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Viruses and Bacteria Antibiotics work on Bacteria, NOT Viruses! Viruses • Protein coat • Requires a host cell to replicate • Smaller than a prokaryote (10400nm) • (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 Infection Lytic 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 streptococcus Bacillus • Shape - 3 Shapes are most common – Cocci - round – Bacilli - rod-shaped – Spirilla - Spiral Spirulina