Viruses

What are viruses?• small, non-living, infectious particles

containing genetic material in the form of DNA or RNA with a protein capsule, called the capsid.

• Non-cellular particle

• Over 4000 virus species classified but scientists believe that there are millions

• Highly specific about what they invade– ex: plant viruses infect only plant cells

• specialize in the cell types they infect.

Features of Viruses:• Viruses have no cytoplasm

• Less than 0.1µm in diameter (100s of 1000s of viruses could fit inside a human cell)

• Cannot grow or reproduce on their own

• Do not produce energy; do not create waste

• Take control of the cell that they infect

Viruses – Living or Non-Living?

• Considered non-living because:– Do not metabolize energy– Do not perform cellular respiration– Cannot grow– Cannot reproduce on their own (act

as parasites)

Shapes of Viruses

Viral Diseases

• Mild viral diseases such as the common cold or chicken pox, or more severe viral diseases such as AIDS or cholera, are all infectious.

• Epidemic: outbreak confined to a geographical region

• Pandemic: widespread, global epidemic

Bacteriophages

Vaccination• preventions against viruses.

• mixtures that contain weakened forms or parts of dangerous virus.

• they trigger a response by the immune system without causing an infection.

• creates a form of chemical ‘memory’ allowing immune system to react quickly if the individual ever comes in contact with the real virus.

3 ways Viruses enter living cells:

1) enter bacterial cells by punching a hole in the cell wall and injecting its DNA

2) enter plant cells through tiny rips in the cell wall

3) enter animal cells by endocytosis

Infectious Cycles

• Viruses must invade a living host cell to reproduce

• There are two ways to do this– 1. Lytic Cycle– 2. Lysogenic Cycle

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 enters the cell, viral DNA integrates with

the host DNA and becomes inactive, the host functions normally

• an environmental change may then cause the virus to enter the Lytic Cycle

Differences Between Lytic and Lysogenic Cycles

In the Lytic Cycle:• Viral DNA destroys Cell DNA, takes over cell functions and destroys

the cell.• The virus replicates and reproduces.• There are symptoms of viral infection.

In the Lysogenic Cycle:• Viral DNA merges with cell DNA and does not destroy the cell.• The virus does not reproduce.• There are no symptoms of viral infection.

The Origin of the Virus

• The origin of the modern virus is unclear.

• Two hypotheses exist:

1)They could be runaway stretches of nucleic acid from a larger organism that detached and became active, therefore new viruses are forming frequently and many do not have ancestors

2)Viruses once lived outside of host cells, but over time due to their parasitic lifestyle, they lost the genes necessary to live outside the host

Videos

• Flu Virus

• Ebola

• HIV/AIDS

Homework:

• Why are viruses considered to be non-living?

• What characteristics do viruses share with all living things?

• Which viral diseases are quite common and associated with the winter season?

• Explain the relationship between a virus’s dormant period in a cell and the appearance of cold sores.