VirusesViruses16.516.5

Why are viruses considered Why are viruses considered non-living?non-living?

• Do they have organelles?• Do they carry out life processes?

– Grow, take in food, make waste?– How do they reproduce?

• Are they cells?

Viruses and bacteria can cause Viruses and bacteria can cause infection.infection.

• Any disease-causing agent is called a pathogen.

• Let’s talk size-– viruses are 100x smaller than bacterial cells.

viruses50-200 nm

prokaryotics cells200-10,000 nm

prion2-10 nm

viroids5-150 nm

eukaryotics cells10,000-100,000 nm

100 nm

1 nanometer (nm) = one billionth of a meter

Virus classificationVirus classification• Type of nucleic acid

– DNA or RNA but not both

• Their shape– Based on the structure of the capsid- outer protein coat.– Two basic shapes-

• Rod• Spherical

• How they reproduce– Lytic ( lysis) cycle– Lysogenic ( latent) cycle

• What they infect-– Organisms/species– Type of cells

Viruses differ in shape and Viruses differ in shape and how they enter host cells. how they enter host cells. • Viruses have a simple structure.

– genetic material- DNA or RNA – Capsid- outer protein shell – maybe a lipid envelope, a protective outer coat

capsid nucleic acid

lipidenvelope

surfaceproteins

capsid

nucleic acid

lipid envelope

Surface proteins capsidsurfaceproteins

nucleic acid

helical(rabies)

polyhedral(foot-and-mouth

disease)

enveloped(influenza)

Basic Viral Structure- T-phageBasic Viral Structure- T-phageBacteriophage- virus that Bacteriophage- virus that

infects bacteriainfects bacteria

capsid

DNA

tail sheath

tail fiber

Viruses enter cells in various ways:Viruses enter cells in various ways: bacteriophages pierce host cells- bacteriophages pierce host cells- injecting their DNAinjecting their DNA

colored SEM; magnifications:large photo 25,000; inset 38,000x

Viruses of eukaryotes -can fuse with Viruses of eukaryotes -can fuse with cell membranes- endocytosiscell membranes- endocytosis

host bacterium

The bacterophage attachesand injects it DNA into a host bacterium.

The host bacterium breaks apart, or lyses. Bacteriophages are ableto infect new host cells.

The viral DNA directs the hostcell to produce new viral parts.The parts assemble into newbacteriophages.

The viral DNA forms a circle.

Viruses cause two types of Viruses cause two types of infectionsinfections. .

• A lytic infection causes the host cell to burst.

The virus may enter thelysogenic cycle, in which thehost cell is not destroyed.

A lysogenic infection does no immediate harm- latent infection- examples: herpes, HIV

The viral DNA ( a prophage) combines withthe host cell’s DNA.

Although the prophage is notactive, it replicates along withthe host cell’s DNA.

Many cell divisions produce acolony of bacteria infectedwith prophage.

At some point-the prophage leaves the host’s DNA and enter the lytic cycle.

Viruses such as a bacteriophage are Viruses such as a bacteriophage are capable of reproducing in two general capable of reproducing in two general ways, the lytic and lysogenic cycles.ways, the lytic and lysogenic cycles.

.

Viruses cause many Viruses cause many infectious diseases infectious diseases

• There are many examples of viral infections.– common cold

– influenza

Viruses cause many Viruses cause many infectious diseases infectious diseases

• There are many examples of viral infections.– common cold

– influenza

Viruses cause many Viruses cause many infectious diseases infectious diseases

• There are many examples of viral infections.– common cold– – Sars

– HIV

HIV-infected whiteblood cell

Viruses cause many Viruses cause many infectious diseases infectious diseases

• There are many examples of viral infections.

• The body has natural defenses against viruses.

HIV-AIDSHIV-AIDS• HIV =virus

– Human Immunodeficiency Virus

• AIDS= disease-– Acquired Immune Deficiency Syndrome

• An immune disease- immune cells attacked- T4 white blood cells.

• Symptoms (damage of host immune cells) occurs when a switch from the lysogenic cycle to the lytic cycle occurs.

– 5-10 years later.

• HIV- retrovirus-RNA virus– Flow: RNA-DNA-RNA– How: they have reverse transcriptase

• An enzymes that synthesizes DNA from RNA

HIV HIV HIV, a retrovirus, uses immune system cells to HIV, a retrovirus, uses immune system cells to

reproduce itself. These host cells are eventually reproduce itself. These host cells are eventually destroyed, weakening the patient's immune system.destroyed, weakening the patient's immune system.

•

Prevention- Prevention- Antibiotics have no effectAntibiotics have no effect• Vaccines can be effective-

Deactivated varieties or small pieces of pathogens that stimulate the immune system to respond by producing a memory response when the actual pathogen is met.

• Edward Jenner developed the first vaccine (cowpox) against smallpox.

• Not all viruses can be prevented with a vaccine because of the rapid mutation rates of these viruses.

Edward Jenner- Edward Jenner- Development of smallpox virus schemeDevelopment of smallpox virus scheme..It worked because smallpox and cowpox are It worked because smallpox and cowpox are very similar –same antigens( cause an very similar –same antigens( cause an antibody response from the body.antibody response from the body.1979- smallpox was eradicated1979- smallpox was eradicated

•

Vaccines are made from weakened Vaccines are made from weakened

(attenuated) pathogens.(attenuated) pathogens. • A vaccine stimulates the body’s own immune

response. • Vaccines prepare the immune system for a future

attack.- memory response

• Vaccines are the only way to control the spread of viral disease.

• What is herd mentality?

Antivirals?Antivirals?• What are they?