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Chapter 24 Viruses Table of Contents Section 1 Viral Structure and Replication Section 2 Viral Diseases In the late 1800’s • Scientists really didn’t know much about what a virus was (no tech) • They found out that it wasn’t a bacteria causing the disease. • They knew it was smaller than a bacteria cell • Scientists began to study viruses • They wanted to know if they were cellular or not • So are Viruses made of cells? Wendell Stanley 1935 • He took a TMV (tobacco mosaic virus) plant and broke it down to it’s smallest part • He was expecting to see a cell, but instead he saw crystals! • So Wendell concluded that viruses are NOT alive! Chapter 24 Section 1 Viral Structure and Replication Characteristics of Viruses • • • • • • • Lack cytoplasm Lack organelles Cannot do metabolism Cannot do homeostasis Do not have cells Have DNA OR RNA, not both Cannot reproduce outside their host cell – They must enter a living cell and use it’s machinery (ribosomes, ATP & enzymes) in order to reproduce Viral Sizes • Extremely small; cannot be seen using a light microscope • Vary in size and shape Virus Structure • Capsid – protein coat that surrounds it’s DNA or RNA • Envelope – some viruses have this 2nd layer. It is made of specific proteins and help new viruses recognize host cells (ex. – chickenpox & HIV) • Bacteriophages (viruses that attack bacteria) have a tail and tail fibers capsid Chapter 24 Section 1 Viral Structure and Replication Virus Click below to watch the Visual Concept. Visual Concept Chapter 24 Section 1 Viral Structure and Replication Parts of a Virus Click below to watch the Visual Concept. Visual Concept Chapter 24 Section 1 Viral Structure and Replication Characteristics of Viruses, continued Viruses can be classified based on: • whether they have RNA or DNA • whether the RNA or DNA is single or double stranded • Whether the RNA or DNA is circular or linear • capsid shape • whether or not they have an envelope. Other things…. • Outside their host, virus’s are lifeless with no control over its movements • Viruses are spread through • Air • Water • Food • Body fluids • In order to replicate, viruses need to recognize the appropriate host cell before it can infect it. An enveloped virus needs to match the host cell’s membrane receptors precisely. Chapter 24 Section 1 Viral Structure and Replication Replication in DNA Viruses • The DNA in some DNA viruses enter a host cell and then it makes mRNA • The DNA in some DNA viruses insert into the host’s chromosomes. Then it is called a provirus. • Some DNA viruses use the host cell enzymes to make new viral DNA and eventually new viruses! Chapter 24 Section 1 Viral Structure and Replication Replication in RNA Viruses • Some RNA viruses enter their host and that RNA serves as mRNA, which then is translated into new viral proteins immediately • Some RNA viruses, called retroviruses contain the enzyme reverse transcriptase in addition to RNA. • Reverse transcriptase uses RNA as a template to make DNA, which then inserts into the host cell’s genome. • Reverse transcriptase reverses the normal process of transcription • HIV is a retrovirus Chapter 24 Section 1 Viral Structure and Replication Replication in Bacteriophages • Remember bacteriophages are viruses that attack bacteria • They have complex capsids • They replicate by one of both of two different methods: – Lytic cycle – Lysogenic cycle Chapter 24 Section 1 Viral Structure and Replication Lytic Cycle • Lytic = lyse = break • This means that the host cell will rupture after replication is completed. • Viruses that do this cycle are called virulent. • There are 5 steps to the lytic cycle 5 Steps to the Lytic Cycle 1. Bacteriophage attaches its tail fibers to specific receptor molecules on the cell surface of a bacteria cell. The DNA is injected into the host cell 2. The ends of the viral DNA attach to each other, forming a circle. It is separate from the host cell’s DNA 3. The viral DNA takes control of protein synthesis and the viral genome is copied. Enzymes and ribosomes are used from the host cell. 4. New Viral pieces are made and put together 5. Lysozyme enzymes digest the cell wall and up to 200 new viruses are released. Chapter 24 Section 1 Viral Structure and Replication Lytic Cycle Click below to watch the Visual Concept. Visual Concept Chapter 24 Section 1 Viral Structure and Replication Lysogenic Cycle • Host cell does not get killed • It allows viruses to hide in the host cell for days, months or even years. • A virus that does this cycle is called temperate. • 5 steps 5 Steps to Lysogenic Cycle 1. Bacteriophage attaches to host cell and injects it’s DNA 2. Viral DNA attaches its ends to make a circle 3. The viral DNA then integrates into the host DNA – now it’s a prophage 4. When the bacteria replicates it’s own DNA, it replicates the viral DNA. So now each bacteria cell will be infected with a prophage. 5. The prophage will exit the host cell – usually caused by radiation or chemicals Section 1 Viral Structure and Replication Chapter 24 Lysogenic Cycle Click below to watch the Visual Concept. Visual Concept Chapter 24 The Lytic and Lysogenic Cycles Section 1 Viral Structure and Replication Chapter 24 Section 1 Viral Structure and Replication The Origin of Viruses • Most scientists think viruses originated from naked pieces of nucleic acid and evolved from that • Viruses mutate quickly, such as influenza and HIV • It is difficult for the immune system to recognize and destroy them, which makes it hard to make good vaccines. • A vaccine is a live virus that is injected so that the body can make antibodies to kill it • Therefore each year, there is a new “flu shot” available, Chapter 24 Section 2 Viral Diseases Vectors of Viral Diseases • Vectors, or hosts, of viral diseases include humans, animals, and insects. • They transfer a pathogen or parasite or virus to another organism Chapter 24 Section 2 Viral Diseases Human Viral Diseases • Viruses cause many human diseases, including the common cold, flu, hepatitis, rabies, chickenpox, certain types of cancer, and AIDS. • They can affect various human organs, such as the brain, liver, heart, lungs and skin Chapter 24 Section 2 Viral Diseases Human Viral Diseases, continued • Chickenpox and Shingles • Chickenpox and shingles are caused by the same varicella-zoster herpesvirus. • It multiplies in the lungs and travels to blood vessels in the skin • Symptoms are fever and skin rash • It is spread thru direct contact with the skin rash and through the air • After having it, people usually won’t get it again, but it can stay in the nerve cells are a provirus. Then if it gets activated, it’s called shingles. Chapter 24 Section 2 Viral Diseases Human Viral Diseases, continued • Viral Hepatitis • Hepatitis, or inflammation of the liver, can be caused by at least five viruses. • Hepatitis A and hepatitis E can be spread by fecally contaminated food and water. • Hepatitis B, C, and D are spread by sexual contact, by contact with infected blood and serum,and by the use of contaminated needles. • Symptoms are fever, nausea, jaundice & liver failure Chapter 24 Section 2 Viral Diseases Human Viral Diseases, continued • Acquired Immune Deficiency Syndrome (AIDS) • The human immunodeficiency virus (HIV) is an RNA virus spread by sexual contact, by contact with infected body fluids, and from mother to fetus. • HIV targets macrophages, which are immune system cell, and thus damages the body’s immune system. The disease called acquired immune deficiency syndrome (AIDS) results. HIV • Virus attaches to the CD4 and CCR5 receptors on cell surface. Binding must occur. • The viral envelope fuses with the membrane and releases the capsid into the host cell • Viral RNA and reverse transcriptase are released and used to make a template to make a double stranded DNA version of the viral genome • The HIV DNA enters the cell’s nucleus and integrates into the cell’s DNA, thus making a provirus • Transcription and translation occurs and viral proteins are made • Then HIV assembles and a new envelope forms around it and eventually is released from the cell Chapter 24 Section 2 Viral Diseases AIDS (Acquired Immunodeficiency Syndrome) Chapter 24 Section 2 Viral Diseases Human Viral Diseases, continued • Viruses and Cancer • Some viruses contain oncogenes that can cause cancer • other viruses convert proto-oncogenes, which usually control cell growth, to oncogenes. • Human papillomavirus (HPV) can cause cervical cancer and hepatitis B virus can cause liver cancer Chapter 24 Section 2 Viral Diseases Emerging Viral Diseases • Emerging viruses usually infect animals isolated in nature but can jump to humans when contact occurs in the environment. • Ebola virus – animals in tropical forests of central Africa • SARS – civet cats • Vaccines are expensive to develop for these infectious diseases Chapter 24 Section 2 Viral Diseases Prevention and Treatment • Vaccinations • A vaccine contains a harmless version of a virus, bacterium, or a toxin that causes an immune response when introduced to the body. • Vaccines have helped to greatly reduce certain viral diseases. • Viral vaccines are made from inactivated viruses, which are viruses that are unable to replicate in a host Attenuated Viruses • Weakened forms of a virus that cannot cause a disease • When made into a vaccine, they provide greater protection from disease • Ex., measles, mumps, rubella, polio, hepatitis a and b and chicken pox Chapter 24 Section 2 Viral Diseases Virods and Prions • Viroids are short, circular, single strands of RNA lacking a capsid that infect plant cells. • Prions are infectious particles containing protein but no nucleic acids. – Prions cause mad cow disease and similar degenerative brain diseases. Chapter 24 Section 2 Important Viral Diseases