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!

Section 1 Viral Structure and Replication Chapter 24 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 helical polyhedral icosahedron

Virus Structure capsid • 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

Section 1 Viral Structure and Replication Chapter 24 Virus Click below to watch the Visual Concept. Visual Concept

Section 1 Viral Structure and Replication Chapter 24 Parts of a Virus Click below to watch the Visual Concept. Visual Concept

Section 1 Viral Structure and Replication Chapter 24 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.

Section 1 Viral Structure and Replication Chapter 24 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!

Section 1 Viral Structure and Replication Chapter 24 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

Section 1 Viral Structure and Replication Chapter 24 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

Section 1 Viral Structure and Replication Chapter 24 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 • 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 • The ends of the viral DNA attach to each other, forming a circle. It is separate from the host cell’s DNA • The viral DNA takes control of protein synthesis and the viral genome is copied. Enzymes and ribosomes are used from the host cell. • New Viral pieces are made and put together • Lysozyme enzymes digest the cell wall and up to 200 new viruses are released.

Section 1 Viral Structure and Replication Chapter 24 Lytic Cycle Click below to watch the Visual Concept. Visual Concept

Section 1 Viral Structure and Replication Chapter 24 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 • Bacteriophage attaches to host cell and injects it’s DNA • Viral DNA attaches its ends to make a circle • The viral DNA then integrates into the host DNA – now it’s a prophage • When the bacteria replicates it’s own DNA, it replicates the viral DNA. So now each bacteria cell will be infected with a prophage. • 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

Section 1 Viral Structure and Replication Chapter 24 The Lytic and Lysogenic Cycles

Section 1 Viral Structure and Replication Chapter 24 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,

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 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.

Chicken pox shingles

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 AIDS (Acquired Immunodeficiency Syndrome)

Section 2 Viral Diseases Chapter 24 Human Viral Diseases, continued • Viruses and Cancer • Some viruses contain oncogenes that can cause cancer • other viruses convertproto-oncogenes, which usually control cell growth,tooncogenes. • Human papillomavirus (HPV) can cause cervical cancer and hepatitis B virus can cause liver cancer

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 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

Section 2 Viral Diseases Chapter 24 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.

Section 2 ImportantViral Diseases Chapter 24

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