Biology Chapter 19 Viruses PDF

Chapter 19 Viruses Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatrick Figure 19.1b © 2021 Pearson Education, Inc. CONCEPT 1...

Chapter 19 Viruses Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatrick Figure 19.1b © 2021 Pearson Education, Inc. CONCEPT 19.1: A virus consists of a nucleic acid surrounded by a protein coat A virus is an infectious particle consisting of genes packaged in a protein coat – Viruses are much simpler in structure than even prokaryotic cells Viruses can cause a wide variety of diseases They cannot reproduce or carry out metabolism outside of a host cell Viruses exist in a shady area between life-forms and chemicals, leading a kind of “borrowed life” © 2021 Pearson Education, Inc. The Discovery of Viruses: Scientific Inquiry Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration In the late 1800s, hypothesized that unusually small bacteria might be responsible Infectious agent did not share features with bacteria - such as the ability to grow on nutrient media In 1935, Wendell Stanley confirmed this latter hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) © 2021 Pearson Education, Inc. Structure of Viruses Viruses are not cells Very small infectious particle consisting of: – nucleic acid – protein coat – in some cases, a membranous envelope The simple structure of viruses make them a useful biological system © 2021 Pearson Education, Inc. Viral Genomes Viruses are classified as DNA viruses or RNA viruses Viral genomes may consist of either – double- or single-stranded DNA or – double- or single-stranded RNA The genome is either a single linear or circular molecule of the nucleic acid Viruses have between 3 and 2,000 genes in their genome © 2021 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 a variety of structures; associated viruses may be referred to as helical or icosahedral viruses © 2021 Pearson Education, Inc.  Some viruses have accessory structures that help them infect hosts Viral envelopes – derived from membranes of host cells – surround the capsids (influenza viruses and many other viruses found in animals) – contain a combination of viral and host cell molecules © 2021 Pearson Education, Inc.  Bacteriophages, also called phages, are viruses that infect bacteria They 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 © 2021 Pearson Education, Inc. CONCEPT 19.2: 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 species that it can infect Some viruses have broad host ranges, while others are able to infect only one species – For example, measles virus only infects humans © 2021 Pearson Education, Inc. General Features of Viral Replicative Cycles The viral genome enters the host cell in a variety of ways 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 © 2021 Pearson Education, Inc. Animation: Simplified Viral Replicative Cycle © 2021 Pearson Education, Inc. Replicative Cycles of Phages Phages are the best understood of all viruses Phages have two alternative reproductive mechanisms: – the lytic cycle – the lysogenic cycle © 2021 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 © 2021 Pearson Education, Inc. Figure 19.5 © 2021 Pearson Education, Inc. Animation: Phage Lytic Cycle © 2021 Pearson Education, Inc.  The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated into the host cell’s chromosome Phages that use both the lytic and lysogenic cycles are called temperate phages – A temperate phage called lambda (l) is widely used in biological research © 2021 Pearson Education, Inc. Figure 19.6 © 2021 Pearson Education, Inc.  The integrated viral DNA is known as a prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Some prophages are expressed during lysogeny, and some cause the host bacteria to secrete toxins that are harmful to humans – Shiga toxins (Stx) in Escherichia coli, cholera toxin (CT) in Vibrio cholerae, and botulinum neurotoxins (BoNTs) in Clostridium botulinum. © 2021 Pearson Education, Inc. Animation: Phage Lysogenic and Lytic Cycles © 2021 Pearson Education, Inc. Bacterial Defenses Against Phages Bacteria have their own defenses against phages: 1. Natural selection favors bacterial mutants with surface proteins that cannot be recognized as receptors by a particular type of phage 2. Foreign DNA can be identified as such and cut up by cellular enzymes called restriction enzymes The bacterium’s own DNA is protected from the restriction enzymes by being methylated © 2021 Pearson Education, Inc.  3. Both bacteria and archaea can protect themselves from viral infection with the CRISPR-Cas system – It is based on sequences called clustered regularly interspaced short palindromic repeats (CRISPRs) – Each “spacer” sequence between the repeats corresponds to DNA from a phage that had infected the cell – Particular nuclease proteins interact with the CRISPR region; these are called CRISPR-associated (Cas) proteins © 2021 Pearson Education, Inc.  The attempt of the phage to infect the cell triggers transcription of the CRISPR region – The resulting RNAs are cut into pieces, bound by Cas proteins, and used to target the invading phage DNA The phage DNA is cut and destroyed The relationship between phage and bacteria is in constant evolutionary flux © 2021 Pearson Education, Inc. Replicative Cycles of Animal Viruses There are two key variables used to classify viruses that infect animals: – An RNA or DNA genome, either single-stranded or double-stranded – The presence or absence of a membranous envelope Whereas few bacteriophages have an envelope or an RNA genome, many animal viruses have both © 2021 Pearson Education, Inc. 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 Usually derived from the host cell’s plasma membrane as the viral capsids exit – Some form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane (Herpes) © 2021 Pearson Education, Inc. Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animals – dsRNA – ssRNA that acts as mRNA – ssRNA that is a template for mRNA synthesis (RNA dependent RNA synthesis) – ssRNA that serves as template for DNA synthesis (RNA dependent DNA synthesis; reverse transcriptase) Retroviruses (HIV) © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Retroviruses and other integrating viruses The viral DNA that is integrated into the host genome is called a provirus – Unlike a prophage, a provirus remains a permanent resident of the host cell 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 virus particles released from the cell © 2021 Pearson Education, Inc. Animation: HIV Replicative Cycle Animation: Retrovirus (HIV) Replicative Cycle © 2021 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 as bits of cellular nucleic acid Candidates for the source of viral genomes include plasmids and transposons – Plasmids, transposons, and viruses are all mobile genetic elements The largest virus identified about 20 years ago is the size of a small bacterium – Its genome encodes proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis There is controversy about whether this virus evolved before or after cells In the past decade several even larger viruses have been identified; how these evolved is an unresolved question © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. CONCEPT 19.3: Viruses and prions are formidable pathogens in animals and plants Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide Smaller, less complex entities called prions also cause disease in animals © 2021 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 © 2021 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, such as smallpox, rubella, mumps, and others Viral infections cannot be treated by antibiotics Antiviral drugs can help to treat, not cure, viral infections by inhibiting synthesis of viral DNA and by interfering with viral assembly © 2021 Pearson Education, Inc. Emerging Viral Diseases Emerging viruses are those that suddenly become apparent HIV, the AIDS virus, is a classic example © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc.  The Ebola virus is one of several emerging viruses that cause hemorrhagic fever, an often fatal illness In 2014, a widespread outbreak (epidemic) of Ebola virus occurred In 2017, 2018, and 2019, smaller outbreaks occurred in the Democratic Republic of the Congo Other examples of emerging viruses include the chikungunya virus and the recently emerging Zika virus (2015) © 2021 Pearson Education, Inc. Causes of rapidly emerging viral disease in humans 1. Mutation of existing viruses into new ones that can spread more easily 2. Spread of a viral disease from a small, isolated human population 3. Spread of existing viruses from other animals – It is estimated that about three-quarters of new human diseases originate in this way © 2021 Pearson Education, Inc. Flu Flu epidemics are caused by type A influenza viruses – infect a wide variety of animals including birds, pigs, horses, and humans Strains of influenza A are given standardized names based on the viral surface proteins hemagglutinin (HA) and neuraminidase (NA) As of 2017 18 types of HA, and 11 types of neuraminidase, have been identified © 2021 Pearson Education, Inc. Flu H5N1 (deadly), because it is very different from influenza strains circulating among people for a long time – It is thus difficult for people to mount an effective immune response to this strain – However, it has not caused an epidemic because it is not transmitted from person-to-person H1N1 (deadly), originally called the swine flu, was not actually transmitted to humans from pigs – was a unique combination of swine, avian, and human influenza genes – An epidemic of H1N1 occurred in 2009, reaching 207 countries, infecting over 600,000 people and killing almost 8,000 A global epidemic like this is called a pandemic © 2021 Pearson Education, Inc. Flu Influenza viruses have nine RNA segments in their genome, leading to many new genetic combinations They also have a high rate of mutation Normal seasonal flu viruses are not considered emerging viruses because variants of these viruses have been circulating among humans for a long time – However, these viruses still undergo mutation and reassortment Variations thought to be most likely to occur each year are selected to generate vaccines © 2021 Pearson Education, Inc. Increasing the Spread Changes in host behavior or the environment can increase the spread of viruses responsible for emerging diseases New roads into a remote area may increase spread of viral diseases The use of insecticides and mosquito nets may help prevent the spread It is possible that global climate change may allow mosquitoes that carry viruses to expand their range © 2021 Pearson Education, Inc. 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 Most known plant viruses have an RNA genome Many have a helical capsid, while others have an icosahedral capsid Plant viruses spread disease by two major routes: – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent © 2021 Pearson Education, Inc. Prions: Proteins as Infectious Agents Prions are infectious proteins that appear to cause degenerative brain diseases in animals Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans are all caused by prions Prions are incorrectly folded proteins, can be transmitted in food, act slowly, and are virtually indestructible © 2021 Pearson Education, Inc.  Prions are somehow able to convert a normal form of a protein into the misfolded version Then several prions aggregate into a complex that can convert more proteins to prions, which join the chain Prions might also be involved in diseases such as Alzheimer’s and Parkinson’s disease There are many outstanding questions about these small infectious agents © 2021 Pearson Education, Inc. Figure 19.UN04 © 2021 Pearson Education, Inc.