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

18.1 Studying Viruses and Prokaryotes

KEY CONCEPT Infections can be caused in several ways.


Viruses, bacteria, viroids, and prions can all cause infection.

• Any disease-causing agent is called a pathogen.

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


• 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


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


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

lipidenvelope

surfaceproteins

capsid

nucleic acid

lipid envelope

Surface proteins capsidsurfaceproteins

nucleic acid

helical(rabies)

polyhedral(foot-and-mouth

disease)

enveloped(influenza)


• Bacteriophages infect bacteria.

capsid

DNA

tail sheath

tail fiber


• Viruses enter cells in various ways.

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

– bacteriophages pierce host cells


– viruses of eukaryotes enter by endocytosis



– viruses of eukaryotes also fuse with membrane



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 infections.

• 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.

The viral DNA is called a prophagewhen it 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.

The prophage may leave thehost’s DNA and enter thelytic cycle.


KEY CONCEPT Some viral diseases can be prevented with vaccines.


Viruses cause many infectious diseases

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


– influenza




– influenza



– SARS


– HIV

HIV-infected whiteblood cell


• There are many examples of viral infections.

• The body has natural defenses against viruses.


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.

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

Documents

studying viruses

prokaryotes influenza

endocytosis viruses

membrane viruses

prokaryotes bacteriophages

prokaryotes vaccines

prokaryotes host bacterium

host cells dna