Bellwork Question: How does the common cold spread Bellwork Question: How does the common cold spread? What precautions can you take to stop yourself getting ill?

Viruses Section 20.1

Why were viral diseases hard to identify? Tobacco mosaic disease caused a large number of issues in the late 19th Century Dimitri Ivanovski inferred that cause of disease came from host plants, but could not pin down exactly what caused it Martinus Bejerink proposed that tiny particles were the cause – he called them viruses Crystals of the virus isolated by Wendell Stanley in 1935

Are Viruses alive? No! Virus – nonliving particle made of proteins, nucleic acid and sometimes lipids They can only reproduce by infecting living cells

Different types of viruses

Common links between viruses All viruses have a protein coat called a capsid Number of genes can be highly varied between viruses ( a few to hundreds)

How do Viruses work? Viruses bind to receptor proteins on host cells Cell is tricked to taking on the virus Most will only infect a specific kind of cell Plant viruses – plant cells Animal viruses – animal cells Bacteria viruses – bacteriophages

What happens after a virus infects a cell? Viruses use their genetic information to make multiple copies of themselves Some viruses will replicate immediately Others will not – remain inactive Two types of infection Lytic infection Lysogenic infection

What happens during a lytic infection? Virus enters cell Copies of virus produced within the cell The cell bursts (lyse) Example – bacteriophage T4

What happens during a lysogenic infection? Viral nucleic acid is inserted into the host cell’s DNA Copied along with the host DNA without damaging the host Embedded bacterial DNA is called a prophage Can remain dormant for many generations An environmental trigger can cause it to ‘activate’ Examples – radiation, heat, specific chemicals Removes itself from the host cells DNA and directs the synthesis of new virus particles Become and active lytic infection

Comparing lytic and lysogenic infections

Comparing lytic and lysogenic infections

RNA viruses Most viruses contain RNA rather than DNA (70%) Common example – common cold! Capsids settle on cell (normally in nose), brought inside, whereby a viral protein makes new copies of viral RNA Hosts ribosomes mistake viral RNA for mRNA, and translate it into capsids and other viral proteins New capsids assemble around viral RNA copies, and within hours, hundreds of new viruses are released

What is HIV? Acquired immune deficiency syndrome is caused by an RNA virus called human immunodeficiency virus HIV virus belongs to a group of viruses called retroviruses RNA from virus is copied into DNA This is inserted into the DNA of the host cell Very similar to lysogenic infections of bacteria Can remain dormant for a long time Once active, it begins to destroy the very system of the body that would normally fight infections How is this different from how the virus that causes common colds works?

Viruses and Cells Viruses are parasites – their existence relies on other organisms

Writing assignment (well kind of…) Using your notes and the textbook, compare the structure of a virus to the structure of both a prokaryotic cell and a Eukaryotic cell. Display the information in a graphic organizer – chart, venn diagram, spider diagram etc… Explain in a paragraph why viruses are not considered living things, paying special attention to the different modes of reproduction between cells and viruses.

Key summary questions? Are viruses alive? What structures do all viruses contain? What are the two main types of viral infections? Explain the different mechanisms Give examples of diseases caused by viruses? How are viruses and cells different?

Summary Viruses are parasites, they are not alive! All viruses are very small and contain a protein coat, called a capsid There are two types of viral infections – Lytic and Lyosgenic Most viruses contain RNA Common Cold and HIV examples of RNA viruses With a common cold, viral RNA does not enter nucleus With HIV virus, genetic material enters nucleus Disease can remain dormant for large periods of time

Bellwork: Why are nitrogen fixing bacteria so important Bellwork: Why are nitrogen fixing bacteria so important? How does this relate to crop rotation?