Viruses and ProkaryotesChapter 18

Virus A non-living thing A nucleic acid surrounded by a protein

coat Doesn’t grow, respond, eat, or possess

cells A tiny disease-causing structure

› 10,000 cold viruses fit on a period on this page.

Requires a host to live and reproduce

Virus Classification

Since they are non-living, they are not placed in a biological category.

Instead, scientists organize viruses in their own non-living category.› Shape (next slide)› Type of nucleic acid contained (RNA or

DNA)› Type of organisms infected› Presence or absence of viral envelope

Viral Types: Shape

Capsid: outer layer of all viruses; made of proteins

Virus Behavior

Active Virus› Immediate

infection› Symptoms within 1

to 4 days› Example: cold and

flu viruses

› Reproduce by the lytic cycle

Latent Virus› Can remain dormant

for months or years› Symptomless› Example: herpes

virus

› Reproduce by the lysogenic cycle; enters lytic cycle when activated

Active Virus1) ATTACHMENT: A specific virus

attaches to the surface of a specific cell

2) INVADE: The nucleic acid (DNA or RNA) of the virus is injected into the cell.

3) COPY: The viral nucleic acid takes control of the cell and begins to make new virus particles.

4) RELEASE: The cell bursts open, hundreds of new virus particles are released from the cell. These virus particles go on to infect other cells.

Example of Active Virus

Coronavirus› Classification:

Family: Coronaviridae Genus: Coronavirus Species: Common cold virus

› Structure: RNA, helical, with envelope

Latent Infections

Acute infection with symptoms symptomless period reactivation of disease

Symptoms of reactivation may be different from original infection

No measurable viral particles during symptomless period

Examples of Latent Viruses

Herpesvirus› Classification:

Family: Herpesviridae Genus: Herpesvirus Species: Simplex type 1 and type 2

› Structure: DNA, helical, with envelope

Examples of Latent Viruses Chickenpox (Varicella)

› Symptoms: Blister-like rash on surface of skin and mucous membranes. Blisters usually appear first on trunk and face, then spread to almost everywhere else.

Shingles (2nd occurrence)› Symptoms: Rash occurs in an

area of the skin that is supplied by the sensory fibers of a single nerve- dermatome. Rash appears as well-defined band on one side of body, or on one side of face, arms or legs.

Body’s Response to Viruses

Vaccines Definition: Mixture of live, modified, or

dead viruses (or their toxins) which when administered into the body stimulates the body’s immune system to produce antigen-specific antibodies

Goal: Future exposure to the virus will trigger a ready response from the body. Specific antibodies will already exist to stop the infection.

Genetic Re-assortment

Results in new virus strains (example: influenza)

Requires new vaccines needed annually

In segmented viruses

Simultaneous infection of one cell with 2 different types of viruses leads to exchange of genetic information

Vaccine and Virus Studies

Animal Models: General Method

A blood test reveals the levels of viruses, T-cells and antibodies.

A “challenge” is given: a highly virulent strain of the virus is injected.

Genetic and biochemical analysis can reveal why vaccine failed.

If the animal is healthy for a “long time,” then human trials can begin.

If the animal becomes sick or dies, more blood tests are taken.

If the reason for vaccine failure is discovered, a new round of design is initiated OR the project ends.

Virus Transmission Simulation

Background: You are hanging out with a group of friends at Sonic. Everyone gets a different drink, and you all take a sip.› Take a test tube.› Exchange #1: Find a friend. Fill your own pipette.

Simultaneously release your pipette into your friend’s test tube.

› Exchange #2: Find another friend and repeat.› Exchange #3: Find another friend and repeat.› Exchange #4: Find another friend and repeat.› Test your fluid. Did you catch a virus while sharing

drinks? If your fluid is pink, then you are infected!› Circle your number on the board if you are

infected.

Avian Flu Pandemic

Starts in L.A. with 9 people…

Day 50

Day 90 – 280 million people infected

Day 130