Presentation on theme: "KEY CONCEPT Infections can be caused in several ways."— Presentation transcript:
1 KEY CONCEPT Infections can be caused in several ways.
2 Viruses, bacteria, viroids, and prions can all cause infection. Any disease-causing agent is called a pathogen.1 nanometer (nm) = one billionth of a meter100 nmeukaryotics cells 10, ,000 nmviroids nmviruses nmprokaryotics cells ,000 nmprion 2-10 nm
3 A virus is made of DNA or RNA and a protein coat. non-living pathogencan infect many organismsA viroid is made only of single-stranded RNA.causes disease in plantspassed through seeds or pollen
4 A prion is made only of proteins. causes misfolding of other proteinsresults in diseases of the brain
5 KEY CONCEPT Viruses exist in a variety of shapes and sizes.
6 Viruses differ in shape and in ways of entering host cells. Viruses have a simple structure.genetic materialcapsid, a protein shellmaybe a lipid envelope, a protective outer coatenveloped (influenza)helical (rabies)polyhedral (foot-and-mouthdisease)capsidnucleic acidlipid envelopesurface proteinscapsidsurface proteinsnucleic acidcapsidnucleic acidlipid envelopeSurface proteins
8 Viruses enter cells in various ways. bacteriophages pierce host cellscolored SEM; magnifications:large photo 25,000; inset 38,000x
9 Viruses enter cells in various ways. viruses of eukaryotes enter by endocytosis
10 Viruses enter cells in various ways. viruses of eukaryotes also fuse with membrane
11 Viruses cause two types of infections. A lytic infection causes the host cell to burst.host bacteriumThe 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 thelysogenic cycle, in which thehost cell is not destroyed.
12 A lysogenic infection does no immediate harm. The viral DNA is called a prophagewhen it combines withthe host cell’s DNA.Although the prophage is notactive, it replicates along withMany cell divisions produce acolony of bacteria infectedwith prophage.The prophage may leave thehost’s DNA and enter thelytic cycle.
13 KEY CONCEPT Some viral diseases can be prevented with vaccines.
14 Viruses cause many infectious diseases There are many examples of viral infections.common cold
15 Viruses cause many infectious diseases There are many examples of viral infections.common coldinfluenza
16 Viruses cause many infectious diseases There are many examples of viral infections.common coldSARSinfluenza
17 Viruses cause many infectious diseases There are many examples of viral infections.HIV-infected whiteblood cellThe body has natural defenses against viruses.HIV
18 Vaccines are made from weakened pathogens. A vaccine stimulates the body’s own immune response.Vaccines prepare the immune system for a future attack.Vaccines are the only way to control the spread of viral disease.