Viruses Chapter 26

Outline The Discovery of Viruses The Nature of Viruses Bacteriophages Cell Transformation and Phage Conversion AIDS The Future of HIV Treatment Disease Viruses Prions and Viroids

The Discovery of Viruses Viruses possess only a portion of the properties of organisms. segments of DNA or RNA wrapped in a protein coat must reproduce within cells vary greatly in appearance and size

Viral Structure

The Nature of Viruses Viral structure - core of nucleic acid surrounded by protein classified by nature of genomes RNA-based viruses - retroviruses nearly all form a protein sheath or capsid around their nucleic acid core Many animal viruses form an envelope around the capsid. Host range - suitable cells for a virus

Bacterial Virus

The Nature of Viruses Viral replication - Viruses can reproduce only when they enter cells and utilize the host’s cellular machinery. genes translated into proteins by the cell’s genetic machinery Viral shape helical - rodlike isometric - spiral icosahedron

Bacteriophages Bacteriophages - viruses that infect bacteria some named as members of a “T” series Lytic cycle tail fiber contacts lipoproteins of host bacterial cell wall tail contracts and tail tube passes through opening in base plate, piercing bacterial cell wall contents injected into host cytoplasm

Bacteriophages Lysogenic cycle Many bacteriophages integrate their nucleic acid into the genome of the infected host cell (prophage). The integration of a virus into a cellular genome is termed lysogeny. lysogenic cycle lysogenic (temperate) viruses

Cell Transformation and Phage Conversion Transformation - genetic alteration of a cell’s genome by the introduction of foreign DNA phage conversion - foreign DNA contributed by bacterial virus disease-causing bacteria Vibrio cholerae usually exists in harmless form bacteriophage that infects V. cholerae introduces into the host bacterial cell a gene that codes for the cholera toxin

AIDS Acquired Immunodeficiency Syndrome (AIDS) was first reported in the US in 1981. estimated over 33 million people worldwide are infected with Human Immunodeficiency Virus (HIV) Infection cycle In normal individuals, white blood cells patrol the bloodstream and attack invading bacteria or viruses.

AIDS In AIDS patients, the virus hones in on CD4+ T cells, infecting and killing them. Without T cells, the body cannot defend against invading bacteria or viruses. Each HIV particle possesses glycoprotein (gp120) on its surface that precisely fits a cell-surface marker protein (CD4) on surfaces of macrophages and T cells.

AIDS After docking onto the macrophage CD4 receptor, HIV requires a second macrophage receptor (CCR5) to cross the cell membrane. Once inside the macrophage, the HIV particle sheds its protective coat. RNA and reverse transcriptase left floating in cytoplasm double strand of DNA, complementary to RNA, produced viruses released via exocytosis

The Future of HIV Treatment Combination drug therapy AZT and protease inhibitors keeps disease in check Vaccine therapy may reduce reproductive capability of HIV Blocking replication chemokines bind to and block receptors CAF prevents viral replication Disabling receptors

Disease Viruses Many human diseases are caused by viruses: influenza, smallpox , chicken pox, herpes Viruses may also play a role in autoimmune diseases such as multiple sclerosis and diabetes. Influenza Flu viruses are animal retroviruses distinguished by their capsid. types A, B, and C subtypes differ in protein spikes

Disease Viruses Recombination Viral genes are readily re-assorted by genetic recombination. novel combinations of H and N spikes unrecognizable by human antibodies inability to make perfect vaccines flu pandemics

Disease Viruses Emerging viruses viruses that originate in one organism and then pass to another and cause disease Ebola Severe Acute Respiratory Syndrome (SARS) Viruses and cancer Viruses are capable of altering growth properties of human cells they infect by triggering oncogene expression.

Prions and Viroids Prions infectious proteins that some believe may be responsible for transmissible spongiform encephalopathies mad cow disease Viroids tiny, naked molecules of RNA that are an important infectious disease agent in plants

How Prions Arise

Summary The Discovery of Viruses The Nature of Viruses Bacteriophages Cell Transformation and Phage Conversion AIDS The Future of HIV treatment Disease Viruses Prions and Viroids