Overview: A Borrowed Life Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Viruses lead “a kind of borrowed life” between life-forms and chemicals The origins of molecular biology lie in early studies of viruses that infect bacteria

Fig. 19-1 Figure 19.1 Are the tiny viruses infecting this E. coli cell alive? 0.5 µm

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat Viruses were detected indirectly long before they were actually seen

RESULTS Extracted sap from tobacco plant with tobacco mosaic disease Fig. 19-2 RESULTS 1 Extracted sap from tobacco plant with tobacco mosaic disease 2 Passed sap through a porcelain filter known to trap bacteria 3 Rubbed filtered sap on healthy tobacco plants Figure 19.2 What causes tobacco mosaic disease? 4 Healthy plants became infected

Structure of Viruses Viruses are not cells Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

Viral genomes may consist of either Double- or single-stranded DNA, or Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Fig. 19-3 RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Capsomere of capsid Tail fiber Glycoprotein Glycoproteins 18  250 nm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm Figure 19.3 Viral structure 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

18  250 nm RNA Capsomere of capsid (a) Tobacco mosaic virus 20 nm Fig. 19-3a RNA Capsomere of capsid 18  250 nm Figure 19.3 Viral structure 20 nm (a) Tobacco mosaic virus

DNA Capsomere Glycoprotein 70–90 nm (diameter) (b) Adenoviruses 50 nm Fig. 19-3b DNA Capsomere Glycoprotein 70–90 nm (diameter) Figure 19.3 Viral structure 50 nm (b) Adenoviruses

Membranous envelope RNA Capsid Glycoproteins 80–200 nm (diameter) Fig. 19-3c Membranous envelope RNA Capsid Glycoproteins 80–200 nm (diameter) Figure 19.3 Viral structure 50 nm (c) Influenza viruses

Head DNA Tail sheath Tail fiber 80  225 nm (d) Bacteriophage T4 50 nm Fig. 19-3d Head DNA Tail sheath Tail fiber 80  225 nm Figure 19.3 Viral structure 50 nm (d) Bacteriophage T4

Concept 19.2: Viruses reproduce only in host cells Viruses are intracellular parasites, which means they can reproduce only within a host cell Each virus has a host range, a limited number of host cells that it can infect

General Features of Viral Reproductive Cycles Once a viral genome has entered a cell, the cell begins to manufacture viral proteins The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses Animation: Simplified Viral Reproductive Cycle

VIRUS Entry and uncoating DNA Capsid Transcription and manufacture Fig. 19-4 VIRUS Entry and uncoating 1 DNA Capsid Transcription and manufacture of capsid proteins 3 2 Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Figure 19.4 A simplified viral reproductive cycle Self-assembly of new virus particles and their exit from the cell 4

Reproductive Cycles of Phages Phages are the best understood of all viruses

Fig. 19-5-1 1 Attachment Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA 1 2 Fig. 19-5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage 3 Synthesis of viral genomes and proteins

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

Attachment Entry of phage DNA and degradation of host DNA Release Fig. 19-5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

Table 19-1a Table 1

Table 19-1b Table 1

Viral Envelopes Many viruses that infect animals have a membranous envelope Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

Capsid and viral genome enter the cell Capsid Fig. 19-7 Capsid and viral genome enter the cell Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins Figure 19.7 The reproductive cycle of an enveloped RNA virus New virus

RNA as Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animals Retroviruses use reverse transcriptase to copy their RNA genome into DNA HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

Fig. 19-8 Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse transcriptase HIV Membrane of white blood cell HIV HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS RNA genome for the next viral generation mRNA New virus New HIV leaving a cell

Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse Fig. 19-8a Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse transcriptase HOST CELL HIV Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS mRNA New virus

Membrane of white blood cell HIV HIV entering a cell Fig. 19-8b Membrane of white blood cell HIV Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS 0.25 µm HIV entering a cell New HIV leaving a cell

Evolution of Viruses Viruses do not fit our definition of living organisms Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid

Viral Diseases in Animals Viruses may damage or kill cells by causing the release of digestive enzymes from lysosomes Some viruses cause infected cells to produce poisons (toxins) that lead to disease symptoms Others have envelope proteins that are toxic

Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual virus Vaccines can prevent certain viral illnesses Viral infections cannot be treated by antibiotics Antiviral drugs can help to treat, though not cure, viral infections

Emerging Viruses Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists Severe acute respiratory syndrome (SARS) recently appeared in China Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

Flu epidemics are caused by new strains of influenza virus to which people have little immunity Viral diseases in a small isolated population can emerge and become global New viral diseases can emerge when viruses spread from animals to humans Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

These strains can cause pandemics, global epidemics The “bird flu” is a virus that recently appeared in humans and originated in wild birds

(a) The 1918 flu pandemic (b) Influenza A H5N1 virus Fig. 19-9 (a) The 1918 flu pandemic 0.5 µm Figure 19.9 Influenza in humans and other animals For the Discovery Video Emerging Diseases, go to Animation and Video Files. (b) Influenza A H5N1 virus (c) Vaccinating ducks

(a) The 1918 flu pandemic Fig. 19-9a Figure 19.9 Influenza in humans and other animals (a) The 1918 flu pandemic

(b) Influenza A H5N1 virus 0.5 µm Fig. 19-9b Figure 19.9 Influenza in humans and other animals (b) Influenza A H5N1 virus

(c) Vaccinating ducks Fig. 19-9c Figure 19.9 Influenza in humans and other animals (c) Vaccinating ducks

Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots

Fig. 19-10a Figure 19.10 Viral infection of plants

Fig. 19-10b Figure 19.10 Viral infection of plants

Fig. 19-10c Figure 19.10 Viral infection of plants

Viroids and Prions: The Simplest Infectious Agents Viroids are circular RNA molecules that infect plants and disrupt their growth Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals Prions propagate by converting normal proteins into the prion version Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions

Original Prion prion Aggregates of prions New prion Normal protein Fig. 19-11 Original prion Prion Aggregates of prions New prion Normal protein Figure 19.11 Model for how prions propagate