Viruses

Question 1

Are viruses alive (why or why not)?

Answer 1

Viruses:

• move and respond to stimulus
• reproduce within a host and kill it
• have DNA or RNA which is passed onto “offspring”
• can adapt to their environment and evolve
• die(can be destroyed)

Not alive because they don’t have all the characteristics of life, and they must rely on other living things.

• are not made of cells
• dont require energy to survive
• can’t reproduce without infection living thing
• have no homeostasis

Question 2

Characteristics of Viruses

Answer 2

• All viruses are parasitic and cannot live independently
• They cannot invade all cell types (e.g. rabies infects mammals; HIV infects only white blood cells)
• They must use the energy of other cells to “live” and reproduce

Question 3

Viral Life Cycle

Answer 3

A. Getting into the cell
Viruses invade cells and force them to make more viruses.

a) Bacteriophage:
attaches to cell surfaces and injects genetic material

b) Animal virus:
triggers cell to engulf or ‘eat’ virus (endocytosis)

c) Plant virus:
through a cell wall injury or when pollination occurs

B. Virus Reproduction

DNA viruses - two types

1. Lytic cycle:
   - virus attaches to cell and injects DNA into the cell
   - viral DNA uses bacterial cell components to manufacture more viruses
   - occurs very fast,destroys cell; viral symptoms are immediate
   - example: Adenovirus
   - causes the common cold
2. Lysogenic cycle:
   - virus DNA combines with cell DNA
   - occurs slowly, viral symptoms are delayed,
   - example: Human Herpes Simplex 3:
   - causes cold sores
   - can hide in nerves and reappear
   - transmitted by close contact
   - lysogenic virus

C. Getting out of the Cell

i) Cell Lysis
- virus particles build up inside cell; cell eventually bursts open
- cell is completely destroyed; very common

ii) Exocytosis
- virus is exports out of cell; no damage to cell structure
- can continue to make viruses; example: flu virus

Question 4

What are vaccines? and how do they work?

Answer 4

-antigen-containing substances obtained from weakened or dead microbes

Examples: 
Pasteur 1885 (Rabies)
Salk 1955 (Polio)

How do they work?

1. antibodies coat free virus particles. the virus envelope cannot fuse with the host cell membrane
2. The antibody-coated virus is recognized and phagocytosed by a macrophage