18.1 Studying Viruses and Prokaryotes KEY CONCEPT Infections can be caused in several ways.

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

18.1 Studying Viruses and Prokaryotes A virus is made of DNA or RNA and a protein coat. –non-living pathogen –can infect many organisms –Viruses attacks specific hosts-ex. Flu attacks respiratory system A viroid is made only of single-stranded RNA. –causes disease in plants –passed through seeds or pollen

18.1 Studying Viruses and Prokaryotes A prion is made only of proteins. –causes misfolding of other proteins –results in diseases of the brain

18.1 Studying Viruses and Prokaryotes KEY CONCEPT Viruses exist in a variety of shapes and sizes.

18.1 Studying Viruses and Prokaryotes 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 capsid nucleic acid lipid envelope surface proteins capsid nucleic acid lipid envelope Surface proteins capsid surface proteins nucleic acid helical (rabies) polyhedral (foot-and-mouth disease) enveloped (influenza)

18.1 Studying Viruses and Prokaryotes Bacteriophages infect bacteria. capsid DNA tail sheath tail fiber

18.1 Studying Viruses and Prokaryotes Viruses enter cells in various ways. colored SEM; magnifications: large photo 25,000; inset 38,000x –bacteriophages pierce host cells

18.1 Studying Viruses and Prokaryotes –viruses of eukaryotes enter by endocytosis—are taken in by the cell Viruses enter cells in various ways.

18.1 Studying Viruses and Prokaryotes –viruses of eukaryotes also fuse with membrane Viruses enter cells in various ways.

18.1 Studying Viruses and Prokaryotes 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. Viruses cause two types of infections. A lytic infection causes the host cell to burst. The virus may enter the lysogenic cycle, in which the host cell is not destroyed.

18.1 Studying Viruses and Prokaryotes

A lysogenic infection does no immediate harm. Ex. HIV- AIDS 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 the host cell’s DNA. Many cell divisions produce a colony of bacteria infected with prophage. The prophage may leave the host’s DNA and enter the lytic cycle.

18.1 Studying Viruses and Prokaryotes

KEY CONCEPT Some viral diseases can be prevented with vaccines.

18.1 Studying Viruses and Prokaryotes Viruses cause many infectious diseases There are many examples of viral infections. –common cold

18.1 Studying Viruses and Prokaryotes –influenza Viruses cause many infectious diseases There are many examples of viral infections. –common cold

18.1 Studying Viruses and Prokaryotes –Influenza –Zika –Ebola Viruses cause many infectious diseases There are many examples of viral infections. –common cold –SARS

18.1 Studying Viruses and Prokaryotes HIV—attacks helper T cells in the immune system HIV-infected white blood cell Viruses cause many infectious diseases There are many examples of viral infections. The body has natural defenses against viruses.

18.1 Studying Viruses and Prokaryotes

Herpes simplex causes cold sores

18.1 Studying Viruses and Prokaryotes Varicella zoster causes chicken pox and shingles

18.1 Studying Viruses and Prokaryotes 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.

13.1 Ecologists Study Relationships How are viruses classified? Living or nonliving?? Obligate Intracellular Parasites