Ch. 19 Viruses & Bacteria: What Is a Virus?

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1 Ch. 19 Viruses & Bacteria: What Is a Virus? A virus is an invective agent consisting of a nucleic acid in a protein coat, able to multiply only within the living cells of a host. A bacteriophage ( bacteria eating ) is a virus that infects specific prokaryotes. Viruses are composed of primarily protein and nucleic acid. The capsid is the viral protein coat. The capsid in this diagram (green) includes everything other than the DNA. The primary functions of the capsid are to contain the viral nucleic acid (Head), attach to the host cell (Tail fibers), and deliver the nucleic acid into the host cell (Tail sheath). The viral modus operandi (MO): OBLIGATE INTRACELLULAR PARASITE. Generally, viruses vary widely in size and structure... 1

2 T4 Bacteriophage Tobacco Mosaic Virus ssrna Head dsdna Tail sheath Tail fiber How are viruses classified? Influenza Virus ssrna Capsid Membrane envelope Capsid Surface proteins Glycoproteins Note the variety of nucleic acid, size, shape, complexity and accessories! The first level of viral classification is the host range, the host(s) the virus infects. The host ranges of these examples are bacteria (E. coli), Plant (Tobacco), and Animal (human). Host range varies from whole groups (rabies virus) to a specific cell (HIV( ds )). The second level of viral classification is genetic material (DNA vs. RNA). The third level of viral classification is the number of genetic material strands (ss vs. ds). The forth level of viral classification is whether the RNA is positive, negative, or retro. A retrovirus inserts a DNA copy of its RNA into the host cell s genome (HIV). The membrane envelope source is the host cell plasma membrane, or nuclear envelope. Surface glycoproteins help the virus attach and enter the host cell... 2

3 Viral Infection Viruses trick the cell by binding glycoproteins to cell surface receptors designed for other functions. In general, once the viral genetic material is inside it is copied and expressed. Depending on the virus, infection may be lytic or lysogenic. In lytic infection, the viral genetic material is expressed right away. In lysogenic infection, the viral DNA becomes part of the host cell genome as a prophage. The most important difference is that lytic infection results in rapid destruction of the host cell, while lysogenic infection results in the prophage being copied and passed to daughter cells for generations! Virulent viruses use lytic infection and make you sick quick (common cold). Temperate viruses use lysogenic infection and may not make you sick for years (HIV)! The latent period of a temperate virus is the period of time the prophage/provirus is not being expressed. An environmental cue or stress triggers a prophage/provirus to be expressed, resulting in destruction of the host cell, and disease. Viral Gene Expression Prophage Expression 3

4 HIV This is a human CD4 Helper T-cell. It helps the immune system recognize and respond to infection. HIV is an animal, dsrna, retrovirus. HIV as a retrovirus because its RNA is reverse transcribed into DNA. HIV is a temperate virus because it incorporates a DNA copy of its RNA into the host cell genome. Without treatment, the latent period of the HIV provirus is about 10 years. HIV glycoproteins bind CD4 receptors. The viral envelope fuses with the cell plasma membrane. The viral capsid enters and breaks down. Viral reverse transcriptase makes a DNA copy of the viral RNA which incorporates into the host cell DNA as a provirus. After a latent period, the viral DNA is expressed, making copies of viral RNA and proteins that self assemble. New virus particles bud through the plasma membrane, surrounded by a membrane envelope derived from the host cell. Viruses probably appeared on Earth after cells evolved because they are OIPs. Viruses probably developed from the genetic material of their host, first as naked DNA/RNA, evolving the capsid to better survive and gain entry to their host cell. Viruses do not spontaneously generate. They evolve from preexisting viruses ( New flu each year!). RNA viruses evolve more quickly due to a higher mutation rate. 4

5 Pandemic Viral Disease in Humans Viral infection does not always produce disease. You have benign infections right now! Viruses cause disease by destroying cells, interfering with normal function/development. Antibiotics have no effect on viral infection because are not living cells, like bacteria. Vaccines PREVENT viral infection by teaching the immune system to recognize the virus, BEFORE infection. There are antiviral drugs that interfere with viral replication, but there are few and can also interfere with normal cellular function. A pandemic is a global epidemic. One virus/disease that the World Health Organization wiped off the face of the Earth (almost) by vaccinating EVERYONE is smallpox. Sadly, we don t have vaccines for all viruses, and vaccinating EVERYONE is difficult! 5

6 Viral Diseases Identify the diseases for which there are vaccines. Vaccines have been developed primarily for DNA viruses because they have a lower mutation rate. Viruses also infect our crops and domestic livestock! Vaccine! (New each year.) Ebola: Nausea, vomiting, diarrhea (may be bloody), red eyes, raised rash, chest pain and cough, stomach pain, severe weight loss, bleeding, usually from the eyes, and bruising (people near death may bleed from other orifices, such as ears, nose and rectum) (dsdna) Vaccine! (dsdna) Vaccine! Zika Ebola Fever, rash, joint pain, red eyes, microcephaly in newborns Hemorrhagic fever (see left) 6

7 Viroids Viroids are bare single-strand RNA loops of around 300 nucleotides. They do not code for protein. The simplest of infectious agents, viroids may be similar to the first virus-like particles and remnants from the RNA world. Potato Spindle Tuber Disease (elongated & misshapen potatoes) Viroids are short polynucleotides of naked RNA (no capsid), that infect only plants. Viroids usually enter their host through damaged tissues/cells because they have no capsid to help them gain entry to cells. Just like viruses, viroid replication interferes with host cell function. In the experiment diagram, the compounds that end in ase are hydrolytic enzymes. Along with the enzyme, each test tube contains sap from an infected plant. The purpose of no treatment is as a control group. Each enzyme in this experiment hydrolyzes a specific class of compounds. Blotches on the leaves represent the transmission of viroid infection. These results suggest that the disease causing agent is RNA only... A viroid! 7

8 Prions Prions are protein only, vs. viroids (RNA) and viruses (nucleic acid + protein). Prions infect only animals. Prion disease is transmitted by eating the tissue of an infected animal because they have no capsid to help gain entry to cells. Prion diseases lead to gradual deterioration of nervous system function. Prions initially form through protein misfolding. Prions misfold ( recruit ) other proteins. Amyloid plaques form in brain tissue as prions bind together to form fibrils. Fractionation of amyloids further accelerates recruitment. Diseases include Bovine Spongiform Encephalopathy (BSE) and Kuru. At this time, the only absolute diagnosis is postmortem visual inspection of the brain. In the figure at right, the prion infected brain is on the left. 8