20.1 Viruses

Viruses can reproduce only by infecting living cells.

Inside living cells, viruses use their genetic information to make multiple copies of themselves. Some viruses replicate immediately, while others initially persist in an inactive state within the host.

• virus (574)

• lysogenic infection (577)

• capsid (575)

• prophage (577)

• bacteriophage (575)

• retrovirus (578)

• lytic infection (576)

20.2 Prokaryotes

Prokaryotes are classified as Bacteria or Archaea—two of the three domains of life.

Prokaryotes vary in their size and shape, in the way they move, and in the way they obtain and release energy.

Prokaryotes are essential in maintaining every aspect of the ecological balance of the living world. In addition, some species have specific uses in human industry.

• prokaryote (580)

• binary fission (583)

• bacillus (582)

• endospore (583)

• coccus (582)

• conjugation (583)

• spirillum (582)

20.3 Diseases Caused by Bacteria and Viruses

Bacteria cause disease by destroying living cells or by releasing chemicals that upset homeostasis.

Viruses cause disease by destroying living cells or by affecting cellular processes in ways that upset homeostasis.

The pathogens that cause emerging diseases are particularly threatening to human health because human populations have little or no resistance to them, and because methods of control have yet to be developed.

• pathogen (586)

• emerging disease (590)

• vaccine (588)

• prion (592)

• antibiotic (588)

Think Visually A flipbook consists of pages of sequential drawings that, when flipped, appear to move. Create a flipbook movie of the steps in a lytic infection. Be sure to show what happens to the bacteriophage at each step. Exchange your flipbook with another student. Look at the other student's movie, and write a review of it.