Viruses

Page
7

Fig. 19-10a

Slide 56

Fig. 19-10b

Slide 57

Fig. 19-10c

Slide 58

Plant viruses spread disease in two major modes:

Horizontal transmission, entering through damaged cell walls

Vertical transmission, inheriting the virus from a parent

Slide 59

Viroids and Prions: The Simplest Infectious Agents

Viroids are circular RNA molecules that infect plants and disrupt their growth

Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

Prions propagate by converting normal proteins into the prion version

Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

Slide 60

Fig. 19-11

Prion

Normal

protein

Original

prion

New

prion

Aggregates

of prions

Slide 61

Fig. 19-UN1

Phage

DNA

Bacterial

chromosome

The phage attaches to a

host cell and injects its DNA

Prophage

Lysogenic cycle

Temperate phage only

Genome integrates into bacterial

chromosome as prophage, which

(1) is replicated and passed on to

daughter cells and

(2) can be induced to leave the

chromosome and initiate a lytic cycle

Lytic cycle

Virulent or temperate phage

Destruction of host DNA

Production of new phages

Lysis of host cell causes release

of progeny phages

Slide 62

Fig. 19-UN2

Time

Time

A

B

Number of bacteria

Number of viruses

Slide 63

Fig. 19-UN3

Slide 64

You should now be able to:

Explain how capsids and envelopes are formed

Distinguish between the lytic and lysogenic reproductive cycles

Explain why viruses are obligate intracellular parasites

Describe the reproductive cycle of an HIV retrovirus

Describe three processes that lead to the emergence of new diseases

Describe viroids and prions