Viruses

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Living Organisms

– Cellular Respiration– Reproduction– Metabolism– Homeostasis– Heredity– Responsiveness– Growth and development

Viruses are not living organismsViruses do not

– Grow– Have homeostasis– Metabolize

Viruses do– Infect cells and use the cell to make more

viruses– Cause disease in many organisms

Parts of a Virion (a virus particle)

Nucleic Acid – RNA or DNA (never both)

Capsid – protein coat that surrounds the DNA or RNA in a virus

Lipid Membrane – a membrane around the capsid in many kinds of viruses; helps the virus enter cells (“enveloped” viruses; without the membrane, the virus is “naked”)– Made of proteins, lipids, and glycoproteins

RNA or DNA?

Viruses with RNA– Human immunodeficiency virus (HIV)– Influenza viruses– Rabies

Viruses with DNA– Warts– Chickenpox– mononucleosis

Virus Shapes

HelicalRodlike with capsid proteins winding around the core in a spiral

Tobacco Mosaic Virus

Virus Shapes

Polyhedral

Has many sides

Most polyhedral capsids have 20 sides and 12 corners

Virus Shapes

Polyhedral capsid attached to a helical tail.

How do viruses replicate?

2 methods of replication:1. Lytic Cycle – the virus enters the cell,

replicates itself hundreds of times, and then bursts out of the cell, destroying it.2. Lysogenic Cycle – the virus DNA integrates with the host DNA and the host’s cell helps create more virus DNA. An environmental change may cause the virus to enter the Lytic Cycle.

In the lytic cycle, the virus reproduces itself using the host cell's chemical machinery. The red spiral lines in the drawing indicate the virus's genetic material. The orange portion is the outer shell that protects it.

In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions.

Viruses Enter Living Cells

Viruses enter bacterial cells by punching a hole in the cells wall and injecting its DNA

Viruses Enter Living Cells

Viruses enter plant cells through tiny rips in the cell wall.

Viruses enter animal cells by endocytosis.

Mutating viruses

Viruses can mutate when they copy the genetic material– Copy something wrong– Mistake proves useful– More powerful virus (more infectious)

Viruses don’t mutate often, except…– Influenza– HIV

Viruses are host cell specific.

Most viruses are restricted to certain kinds of cells (those that infect plants cannot infect animal cells).

Why?

Scientists think that viruses originated from escaped genetic material from host cells.

Viruses can be beneficial…

Bacteriophages – attack & destroy bacteria

Baculovirus – ebola-like virus that attacks insects– Could use for pest control in crops

• Cabbage loopers eat

cabbage crops

• Virus can kill pests in days– (it’s really gross)

… and then there are those that are not so good….

Also important…

Any agent (not just viruses) that causes disease is a pathogen.

When a virus inserts its genetic material into a host’s DNA, it is called a provirus.

Some viruses replicate very slowly and only cause damage when the conditions are “right”. (cold sores)

Human Immunodeficiency Virus (HIV)

Acquired Immuno Deficiency Syndrome (AIDS)

Basic Structure

• Viral envelope – lipid bilayer; glycoproteins protrude from surface– Glycoproteins enable virus to recognize

surface proteins of special immune cells and to enter the cell (like a key to the cell’s door)

• 2 strands RNA – only 9 genes; 3 are found in many viruses (structural proteins)

• Reverse Transcriptase – turns RNA into DNA (this makes HIV a retrovirus); DNA instructs cell to make more viruses

• Virus enters cell through endocytosis

• Virus replicates RNA to DNA with reverse transcriptase

HIV Making Factories

• DNA enters nucleus & binds with host DNA

• New virions exit cell through exocytosis to infect other cells (notice cell isn’t destroyed)

•mRNA is created (carries instructions for making new viral proteins) and leaves nucleus•Uses host cell’s enzymes to make new viruses

How Is HIV Spread?

• Sexual contact

• Sharing contaminated needles

• Blood transfusions

• Breast feeding (mother to baby)

• Mother to baby during pregnancy or birth

Think about it…• In the US, there is better than a 1/1000 chance of

contracting HIV during unprotected sex• A person can be contagious for more than 10 years before

any sign of the disease is apparent• HIV becomes AIDS when the number of immune cells drops

below a predetermined number• No one dies from HIV or AIDS; people die from secondary

infections (ranging from the common cold to cancer)• More than 3 million people (size of Chicago) die each year

• There are approx. 14,000 new cases of HIV worldwide every day