Transcript Viruses
21.9 Viruses Viruses are small particles of RNA and DNA containing 3 to 200 genes. The Epstein–Barr virus (EBV), herpesvirus 4, causes cancer in humans. Learning Goal Describe the methods by which a virus infects a cell. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Viruses Viruses • are small particles of DNA or RNA that require a host cell to replicate and cause infection. • can replicate only in cells, by taking over the machinery and materials necessary for protein synthesis and growth. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Viral Infections A virus causes infection when • an enzyme in the protein coat of the virus makes a hole in the outside of the host cell; • the virus enters the cell and the viral nucleic acid mixes with the materials in the host cell; • a protease processes proteins to produce a protein coat encasing the new viral RNA or DNA; and • the new virus particles are released from the cell, ready to infect more cells. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Diseases Caused by Viruses General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Reverse Transcription In reverse transcription, • a retrovirus, which contains viral RNA but no viral DNA, enters a cell. • the viral RNA uses reverse transcriptase to produce a viral DNA strand. • the viral DNA strand forms a complementary DNA strand. • the new DNA uses the nucleotides and enzymes in the host cell to synthesize new virus particles. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. HIV Virus and AIDS The HIV-1 virus • is a retrovirus that infects and destroys T4 lymphocyte cells. • leaves the immune system unable to destroy harmful organisms. • is associated with an increased chance of developing pneumonia and skin cancer associated with AIDS. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Retroviruses After a retrovirus injects its viral RNA into a cell, it forms a DNA strand by reverse transcription. The single-stranded DNA forms a double-stranded DNA called a provirus, which is incorporated into the host cell DNA. When the cell replicates, the provirus produces the viral RNA needed to produce more virus particles. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Retroviruses General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. AIDS Treatment Treatment for AIDS is based on • attacking the HIV at different points in its life cycle. • developing nucleoside analogs that mimic the structures of the nucleosides used for DNA synthesis. Drugs based on nucleoside analogs used to treat HIV/AIDS include • AZT (3ʹ-azido-2ʹ-deoxythymidine), which is similar to thymidine. • ddI (2ʹ, 3ʹ–dideoxyinosine), which is similar to guanosine. • ddC (2ʹ,3ʹ-dideoxycytidine). • d4T (2ʹ,3ʹ-didehydro-2ʹ,3ʹ-dideoxythymidine). General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. AIDS Treatment, Nucleoside Analogs When a nucleoside analog such as AZT, ddI, ddC, or d4T is incorporated into viral DNA, • the lack of a hydroxyl group on the 3ʹ-carbon in the sugar prevents the formation of the sugar– phosphate bonds. • the production of viral DNA is stopped. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. AIDS Treatment Current treatment for HIV and AIDS involves a combination of drugs that include • entry inhibitors. • reverse transcriptase inhibitors. • protease inhibitors. Lexiva metabolizes slowly to provide amprenavir, an HIV protease inhibitor. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Cancer When cells begin to grow and multiply without control, they invade nearby cells and appear as a tumor. Tumors • that are limited are called benign. • that invade other tissues, interfering with normal body functions, are cancerous. Cancer can be caused by • chemical and environmental substances. • ultraviolet or medical radiation. • oncogenic viruses. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake Epstein–Barr virus (EBV), herpesvirus 4, causes cancer in humans. © 2016 Pearson Education, Inc. Chemistry Link to Health: Cancer Most cancers are caused by environmental and chemical substances (carcinogens), including aniline dyes, cigarette smoke, and asbestos. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Chemistry Link to Health: Cancer Oncogenic viruses cause cancer when cells are infected. Some cancers such as retinoblastoma and breast cancer appear to occur within families. There is some indication that a missing or defective gene may be responsible. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Match the following terms to their definitions: 1) virus 2) retrovirus 3) protease inhibitor 4) reverse transcription A. a virus containing RNA B. small particles requiring host cells to replicate C. a substance that prevents the synthesis of viral proteins D. using viral RNA to synthesize viral DNA General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Match the following terms to their definitions: 1) virus 2) retrovirus 3) protease inhibitor 4) reverse transcription A. a virus containing RNA B. small particles requiring host cells to replicate C. a substance that prevents the synthesis of viral proteins D. using viral RNA to synthesize viral DNA General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2) retrovirus 1) virus 3) protease inhibitor 4) reverse transcription © 2016 Pearson Education, Inc. Concept Map General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.