Viruses and Bacteria

Antibiotics work on Bacteria, NOT Viruses!

Viruses

• Protein coat• Requires a host

cell to replicate• Smaller than a

prokaryote (10-400nm)

• (ex. HIV, influenza/flu)

• Viruses binds to receptors on a cell’s plasma membrane– Viruses are organism

specific because of these receptors!

– Viruses have attachment proteins on their surface.

– Examples – a plant virus cannot infect an animal cell, a T4 Bacteriophage cannot infect a skin cell

Viral InfectionLytic Cycle of a Virus

• Viruses are NOT alive!– use the cellular machinery

of another organism’s living cells to multiply themselves

• Tail of the virus attaches to host– injects its DNA or RNA

into the host.• Host cell cannot tell the

difference between its own DNA and that of the virus

• Cell replicates the viral DNA as if it were its own.

• Thousands of copies of the viral DNA and protein coat are made.

• Eventually the cell becomes too full and lyses (bursts) releasing hundreds of virus particles that can now infect other cells.

Lysogenic Cycle of Viruses

• The virus reproduces itself, but does NOT lyse the cell.

• Viral DNA is injected and becomes incorporated into the cell’s DNA unknowingly.

• The inserted viral DNA is called a Provirus.

• The virus can remain inactive like this for long periods of time– When the cell replicates,

so does the virus!

• Sudden changes (temperature or availability of food) may cause the DNA of the prophage to become active.

• Now new viral DNA could be synthesized like in the Lytic cycle

Retroviruses (backwards)

• Instead of DNA, Retroviruses infect cells with RNA.– RNA is then copied into DNA by an enzyme

called Reverse Transcriptase and inserted as a prophage like the lysogenic cycle.

– HIV is a retrovirus– Retroviruses mutate very easily, making them

resistant to treatment - the problem with HIV

Viruses are not always bad!!• Viruses are commonly used in genetic research.

– The viral genetic info can be removed and replaced with new genetic information

– These modified viruses - called vectors - can then be inserted into other organisms, allowing the virus to inject its new (good) genetic info into the target cell.

Bacterial Structure (prokaryote)• Lacks membrane

bound organelles

• Except they do have small ribosomes!

DNA is typically circular, not in chromosome form

Identifying Bacteria• Gram Staining - Stain

affects bacteria differently based on their cell wall structure– Gram positive = Purple– Gram negative = Pink

• Shape - 3 Shapes are most common– Cocci - round– Bacilli - rod-shaped– Spirilla - Spiral

streptococcus

Bacillus

Spirulina