VirusesAre they alive?

The properties of life

● You already know that living things:1. Made of cells2. Grow and reproduce3. Use information from DNA

What is a Virus?

●A virus is composed of segments of nucleic acid (DNA or RNA) in a protein coat.

●They are NOT cells.●Viruses are pathogens – agents that cause

disease●Viruses replicate by infecting a cell and

using that cell to make more viruses.

So, are viruses alive?

●What do you think? Explain your answer using the characteristics of life.

●They do not grow●They do not maintain

homeostasis●They cannot reproduce on their own●They are not made of cells●Viruses are NOT ALIVE!!!!

Objectives

1A: I can explain why viruses are considered not living1B: I can define what a virus is 1C: I can describe the 3 basic structures of a virus1D: I can describe the 3 different types of viral infections: Lytic Infections, Lysogenic Infections & Retroviruses1E: I can explain how the HIV virus invades and affects a person1F: I can give examples of diseases caused by viruses

Viral Structure- Spherical Viruses

● Capsid – protein coat (may contain RNA or DNA)

● Envelope – membrane that surrounds the capsid. This helps the virus enter cells

● Glycoproteins - proteins on the envelope that are attached to carbohydrate molecules

●Nucleic acid- In the center of the virus is the DNA or RNA

Viral Structure- Helical Viruses

● Capsid – capsid proteins are arranged helically around the center

● Nucleic acid- The DNA or RNA is also arranged helically in the center of the capsid

Bacteriophage

●A virus that infects bacteria

●More complex structure

How does a virus enter the cell?

●They must use other living cells to reproduce

●They enter the cell at points of injury, are taken into the cell by endocytosis or they can even punch a hole in the cell.

Viruses are Specific

●Viruses are restricted to certain kinds of cells.

●TMV (tobacco mosaic) infects tobacco and related plants, but not animals.

●Biologists think there are as many viruses as there are kinds of organisms.

Viral Replication

Viral Replication

●Lytic Cycle- results in the destruction of the infected cell and its membrane

●Lysogenic Cycle- the viruses DNA is inserted into the host bacteria’s DNA where it lies dormant and is replicated until it is activated and then enters the lytic cycle.

●Retroviruses- Viruses that contain RNA instead of DNA. They insert the RNA into the cell, use the cell to make DNA from the RNA and then insert that DNA into the cells DNA to make more copies of itself.

Comparing the 3 Cycles●Lytic Cycle

◦Viral DNA destroys the Cells DNA, takes over cell functions and eventually destroys the cell◦The virus replicates and produces more viruses◦There are symptoms of a viral infection

●Lysogenic Cycle◦Viral DNA merges with the cells DNA and does not destroy the cell◦The virus does not produce more viruses◦There are no symptoms of a viral infection

●Retroviruses◦Viral DNA merges with the cell DNA and does not destroy the cell◦The virus uses the cell to make more viruses◦The immune system is severely compromised

Structure of HIV

●HIV causes AIDS●Human Immunodeficiency Virus●Genetic material – 2 single strands

of RNA

HIV Infection

●Glycoproteins on surface of HIV fits into the receptor

●Immune system cells have this receptor

Replication (NOT reproduction)

●Inside the cell, the capsid comes apart and viral RNA is released.

●Reverse transcriptase is also released (enzyme)

●Makes DNA version of viral RNA

Replication Continued●Viral DNA

integrates with cell DNA

●Cell makes copies of the HIV

●Leaves the cell via budding, does not destroy the cell

AIDS

●The glycoproteins change and fit T cells●Virus destroys T cells●The destruction

of T cells does not allow the body to fight against disease

Viral Diseases

●Influenza is probably most lethal virus – this is a respiratory disease

●Certain viruses cause types of cancer Ex. Hepatitis B – liver cancer

human papilloma virus – cervical cancer

Diseases caused by Viruses●Ebola – high fever, uncontrollable bleeding●Hepatitis – flu-like symptoms, painful joints,

swollen liver●Mumps – swelling in salivary glands●Polio – stiff neck, paralysis●Rabies – paralysis, convulsions, fatal●Yellow Fever – Yellow skin, often fatal●Smallpox – blisters, lesions, blindness, often

fatal