CAMPBELL BIOLOGY IN FOCUS © 2014 Pearson Education, Inc. Urry Cain Wasserman Minorsky Jackson Reece Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge 17 Viruses

© 2014 Pearson Education, Inc. Overview: A Borrowed Life  A virus is an infectious particle consisting of little more than genes packaged into a protein coat  Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals

© 2014 Pearson Education, Inc. Figure  m

© 2014 Pearson Education, Inc. Viral Genomes  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

© 2014 Pearson Education, Inc. Capsids and Envelopes  A capsid is the protein shell that encloses the viral genome  Capsids are built from protein subunits called capsomeres  A capsid can have various structures

© 2014 Pearson Education, Inc. Figure 17.2 RNA Capsomere of capsid Glycoproteins Capsomere Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber Glycoprotein 80  225 nm 80–200 nm (diameter) 70–90 nm (diameter) 18  250 nm (a) Tobacco mosaic virus 20 nm (b) Adenoviruses 50 nm (c) Influenza viruses 50 nm (d) Bacteriophage T4 50 nm

© 2014 Pearson Education, Inc. Figure 17.2a RNA Capsomere of capsid Capsomere DNA Glycoprotein 70–90 nm (diameter) 18  250 nm (a) Tobacco mosaic virus 20 nm (b) Adenoviruses 50 nm

© 2014 Pearson Education, Inc.  Some viruses have membranous envelopes that help them infect hosts  These viral envelopes are derived from the host cell’s membrane and contain a combination of viral and host cell molecules

© 2014 Pearson Education, Inc.  Bacteriophages, also called phages, are viruses that infect bacteria  They have the most complex capsids found among viruses  Phages have an elongated capsid head that encloses their DNA  A protein tail piece attaches the phage to the host and injects the phage DNA inside

© 2014 Pearson Education, Inc. Concept 17.2: Viruses replicate only in host cells  Viruses are obligate intracellular parasites, which means they can replicate only within a host cell  Each virus has a host range, a limited number of host cells that it can infect

© 2014 Pearson Education, Inc. General Features of Viral Replicative 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  These exit from the host cell, usually damaging or destroying it Animation: Simplified Viral Replicative Cycle

© 2014 Pearson Education, Inc. Figure 17.3 VIRUS Replication Entry and uncoating DNA Capsid Transcription and manufacture of capsid proteins HOST CELL Viral DNA mRNA Capsid proteins Self-assembly of new virus particles and their exit from the cell

© 2014 Pearson Education, Inc. Replicative Cycles of Phages  Phages are the best understood of all viruses  Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

© 2014 Pearson Education, Inc. The Lytic Cycle  The lytic cycle is a phage replicative cycle that culminates in the death of the host cell  The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses  A phage that reproduces only by the lytic cycle is called a virulent phage  Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Animation: Phage T4 Lytic Cycle

© 2014 Pearson Education, Inc. Figure Attachment 1

© 2014 Pearson Education, Inc. Figure Attachment 1 Entry of phage DNA and degradation of host DNA 2

© 2014 Pearson Education, Inc. Figure Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3

© 2014 Pearson Education, Inc. Figure Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3 Assembly Phage assembly Head Tail Tail fibers 4

© 2014 Pearson Education, Inc. Figure Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3 Assembly Phage assembly Head Tail Tail fibers 4 Release 5

© 2014 Pearson Education, Inc. The Lysogenic Cycle  The lysogenic cycle replicates the phage genome without destroying the host  Phages that use both the lytic and lysogenic cycles are called temperate phages  The viral DNA molecule is incorporated into the host cell’s chromosome  This integrated viral DNA is known as a prophage

© 2014 Pearson Education, Inc.  Every time the host divides, it copies the phage DNA and passes the copies to daughter cells  A single infected cell can give rise to a large population of bacteria carrying the virus in prophage form  An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Animation: Phage Lysogenic and Lytic Cycles

© 2014 Pearson Education, Inc. Figure 17.5a The phage injects its DNA. Phage DNA and proteins are synthesized and assembled. The cell lyses, releasing phages. Lytic cycle Phage DNA circularizes. Phage DNA Phage Bacterial chromosome

© 2014 Pearson Education, Inc. Figure 17.5b Daughter cell with prophage Many cell divisions create many infected bacteria. Prophage is copied with bacterial chromosome. Phage DNA integrates into bacterial chromosome. Lysogenic cycle Prophage exits chromosome. Prophage

© 2014 Pearson Education, Inc. Replicative Cycles of Animal Viruses  There are two key variables used to classify viruses that infect animals  The nature of the viral genome (single- or double- stranded DNA or RNA)  The presence or absence of an envelope

© 2014 Pearson Education, Inc. Viral Envelopes  An animal virus with an envelope uses it to enter the host cell  The envelope is derived from the plasma membrane of a host cell, although some of the molecules on the envelope are specified by the genome of the virus

© 2014 Pearson Education, Inc. Figure 17.6 Capsid RNA Envelope (with glycoproteins) HOST CELL (RNA) New virus Copy of genome Viral genome (RNA) Template mRNA ER Glycoproteins Capsid proteins

© 2014 Pearson Education, Inc. 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)

© 2014 Pearson Education, Inc.  Viral DNA that is integrated into the host genome is called a provirus  Unlike a prophage, a provirus is a permanent resident of the host cell  The host’s RNA polymerase transcribes the proviral DNA into RNA molecules  The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new viruses released from the cell Animation: HIV Replicative Cycle

© 2014 Pearson Education, Inc. Figure 17.7 Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA HIV Membrane of white blood cell HIV entering a cell 0.25  m Provirus New virus New HIV leaving a cell

© 2014 Pearson Education, Inc. Figure 17.7a Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA Provirus New virus

© 2014 Pearson Education, Inc. Figure 17.7aa Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA

© 2014 Pearson Education, Inc. Figure 17.7ab NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA Provirus New virus

© 2014 Pearson Education, Inc. Figure 17.7b HIV Membrane of white blood cell HIV entering a cell 0.25  m New HIV leaving a cell

© 2014 Pearson Education, Inc. Evolution of Viruses  Viruses do not fit our definition of living organisms  Since viruses can replicate only within cells, they probably evolved after the first cells appeared  Candidates for the source of viral genomes are plasmids (circular DNA in bacteria and yeasts) and transposons (small mobile DNA segments)  Plasmids, transposons, and viruses are all mobile genetic elements

© 2014 Pearson Education, Inc. Concept 17.3: Viruses are formidable pathogens in animals and plants  Diseases caused by viral infections afflict humans, agricultural crops, and livestock worldwide

© 2014 Pearson Education, Inc. Viral Diseases in Animals  Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes  Some viruses cause infected cells to produce toxins that lead to disease symptoms  Others have molecular components such as envelope proteins that are toxic

© 2014 Pearson Education, Inc.  A vaccine is a harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen  Vaccines can prevent certain viral illnesses  Viral infections cannot be treated by antibiotics  Antiviral drugs can help to treat, though not cure, viral infections

© 2014 Pearson Education, Inc. Emerging Viruses  Viruses that suddenly become apparent are called emerging viruses.  HIV is a classic example  The West Nile virus appeared in North America first in 1999 and has now spread to all 48 contiguous states

© 2014 Pearson Education, Inc.  In 2009 a general outbreak, or epidemic, of a flu- like illness occurred in Mexico and the United States; the virus responsible was named H1N1  H1N1 spread rapidly, causing a pandemic, or global epidemic

© 2014 Pearson Education, Inc. Figure 17.8 (a) 2009 pandemic H1N1 influenza A virus (b) 2009 pandemic screening 1  m

© 2014 Pearson Education, Inc. Figure 17.8a (a) 2009 pandemic H1N1 influenza A virus 1  m

© 2014 Pearson Education, Inc.  Three processes contribute to the emergence of viral diseases  The mutation of existing viruses, which is especially high in RNA viruses  Dissemination of a viral disease from a small, isolated human population, allowing the disease to go unnoticed before it begins to spread  Spread of existing viruses from animal populations; about three-quarters of new human diseases originate this way

© 2014 Pearson Education, Inc.  Strains of influenza A are given standardized names  The name H1N1 identifies forms of two viral surface proteins, hemagglutinin (H) and neuraminidase (N)  There are numerous types of hemagglutinin and neuraminidase, identified by numbers

© 2014 Pearson Education, Inc. Viral Diseases in Plants  More than 2,000 types of viral diseases of plants are known; these have enormous impacts on the agricultural and horticultural industries  Plant viruses have the same basic structure and mode of replication as animal viruses  Most plant viruses known thus far have an RNA genome and many have a helical capsid

© 2014 Pearson Education, Inc.  Plant viral diseases spread by two major routes  Infection from an external source of virus is called horizontal transmission  Herbivores, especially insects, pose a double threat because they can both carry a virus and help it get past the plant’s outer layer of cells  Inheritance of the virus from a parent is called vertical transmission

© 2014 Pearson Education, Inc. Figure 17.UN01

© 2014 Pearson Education, Inc. Figure 17.UN02a A/California/07/2009 Group 1 A/Taiwan/1164/2010 Group 3 A/Taiwan/T0724/2009 Group 6 Group 7 Group 9 A/Taiwan/1018/2011 A/Taiwan/552/2011 Group 8 Group 8-1 Group 10 Group 11 A/Taiwan/T1773/2009 A/Taiwan/T1338/2009 A/Taiwan/T1821/2009 A/Taiwan/940/2009 A/Taiwan/937/2009 A/Taiwan/T1339/2009 A/Taiwan/7418/2009 A/Taiwan/8575/2009 A/Taiwan/4909/2009 A/Taiwan/8542/2009 A/Taiwan/2826/2009 A/Taiwan/T0826/2009 A/Taiwan/1017/2009 A/Taiwan/7873/2009 A/Taiwan/11706/2009 A/Taiwan/6078/2009 A/Taiwan/6341/2009 A/Taiwan/6200/2009 A/Taiwan/5270/2010 A/Taiwan/3994/2010 A/Taiwan/2649/2011 A/Taiwan/1102/2011 A/Taiwan/4501/2011 A/Taiwan/67/2011 A/Taiwan/1749/2011 A/Taiwan/4611/2011 A/Taiwan/5506/2011 A/Taiwan/1150/2011 A/Taiwan/2883/2011 A/Taiwan/842/2010 A/Taiwan/3697/2011

© 2014 Pearson Education, Inc. Figure 17.UN03 Phage DNA Prophage Bacterial chromosome The phage attaches to a host cell and injects its DNA. Lysogenic cycleLytic cycle Virulent or temperate phage Destruction of host DNA Production of new phages Lysis of host cell causes release of progeny phages Temperate phage only Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle