1. VIRUSES Characteristics NOT considered living things
Instead considered “biological entities”
Do contain genetic material – either DNA or RNA
Depend on a cell (host) to survive
A virus that specifically infects a bacterium is called a bacteriophage
Why are they not considered living things?
No metabolism (on their own)
Cannot reproduce (on their own)
Not made of cells
It can also be argued that they don’t respond to stimuli AND cannot adapt to the environment either!
DNA or RNA that they can contain can be: double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA
NOTE: ANY cell can be a host to a virus, but specific viruses only infect specific cells. Ex. HIV infects human T cells

2. VIRUSES Viruses take on many shapes and sizes, but all viruses have:
Structure
Viruses take on many shapes and sizes, but all viruses have:
Capsid = protein coat on the outside
Nucleic acid core (either DNA or RNA)
No cell membrane, cytoplasm or ribosomes (because it isn’t a cell!!)

3. VIRUSES
Uses
Scientists are conducting research to find ways to use viruses for good in the treatment of diseases like cancer and genetic disorders. They are also being researched as a potential replacement for antibiotics, which are used to treat bacterial diseases.
Photo Credit: James Gathany, courtesy of the CDC:

4. VIRUSES
Reproduction
Attach: Capsid binds to receptor proteins on host cell
Inject: Virus injects its DNA or RNA into the host cell
Assemble: Viral genes are transcribed, translated and expressed  host becomes a virus-creating factory
Repeat: Host cell bursts  newly formed viruses
This is describing a lytic life cycle of a virus, but there are actually two different life cycles.

5. VIRUSES Lytic Reproduction
When viral DNA (or RNA) is injected it immediately starts being transcribed and translated
Host cell bursts (lyses)  viruses affecting more cells
Remind students that a bacteriophage is specifically when a virus is injected into a bacteria host cell  defined originally on slide 9

6. VIRUSES Reproduction Lysogenic
When viral DNA (or RNA) is injected it becomes part of the host cell’s DNA
Every time the host cell copies, it also copies the viral DNA
This allows the virus to be “dormant” in your body for years until it is expressed
Ex. Chicken pox virus comes up again as shingles later in life
Another example is cold sores. The virus that causes these can go into a lysogenic cycle for years in your nerve cells. Then when your immune system is weakened (due to stress or exposure to other environmental factors) the virus can transition to a lytic cycle where it starts acting and producing cold sores.

7. Compare and Contrast BACTERIA VIRUSES
I like to have students do this AFTER we have done the Bacteria and Virus Task Cards, as a way to review what was covered and summarize the Concept.