VIRUSES What are they & Where do they come from?

What Are Viruses? They are acellular (not cells) They aren’t alive (or are they?) They can’t grow or Reproduce or Metabolize outside of a host cell But that is true for some bacteria also They can be crystallized

What Are Viruses? They are smaller than a bacterium.

What Are Viruses? They are nucleic acids (DNA or RNA) Wrapped in a protein coat (capsid) Some have a surrounding envelope stolen from the host cell They are intracellular parasites

What Are Viruses? They are nucleic acids (DNA or RNA) Wrapped in a protein coat (capsid) Some have a surrounding envelope stolen from the host cell They are intracellular parasites; they can’t grow or reproduce outside the host They are much smaller and more numerous than bacteria

What Are Viruses? Viruses cause disease Polio, measles, herpes, flu, small pox, AIDS, Ebola All organisms are subject to viral infection Antibiotics have no effect on viruses Viruses are being tested to attack cancer cells

Healthy people are full of viruses Test: The number of distinct viruses in stool samples ranged from 52—2773 ! Nasal swabs: 10,000 vial sequences in children with fever Without fever = 1,000

What Do Viruses Look Like?

Characteristics Used to Classify Viruses Type of nucleic acid (DNA or RNA) Single or double stranded Shape of virus Number of units (capsomeres) making up the capsid Naked or with envelope (envelope is part of the host membrane around virus) Size of virus particle (=virion) Site of reproduction (host cytoplasm or nucleus) Biochemical steps involved to produce mRNA

Hepatitis B Herpes Phages HIV Ebola Gastroenteritis in infants Rubella

Lytic Life Cycle ssDNA dsDNA mRNA Attachment Penetration Replication Assembly Release

Lytic Cycle Again—20 minutes--> 300 virions

Phage therapy uses viruses to attack some bacterial infections For every bacterium in your body there are ~100 bacteriophages. Estimated 10 billion phages packed into each gram of human stool. Phage therapy uses viruses to attack some bacterial infections

Lysogenic Life Cycle Integration Attachment Penetration Lytic Cycle Replication Lytic Cycle

Lysogenic Life Cycle Again

Four Major Ways Virus Enters Host Cells Injection e.g. bacteriophage

Four Major Ways Virus Enters Host Cells Endocytosis (envelope viruses; e.g. ebola)

Four Major Ways Virus Enters Host Cells Fusion (envelope viruses - e.g. herpes, HIV)

Four Major Ways Virus Enters Host Cells Wound Entry—Plant viruses generally passed by insects as they eat plants.

Two Major Ways Viruses Escape Host Cells Budding Bursting LYSIS

Budding off HIV Virions in host cell

Ebola virus

Hepatitis B Herpes Phages HIV Ebola Gastroenteritis in infants Rubella

Complete Virus Life Cycle -ssRNA ebola

Human Immunodeficiency Virus AIDS HIV Human Immunodeficiency Virus

HIV attacks cells of the immune system HIV infects Macrophages & T cells and destroys them. Then other infections can invade and kill the victim.

Hepatitis B Herpes Phages HIV Ebola Gastroenteritis in infants Rubella

Retro Viruses RNA viruses make DNA e.g. HIV +ssRNA/DNA hybrid

Life cycle of HIV Retro-virus RNADNA

VIRUSES PASS GENES Viruses are the most common biological agents in the sea 1010 per liter in surface water 5—25 times the number of bacteria! Viruses cause major mortality in the sea microbes. Produce cellular debris. Probably eaten by bacteria Transformation DNA Via water Bacteria

VIRUSES PASS GENES Recombination Different kinds of viruses infecting the same host can exchange genes occasionally. e.g. 1918 flu epidemic bird + human virus e.g. H1N1 virus is combination of 2 pig + bird + human virus

VIRUSES PASS GENES Between Species Transduction = virus carries host DNA along with its own to another host cell. Host Species 1 Virus carries sp.1 genes Host Species 2 (Bacteria sp. 1) (Bacteria sp. 2) Recombination occurs in bacterial DNA We use viruses to inject new genes into plants & animals—genetic engineering

Probably Multiple Origins ORIGIN OF VIRUSES Viruses-first Hypothesis Viruses evolved from protein and nucleic acid at the same time as the first cells were evolving. Degeneracy Hypothesis Viruses were once small cells that parasitized larger cells. Over time they lost those genes not required. Escaped-genes Hypothesis Viruses evolved from bits of naked RNA or DNA that escaped from cells or as plasmids or transposons. Probably Multiple Origins