Viruses Runaway Genes?

Not Usually Considered Alive Because… They aren’t made of cells (order) They can’t reproduce on their own Don’t grow or develop

What Are Viruses? We’re not sure exactly Pieces of nucleic acid wrapped in a protein (and sometimes lipids) No ribosomes or organelles

Viral Genomes Can be DNA or RNA Can be single or double stranded Usually short and circular

Examples of Viruses Cold Influenza (flu) HIV West Nile Virus Chicken pox Small pox Ebola Herpes HPV Rabies Polio Meningitis Measles Mumps Tobacco mosaic virus Also affect plants, bacteria and archaea – extremely widespread

Viruses Depend on a Host Cannot live independently Have no machinery for reproduction or metabolism

Viral Reproduction 2 “phases” of viral reproduction: Lytic Lysogenic Use the hosts to replicate RNA viruses use transcription DNA viruses use DNA replication 2 “phases” of viral reproduction: Lytic Lysogenic

Lytic Cycle Host copies and assembles new viruses New viruses burst free from the cell Makes a virus virulent

Lysogenic Cycle Virus incorporates itself into genome Gets copied along with the host genome Can alter the function of the host

Retroviruses RNA viruses like HIV must first make DNA from RNA to enter lysogenic cycle Reverse Transcriptase accomplishes this so DNA can be inserted Drugs like AZT fight this action

Very Difficult to Fight Viruses There’s nothing to attack! Almost everything they use belongs to the host Only mechanisms are vaccines to prevent viruses from entering cells and preventing reverse transcription

Viral Genetics Viruses can reproduce extremely rapidly by using hosts

Viruses Evolve Quickly Reproduce rapidly Very little proofreading of errors (especially RNA viruses) 2 viruses in the same host can combine

Where Did they Come From? We don’t know! 3 main hypotheses, each with some support Cells that through evolution lost other genes and structures necessary for independent survival Evolved from plasmids or transposons (jumping genes) Co-evolved with the first forms of life

Transposons Jumping genes Can copy and paste or be cut and paste Introduces repetition in patterns in addition to changing genes

What we Think Viruses are ancient and probably as old as life itself Viruses probably do not share a common ancestor There are millions of viruses

Why Should We Care About Viruses? Effects on human health Role in ecosystems Important role in genetics and evolution

Role in Ecosystems Viruses kill bacteria (estimated to kill about 20% of the biomass of the ocean every day) Recycle nutrients and materials, prevent overgrowth of bacteria

Role in Genetics/Evolution Viruses carry genes from one individual to another (can be different species!) Transduction Hugely important in increasing genetic diversity (especially before sex evolved)

The More Realistic Tree of Life Genes have been transferred horizontally Viruses account for much (not all) of this transfer

Impact on Humans Much of our genome came from viruses (how much is debated, but current estimate is 8%) May have inserted things like schizophrenia? http://www.popsci.com/science/article/2010-01/8-percent-human- dna-comes-virus-causes-schizophrenia Cancer? http://www.sci-news.com/medicine/science-ancient-viruses- human-genome-cancer-01311.html

Crazy Virus Facts Every day 1016 copies of HIV are made 1 million viruses per ml of seawater 1030ish on the planet End to end would be 200 million light years long Smallest virus studied was 1,700 nucleotides long- made only 2 proteins Some biologists believe the nucleus formed from a virus that invaded a bacterium (others think it was an archaean cell)