Virus: Latin for “poison” Viruses Virus: Latin for “poison”

Viruses can infect all living things These are all caused by viruses Cold Flu Herpes Some Cancers HIV Mumps Chickenpox

Are all viruses bad? No!!! The tulip mosaic virus Produces highly prized variegated tulips

History 1872 – Mayer and Koch found that a filterable agent caused disease in tobacco plants. - The disease passed through filters that would trap bacteria - What they found could not be grown in a laboratory - Invisible even with the aid of a microscope

More History….. 1935 The first virus was isolated from diseased tobacco plants The Tobacco Mosaic Virus

Are they living or not? Living They REPRODUCE at a fantastic rate, but only in living host cells. They can mutate. Non-Living They are ACELLULAR, that is, they contain no cytoplasm or cellular organelles. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. They possess DNA or RNA but never both. They are NOT CONSIDERED LIVING by many scientists.

Structure? Inside: DNA or RNA Outside: protein coat Spherical rodlike cubical polyhedron May have an outside membrane Up close and personal with the flu virus

Size? too small to be seen with a light microscope must use an electron microscope much smaller than bacteria Ex. Parvovirus 10nm Poxvirus 250 nm

Examples HIV Polio tobacco mosaic bacteriophage

Viral activities? Viruses do not carry out respiration or other metabolic activities Viruses can only reproduce when inside a host cell (parasitic)

Virus cycles Virus Motto: Get In – Take Over – Get Out lytic cycle- virus takes over all metabolic activities of host, makes copies of itself & destroys host cell

5 Steps in the Lytic Cycle Attachment to host Entry of DNA into host Replication Assembly of new viruses LYSIS: cell breaks open and releases new viruses into the organism or environment

Lytic cycle?

Virus Cycles lysogenic cycle- viral DNA becomes integrated with host DNA; provirus Virus Motto: Get In – (New Step of Hiding inside host) - Take Over – Get Out

8 Steps in the Lysogenic Cycle Attachment Entry Integration into the host cell’s DNA (prophage or provirus) Replication of cells (including virus DNA) Trigger to enter the Lytic Cycle Replication of Virus Assembly Lysis

Lysogenic cycle?

Summary of Both

RNA Viruses (Retroviruses) ex. HIV virus with RNA that can invade a cell with DNA RNA undergoes reverse transcription to produce double-stranded DNA & become a provirus inside the host's nucleus Normally: Viral DNA  RNA  Protein  Retroviruses go the other way

RNA Viruses and Retroviruses These viruses have a HIGH MUTATION RATE There is no good PROOFREADING mechanism like there is when going from DNA to RNA These means there are many different STRAINS of RNA viruses and Retroviruses Makes VACCINES very difficult to develop

Review Questions A virus is composed of two parts, what are they? List some types of viruses that you know. At least 3 Can viruses reproduce on their own? What are the two cycles of reproduction for a virus? Why is the lytic cycle so harmful to a cell?  

What are the characteristics of living things? According to the characteristics, are viruses considered to be living? Why or why not. Can a virus be cured with antibiotics? What is used to protect against contraction of a virus? Do viruses evolve? How do you know?