1. Viruses

2. Viruses
Not considered living organisms because they are missing characteristics of living organisms
Do not grow, reproduce, maintain homeostasis, or metabolize
Assembled to their full size within a cell
Have a major impact on the living world

3. Viral Structure All viruses have nucleic acid and a capsid
Some may also have an envelope or tail fibers
Nucleic Acid--Can be DNA or RNA
DNA Viruses—can insert their DNA into the host cell’s DNA or keep it separate
Copies of the viral DNA are made using materials from the host

4. Viral Structure, cont. Capsid RNA Viruses—can be reproduced in 2 ways:
Viral RNA can be used to make more viral RNA
Viral RNA can be inserted into the host cell’s DNA to make more viruses
Viruses that are reproduced this way are called retroviruses
Capsid
The protein coat that surrounds a virus’s genetic material
Proteins on capsids fit proteins on host cells like keys in locks
Have 4 shapes:
T-phage, helical, polyhedral, and round

5. Other Viral Structures
Envelope—membrane surrounding the capsid of some viruses (HIV has an envelope)
Have receptors that help the virus enter cells
Tail Fibers—found on viruses that infect bacteria
Work with structures called tails like a syringe to inject viral DNA into bacteria

6. Life Cycles Lytic Life Cycle Lysogenic Life Cycle
viruses infect the host cell, reproduce (staying separate from the host’s DNA), and kill the host cell
Called proviruses
Lysogenic Life Cycle
Viruses that become part of the host’s DNA to reproduce
Does not kill the host cell, but can enter a lytic life cycle later
Called temperate viruses

7. Viroids Single strands of RNA without capsids
Disrupt host cell’s regulation of growth
Cause abnormal development and stunted growth in plants

8. Prions Misshapen versions of proteins in the brain
Attach to normal proteins and cause them to take on the shape of the prion
This causes the protein to stop working correctly
Misfolding spreads and destroys brain tissue (mad cow disease)
Can be transmitted by eating food contaminated with infected brain tissue