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Are viruses living organisms? For several reasons, viruses are not considered living organisms. • Viruses – are not made of cells – cannot reproduce on their own – do not grow or divide – do not use energy – lack machinery for protein synthesis However, viruses infect living things & can pass from host to host. Compare these viruses. How are the structures similar? How are they different? All viruses have the same major parts, but each virus infects a specific type of host. Why are viruses so specific? Viral Structure & Reproduction Viruses have a very simple structure. • All viruses have: – Nucleic Acids: DNA or RNA – Capsid: protein coat that determines the shape & infection process & houses nucleic acids Viral Structure • Some viruses have a lipid envelope, a protective outer coat. Influenza Rabies What type of nucleic acids do viruses use? A. DNA B. RNA C. Can be DNA or RNA D. Carries both DNA & RNA E. Protein What macromolecule are capsids made of? A. Carbohydrates B. Nucleic Acids C. Lipids D. Proteins What is one function of the viral capsid? A. B. C. D. Produce toxins Gives the virus a shape Synthesize proteins Communicate with other viruses The viral shape is determined by the capsid. Helical Polyhedral Spherical Bacteriophage Which type of cells do bacteriophages infect? A. B. C. D. Immune Cells Phage Cells Viral Cells Prokaryotic Cells Bacteriophage The name tells you that this virus infects bacteria! What are the effects of being infected by this virus? A. B. C. D. You will get AIDS. You will get the flu. You will get a cold sore. You will get the Bubonic Plague. This name tells you that this virus causes influenza (the flu)! A cell’s membrane won’t let just any ol’ particle in, so why do cells allow viruses to infect them? They have no choice! • Viruses mimic molecules a host cell wants & needs to take in. • Viruses “trick” the host cell into letting the virus in. Viral Attachment • The specificity of membrane receptors forces viruses to have specifically shaped antigens on their surface. This is what makes viruses species-specific! Which type of cell would be infected by this virus? THE BRAIN CELL Compare the proteins on the surface of the cell & the virus. They match like puzzle pieces! A B C D How do viruses reproduce? Viruses have to insert their genetic material into the host cell. What happens once the virus is inside? Viral Reproduction Viral Reproduction • Once a virus enters a host cell, 1 of 2 processes will occur: – Lysogenic Cycle – Lytic Cycle The Lytic Cycle The Lytic Cycle 1. 2. 3. 4. 5. Viral nucleic acid is injected into the host cell. Host cell begins to make mRNA from the viral DNA. Viral proteins shut down the host cell’s regular functions. Cell is used to make thousands of viruses. Host cell bursts (lyses) & releases viruses to infect other cells. This is a cell lysing with new viruses. Viruses that cause a host cell to burst are referred to as virulent viruses. The Lysogenic Cycle The Lysogenic Cycle 1. 2. 3. 4. Viral nucleic acid is injected into the host cell. DNA is integrated (joined) into the host DNA. Host cell continues dividing with the virus DNA. Eventually, the viral DNA can be triggered to enter the lytic cycle. Viruses that use this cycle are called temperate viruses. Ever had a cold sore? • Cold sores are caused by a strain of the Herpes virus. • This virus enters the lysogenic cycle, but can enter the lytic cycle if your immune system is weakened. This is when you get a cold sore! Which cycle will spread viral DNA to other cells? A. B. C. D. Lytic Cycle Lysogenic Cycle Both Neither Which cycle stops the host cell’s functions? A. Lytic Cycle B. Lysogenic Cycle C. Both D. Neither What is the correct sequence of events of the lytic cycle? Try to do this without looking at your notes. Use logic! A. B. C. D. Injection, Transcription, Translation, Assembly, Lysis Injection, Assembly, Translation, Transcription, Lysis Lysis, Assembly, Injection, Transcription, Translation Assembly, Transcription, Translation, Lysis, Injection DNA vs. RNA Viruses • DNA viruses don’t mutate as frequently, because they utilize the host cell’s DNA polymerase. Why would that matter? Remember – DNA polymerase can proofread & correct errors during DNA replication. DNA vs. RNA Viruses • RNA viruses have high mutation rates, because when the cell copies RNA for new viruses, the enzyme cannot fix mutations. You only need a vaccine for DNA viruses once. For example, smallpox or HPV. You need vaccines for RNA viruses every year, because they mutate so quickly. For example, the flu. RNA viruses are called retroviruses. • Some cancers, the flu, & AIDS are caused by retroviruses. • We know RNA is made from DNA, but not the other way around. (RNA DNA ???) How can an RNA virus take over a host cell’s DNA? Retroviruses • Retroviruses also need to inject reverse transcriptase. • Reverse transcriptase is an enzyme that makes a DNA copy of viral RNA. – RNA DNA – The lytic or lysogenic cycle can then proceed normally.