Viruses- Chapter 18

Objectives

• Know the structure of a virus

• Know some scientists that studied viruses

• Are viruses living or non-living

• Know how viruses reproduce

• Know some examples of viruses and the disease they cause

Viruses

I. Virus - a tiny, parasitic particle that consists of nucleic acids (DNA or RNA) surrounded by a protein coat. They fall somewhere between living cells and non-living things.

HeadDNA

Collar

Neck

Tail FiberBase Plate

Sheath

Viruses: StructureNOT CELLS!!NOT CELLS!!Viruses contain

1.Nucleic Acids (DNA/ RNA)

2.A Protein Coat (CAPSID)

Some have structures designed to attach to host cells (tail fibers; envelope proteins, spikes, etc)

All viruses are made of 2 things:

1. DNA/ RNA 2. Protein Coat (capsid)

Comparing the size of a virus, a bacterium, and an animal cell

0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

Why non-living:

1. Viruses cannot reproduce on their own

((Must have a living host-cellMust have a living host-cell))

2. Viruses cannot metabolize (obtain and use energy)

3. Viruses are not made of cells

4. Viruses do not grow/ develop

5. Viruses do not respond to stimulus

6. But Viruses DO have genetic material

Scientists1st discovered in 1892 (Iwanowski)• Tobacco Mosaic virus• Originally thought to be a bacterial disease• Disease could be caused even after sap was

filtered through extremely small filters

Smallpox virus

1796 - Edward Jenner - 1st vaccination• Injected cowpox extract into people to immunize

them against smallpox..1st vaccination

1885 - Louis Pasteur• developed rabies vaccination

1936 - Dr. William Stanley• isolated tobacco mosaic viruses for the 1st

time; obtained them as crystals• Living organisms do not crystallize, so

concluded were not living

Sizes of Viruses

Extremely small; largest virus is 1/10 size of smallest bacteria. Range in size from 20 - 200 nm (nanometer); 1 nanometer = 1 millionth of a meter

Shapes of Viruses1. Helical - rod-like appearance.

Ex: Tobacco mosaic virus

2. Isometrical - spherical or polyhedral shaped; some appear to have spikes for cellular recognition and attachment

Ex: Polio or HIV

Shapes (cont.)

3. Geometric - usually a geometric pattern; Ex: bacteriophage

4. Filovirus - filamentous strands

Ex: Ebola

Characteristics of Viruses

• non-cellular; do not respire, grow/ develop, respond to stimuli

• Parasitic on living cells - nucleic material in viruses give information for production of new viruses

• viruses are spread from infected cells by direct contact or indirectly through air, water, food, fecal material

Simple Lytic and Lysogenic Cycle to Draw

in notes:

Viral Reproduction: requires host cell

A. Lytic Cycle - the viral replication process that rapidly kills host cell

1. A virus invades a host cell and takes control

2. Begins to replicate immediately producing many new viruses

3. Host cell lyses (breaks apart), releasing the newly made viruses

4. New viruses may then enter other cells and repeat the cycle.

Lytic Cycle

B. Lysogenic Cycle - viral replication in which a virus does not immediately kill a host cell

1. virus attaches to host cell and injects nucleic acid; viral DNA attaches to host cell's chromosome

2. The nucleic acid becomes part of the host chromosome (prophage/provirus)

3. Host cell divides, replicating viral DNA along with its own DNA

4. The viral DNA is activated and enters into the lytic cycle

Lysogenic Cycle

Lytic vs Lysogenic

Lytic vs lysogenic

• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html

Fig. 19-6

PhageDNA

Phage

The phage injects its DNA.

Bacterialchromosome

Phage DNAcircularizes.

Daughter cellwith prophage

Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.

Cell divisionsproducepopulation ofbacteria infectedwith the prophage.

The cell lyses, releasing phages.

Lytic cycle

Lytic cycleis induced or Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

New phage DNA and proteinsare synthesized andassembled into phages.

See Review SheetSee Review Sheet

***Retrovirus – virus contains RNA and enzyme called reverse transcriptase1.virus attaches to target cell and inject RNA and reverse transcriptase2.the reverse transcriptase “reads” the RNA and produces DNA3.the new viral DNA begins the lytic cycle4.when the viruses emerge from the infected cell, they often surround themselves with the cell’s plasma membrane material – making it harder for the body’s defenses to recognize it. Ex: HIV

Reverse Transcription video

RNA DNA RNA Protein

Viral RNA is converted into host DNA (reverse transcription), which is then

integrated into the host genome. Then when normal transcription occurs, the viral

“message” is expressed, synthesizing a

viral protein.

Types of viruses1. Bacteriophage – infects bacteria2. Papovavirus – (DNA) causes warts3. Adenovirus - (DNA) respiratory infections4. Herpesvirus - (DNA) causes herpes,

chicken pox, shingles5. Parvovirus – (DNA) infects dogs6. Rhinovirus – (RNA) common cold7. HIV – Human immunodeficiency virus

– retrovirus (RNA) – causes AIDS (Auto Immune Deficiency Syndrome)

T4 bacteriophage infecting an E. coli cell

0.5 m

Viruses are pathogenic• Treatment: Really no treatment for viral

diseases… Interferon therapy (IF) has some beneficial effects

• PREVENTION IS CRUCIAL: Vaccination; sterilization, proper hygiene, avoidance of contagious individuals

Ways to prevent viral infection

• Sterilization – with disinfectants (cleaners)– Disinfectants are used to sterilize hospitals

• Avoid contact

• Vaccinations – build up antibodies for future infection

Herd Immunity

Human Immunodeficiency Virus

• Retrovirus– Information is stored in

RNA (mutates easily); hijacks host-cell DNA

• Attacks helper T cells in the immune system and lowers the immune system

• Causes Auto Immune Deficiency Syndrome

Defenses Against Viral Infection

1st line of defense – skin, mucous membranes, tears– Most viruses enter body through nose or

mouth

2nd line of defense (once inside body)1. White blood cells called phagocytes engulf

and destroy invading virus2. Immune response – body detects foreign

proteins called “antigens.” The body then produces antibodies to attack invader

Vaccine – a solution of weakened or killed pathogens (disease-causing organism) that causes   the body to produce antibodies without causing the disease. The immune system “remembers” the pathogen in case it encounters it again.

3rd Interferon (IF) – a protein that interferes with viral replication – it is produced by cells under viral attack which triggers healthy cells to produce protective enzymes. IF works faster than antibody production.

Viroids & PrionsViroids – single stranded RNA molecules that have NO surrounding capsids. Infect many plants, including potatoes, tomatoes, apples, and citrus fruits.

Prions – protein infectious particles. Bovine Spongiform Encephalopathy (mad cow disease) and Creutzfeldt-Jakob disease, a similar disease in humans, may be caused by prions.

Flu Attack! How a Virus Invades your Body

• http://www.youtube.com/watch?v=Rpj0emEGShQ

Lytic & Lysogenic Cycle and Virus vs Cells Diagrams

• Get a textbook and a sheet of paper

• Turn to page 481:On ONE side of the paper, draw the diagram in figure 19-10 comparing the lytic and lysogenic cycles. Be sure to include all text/ info

• Turn to page 483: On the OTHER side of the paper, draw Figure 19-11 comparing viruses and cells. Be sure to include all text/ info