of a T- cell. Notice the membrane coating they receive.

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Ebola Virus

• Some therapies that have worked have been when patients receive blood from convalescent patients

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