Chapter 19

VIRUSES AND SIMPLE INFECTIOUS AGENTS

1) 1892, use porcelain filter to filterize tobacco leaves extratfilterable agent

2) 1898, Martinus Beijerinck called it virus, which means “poison” in Latin.

3) In 1939, electron microscope to capture the first images of tobacco mosaic virus.

1) Plaques

2) Gene therapy

Tiny, infectious agents that: have a nucleic acid core (RNA or

DNA) have a protein coat (capsid)

composed of capsomers may have an envelope, which is

derived from the host cell membrane

HIV (enveloped virus)

Adenovirus (naked virus)

Viral structure

various shape…

Basic structural unit is the virion (a single viral particle).

Viruses are NOT cells.[no nucleus, organelles, or cytoplasm]

Viruses are NOT considered to be living.[do not metabolize, respond or reproduce on their own]

Viruses must infect cells (prokaryote or eukaryote) to replicate.

Step 1: Binding to host cellVirus attaches to specific receptors on cell surface.

Viral Infection & Replication

Step 2: Entry into host cell

Bacteriophages inject nucleic acid.

Animal viruses enter by receptor-mediated endocytosis & then uncoat.

Plant viruses enter through mechanical damage (insect bite, mechanical damage by farm equipment, wind, or nibbling animals).

plasmodesma

cell membrane

cell wall

Within a plant, virions easily spread from cell to cell through the plasmodesma

Step 3: Replication

BindingViral entry

Virus directs host cell to produce viral nucleic acids & proteins

Progeny virions are assembled & released as host cell ruptures

Viruses follow two major strategies to replicate

Lytic lifestyle (most viruses)Virus invades host cell, replicates, and

progeny virions are released.

Lysogenic lifestyle (some bacteriophages)Virus invades host cell and integrates it’s

DNA into host DNA.Virus “hides” in host cell for indefinite

period of time.Virus reverts to lytic lifestyle.

Viral Lifestyles

Replication of HIV

(human immunodeficiency virus)

- viruses that enter a period of dormancy after invading a host cell.

Ex. Herpesvirus that causes cold sores Epstein-Barr virus is so efficient at

remaining undetected that more than 80% of the human population carries it. Because latent viruses persist by signaling their host cells to divide continuously, they may cause cancer.

Latent Viruses

Viral Pathogenicity

Pathogenicity depends upon the ability of the virus to infect its host, and the condition of host defenses.

Epidemiology

Host ranges

Ex. rabies virus infects many mammals; humans, skunks (臭鼬 ), and bats

Ex. small pox virus infects humans only: can be controlled by vaccination

Ex. Ebola virus causes hemorrhagic fever.

Certain hosts develop illness.

Certain hosts show no symptoms - function as reservoirsEx. pigs & ducks serve as reservoirs for influenza (flu) viruses.Influenza can jump from either animal to humans (zoonosis).

Animal cells employ an immune system: Antibodies coat viral particles so they can’t

adhere to target cells. Certain cells rupture virus-infected cells before

progeny viruses can be released. Virus-infected cells release chemicals that

protect surrounding uninfected cells.

Antiviral drugs like AZT & ddC have been developed to slow viral replication.

Defense against Viral Infection

Other Infectious Agents

. Viroids (naked RNA) - infectious RNAs that affect plants.

Ex. Avocado sunblotch, Tomato bunchy top, Chrysanthemum stunt disease

prion protein (PrP)

Prions (proteinaceous infectious particle) - infectious proteins associated with slow-virus spongiform encephalopathy.

A prion disease: bovine spongiform encephalopathy (BSE)= mad cow disease