Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
6 Viruses At the Threshold of Life The Nature of Viruses • Not really alive • Not really completely inert • Existence somewhere between living objects and chemical compounds The Discovery and Structure of Viruses • The development of virology – 1892 Dmitri Iwanoski • Tobacco mosaic virus (TMV) • Bacteria-free filtrate still infectious • 1898 Martinus Beijerinck supported these data – 1930s Wendell Stanley • Crystals of TMV indicated chemical molecule • Body responses indicated infectious microbes • The debate continued… – 1933 Ernst Ruska • Invention of electron microscope • Finally allowed visualization of viruses © National Library of Medicine Figure 6.1a: The Russian pathologist Dmitri Iwanowski The Discovery and Structure of Viruses • The structure of viruses – Size • As small as 27 nm (0.027 mM); poliovirus • At least as large as 250 nm (0.25 mM); smallpox • At least 500 could fit inside single bacterium! The Discovery and Structure of Viruses • The structure of viruses – Symmetry • Helix • Icosahedron • Complex The Discovery and Structure of Viruses The Discovery and Structure of Viruses • The components of viruses – Minimum of nucleic acid and protein = naked – Some have an outer envelope – Nucleic acid • • • • • Called the “Genome” RNA or DNA, not both Segmented or non-segmented Circular or linear Single-stranded or double-stranded Fig. 6.4a: naked Icosahedral virus – Protein • Capsid • Capsomeres – Nucelocapsid = genome + capsid Fig. 6.4b: A naked helical virus The Discovery and Structure of Viruses • The components of viruses – Envelope • Lipids • Spikes – No cytoplasm – No metabolism – No increase or decrease in size Fig. 6.4 Icosahedral and helical viruses Figure 6.COinsert: A TEM of the ultra structural details of a number of influenza virus particles, or “virions” Courtesy of Dr. F.A. Murphy/CDC Viral Replication: Lytic Cycle Viral Replication • Viral replication and disease – Destruction of healthy cells – EXAMPLES • Viral hepatitis – Destruction of liver cells by infection – Loss of liver function • AIDS – Destruction of immune cells – Loss of immune control over other infectious agents • Rabies – Destruction of nerves – Loss of control by nervous system Viral Replication • Defense against viruses – Immune system • Antibodies: protein molecules made by B cells in the immune system, in response to the presence of a virus. • T cells: attack infected cells – Antiviral drugs • Acyclovir • Amantadine • Azidothymidine (AZT) – Interferons: produced by cells when attacked (about 20) Viral Replication • Viral vaccines – Stimulation of immune response without risking pathogenic infection – Inactivated • Chemically destroyed virus genome, capsid intact • Example: Salk polio vaccine – Attenuated • Live, but weakened virus • Example: Sabin polio vaccine, chickenpox, MMR – Genetically engineered • Production of viral proteins by recombinant organism • Example: Hepatitis B vaccine Viral Replication • When viruses don’t replicate – Lysogeny • Integration of viral genome into host’s genome • Viral genome become dormant • Integrated viral genome is known as provirus • Human equivalent is retrovirus, HIV • Typical for many plant viruses Figure 6.9: The formation of a provirus Viral Replication • Viroids and prions – Viroids • Single-stranded RNA • No capsid • Infect plants – Prions Proteinaceous infectious particles No nucleic acid Cause mad cow disease Normal cellular protein Change in conformation creates Prion Results in spongiform encephalopathy © Ulf Sirborn/AP Photos • • • • • • Figure 6.11a: Stanley Prusiner, winner of 1997 Nobel in Physiology or Medicine for his work on prions as agents of infectious disease Viruses and Cancer • The development of cancer – Cancer • Uncontrolled mitotic reproduction of cells • Faster replication than normal cells • Result is mass of cells called tumor – – – – – – Benign tumors Malignant tumors Oncology Dedifferentiation Loss of contact inhibition Modes of disease Viruses and Cancer • The involvement of viruses – Carcinogens – Some viruses can induce tumors – Examples • Epstein-Barr virus • Human T cell leukemia viruses (HTLVs) • How viruses transform cells – Michael Bishop and Harold Varmus • Oncogenes • Proto-oncogenes • How viruses may turn proto-oncogenes into oncogenes Viruses and Cancer: How Viruses Transform Cells Figure 6.12: The oncogene theory