Viruses Evolution Objectives: Explain why a virus is not considered to be “living”. Be able to describe and identify the four basic virus structures. Describe the lifecycle of a virus. Explain what lysogeny is and how the lysogenic cycle differs from the lytic cycle. Explain what Retroviruses are. Describe how vaccinations work. Evolution

What Are Viruses? Microscopic, non-living particles that reproduce within a living cell viruses have no cellular structures, no cytoplasm, no organelles, and no cell membrane

Are Viruses Living? viruses exhibit some characteristics of living things and some of non-living things: do not respire do not respond to stimulus do not grow DO reproduce DO evolve they are little more than mobile genes that parasitize cells

Origin of Viruses? must have evolved after the first cells came into existence fragments of nucleic acid that escaped cells that then became parasites on similar cells each type of virus is probably more related to its host cell than to other viruses

Virus Structure viruses consist of two parts: outer protein coat = capsid nucleic acid – double stranded DNA or single stranded RNA; linear or circular

Classifying Viruses viruses are often classified according to their shape: cylindrical polyhedral circular complex

Viral Replication viruses must reproduce inside a living cell each kind of virus is specific to a specific type of cell E.g.: influenza viruses only affect the lungs, hepatitis viruses affect liver cells

Lytic Cycle during reproduction, the virus kills the host cell which results in the victim showing symptoms of disease (E.g. Rhinovirus – the common cold) 1. Attachment virus attaches to host cell membrane at a specific receptor site (hijacks the cell)

Lytic Cycle 2. Entry virus releases enzyme to break down cell wall/cell membrane or the cell membrane folds to bring the virus in nucleic acid passes into host; leaves empty capsid outside

Lytic Cycle

Lytic Cycle 3. Replication viral nucleic acid is copied (a lot!) using DNA and/or RNA of host protein for capsid is also produced

Lytic Cycle

Lytic Cycle 4. Assembly viral nucleic acids and proteins are assembled to complete the virus particle

Lytic Cycle

Lytic Cycle 5. Release new viruses release an enzyme to break cell membrane of host and are released

Lytic Cycle

Lysogenic Cycle some viruses may remain inactive for many generations (herpes, HIV) these viruses are known as proviruses

Lysogenic Cycle 1. Attachment nucleic acid of virus attaches to DNA of host 2. Replication host cell replicates and replicates viral DNA along with it – viral DNA stays within the host DNA so no new viruses are made this can go on for many years with no effect on the host, but now viral DNA is in many cells 3. Activation virus DNA begins lytic cycle, cells die the trigger for this is still unknown

Lysogenic Cycle

Retroviruses viruses that have RNA (not DNA) are termed retroviruses (70% of viruses are retroviruses) RNA viruses have higher mutation rates than DNA viruses (single stranded) HIV virus is a provirus that also happens to be a retrovirus HIV Virus Source: www.rkm.com.au

Vaccinations RNA vaccines are mixtures containing weakened forms or parts of a dangerous virus exposure triggers an immune response without causing infection this results in a form of “chemical memory” that allows the immune system to react quickly if exposed to the virus again Eradication - reduction of an infectious disease's prevalence in the global host population to zero the virus has ceased to exist on Earth – small pox (humans) and Rinderpest (cows) are the only two so far, many more are being worked on with Polio being the front runner (Rotary International)

Viruses for Cancer Treatment 1.How are viruses being used to treat disease?

Viroids and Prions What are viroids and prions? Why are these particles problematic?

Consolidate the Concepts Read pages 54 – 59 (and of course, annotate your notes) Do questions on page 59 #1-11