KEY CONCEPT Infections can be caused in several ways.

Viruses and Prokaryotes Chapter 18

How are infections causes?

Viruses, bacteria, viroids, and prions can all cause infection. Any disease-causing agent is called a pathogen. 1 nanometer (nm) = one billionth of a meter 100 nm eukaryotics cells 10,000-100,000 nm viroids 5-150 nm viruses 50-200 nm prokaryotics cells 200-10,000 nm prion 2-10 nm

A virus is made of DNA or RNA and a protein coat. non-living pathogen can infect many organisms A viroid is made only of single-stranded RNA. causes disease in plants passed through seeds or pollen

A prion is made only of proteins. causes misfolding of other proteins results in diseases of the brain

Think back about computer viruses… At what point does the virus become dangerous?

How do viruses enter the human populations??

viruses can only replicate after they have infected host cells Packaged genes that move from host to host

Viruses differ in shape and in ways of entering host cells. Viruses have a simple structure. genetic material capsid, a protein shell maybe a lipid envelope, a protective outer coat polyhedral (foot-and-mouth disease) enveloped (influenza) helical (rabies) capsid nucleic acid lipid envelope surface proteins capsid surface proteins nucleic acid capsid nucleic acid lipid envelope Surface proteins

Bacteriophages infect bacteria. capsid DNA tail sheath tail fiber

Viruses enter cells in various ways.

Flu video

bacteriophages pierce host cells colored SEM; magnifications: large photo 25,000; inset 38,000x

Viruses of eukaryotes also fuse with membrane

Viruses cause two types of infections. A lytic infection causes the host cell to burst. host bacterium The bacterophage attaches and injects it DNA into a host bacterium. The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The viral DNA forms a circle. The virus may enter the lysogenic cycle, in which the host cell is not destroyed.

A lysogenic infection does no immediate harm. The viral DNA is called a prophage when it combines with the host cell’s DNA. Although the prophage is not active, it replicates along with Many cell divisions produce a colony of bacteria infected with prophage. The prophage may leave the host’s DNA and enter the lytic cycle.

What is the difference between getting a vaccination shot and taking an antibiotic?

Viruses cause many infectious diseases

There are many examples of viral infections. common cold Over 200 known types

Influenza (FLU)

SARS Severe acute Respiratory syndrome

HIV Causes AIDS HIV-infected white blood cell

Some viruses like the flu can spread very quickly and cause the and epidemic Rapid outbreak More than 20 % of the population is infected Viruses usually infect humans but their have also been viruses that have infected pigs, birds, whales horses and seals Some of these have jumped species (Bird, swine

How do we protect ourselves?

The body has natural defenses against viruses. Skin Outer skeleton Cell wall Viruses can penetrate the skin through and opening Once inside the body the virus finds a target cell, the cell “welcomes” it in

Vaccines – stimulates immune system Made of the weakened or parts of the virus Only way of preventing the spread of viral infections