Viruses PDF

Viruses  Learning Outcomes To understand the basic structure of viruses, and the function of those structures. To understand the different replicative processes of selected example viruses. To provide examples of transmission routes for selected example virus...

Viruses  Learning Outcomes To understand the basic structure of viruses, and the function of those structures. To understand the different replicative processes of selected example viruses. To provide examples of transmission routes for selected example viruses.  Headline grabbing! In recent years, outbreaks of viral infections of humans & animals have caught the headlines…. But, viruses can also cause significant disease in plants as well. All of this means either suffering & disease, or a decrease in available food. All of which will have significant effects on our lives & livelihoods. BBC  What are viruses? Viruses are very small infectious agents. They are obligate intracellular parasites – What do you think this means? Viruses have a very different structure to prokaryotic & eukaryotic cells. They have no cytoplasm, organelles or nucleus. But, they do have some unique features… Viral structure: key features EEPA  Viral nucleic acid The viral genome can consist of either DNA or RNA, but not both. You can also have double stranded or single stranded DNA or RNA viruses. Therefore… dsDNA, ssDNA, dsRNA, or ssRNA.  Examples of different viruses, classified by nucleic acid dsDNA ssDNA dsRNA ssRNA Virus name Adenovirus Aveparvo- Hypo- Rubella virus viridae Host Humans Chickens Fungi Humans organism Disease Colds, plus Gastro- (Decreased Rubella name others enteritis virulence)  Viral acquisition of an envelope In addition to genetic variability, some viruses possess a an envelope. The envelope is composed of phospholipids and glycoproteins. They virus “hijacks” part of the host cell membrane when it’s replication cycle is complete, and as it leaves the host cell. Therefore, it takes part of the host cell with it.  Attachment (or adherence) Viruses are non-motile. They are passively transported until contact with a suitable host (e.g. through aerosols in sneezes, blood flow, sexual contact). Enveloped viruses: during replication, their nucleic acid codes for a receptor which the host cell produces, and expresses on its’ cell membrane. Non-enveloped viruses: a protein on the viral capsid functions as the receptor.  Viral attachment & penetration Viruses must adhere to a host cell in order to begin the infection cycle of the host; also known as the life cycle of the virus. Different viruses do this in different ways. Once adherent, the virus must penetrate through the membrane in to the host cell cytoplasm.  Non-enveloped viral attachment to host cells Non-enveloped viruses enter the host through endocytosis. This is where the binding of the virus to the membrane receptor triggers transportation in to the cell within a vesicle. Once inside, the vesicle breaks down, and the virus begins to replicate. Nature  Enveloped viral attachment to host cells Enveloped viruses usually enter by fusion with the membrane. The fusion of the two plasma membranes leads to the release of the viral capsid inside the host cell, ready to begin replication. Nature  Viral “uncoating” & replication Both enveloped or non-enveloped viruses contain their nucleic acid within a capsid. This must be degraded before replication can begin. This process known as uncoating, can be triggered in the transport vehicles which maintain an acidic environment, or by host cell enzymes designed to destroy proteins (proteases).  Viral replication Next, the two key stages are: 1. nucleic acid replication; 2. capsid protein replication. As the viral proteins assemble within the host cell, they assemble in to complete virus particles (virions) around the newly replicated nucleic acids. These are now referred to as nucleocapsids, until release. Colorado.edu  Viral exit by budding Image result for nucleocapsid budding alpfmedical  Classification & diseases DNA viruses RNA viruses Single stranded Double stranded Single stranded Double stranded DNA viruses DNA viruses RNA viruses RNA viruses Blue tongue virus bacteriophage (Human) Herpes virus  X174 type 1 Rhinovirus Cold sores Blue Tongue disease Infects Escherichia coli Genital herpes Common cold  Bacteriophage stats Sizes range from 10- 200 nm T4 is among the largest phages; it is approximately 200 nm long and 80-100 nm wide. Other phages are smaller. Most phages range in size from Magnification = 25,000X 24-200 nm in length. http:// www.zyvexlabs.com/EIPBNuG/200 5MicroGraph.html Effect of X174 on lawn of Escherichia coli  Classification of viruses – based on nucleic acids DNA RNA viruses viruses Double Single Single Double stranded DNA stranded stranded RNA stranded RNA viruses DNA viruses viruses viruses Negative Positive sense sense Retroviruses Tobacco Mosaic Human Virus Canine Immunodeficiency Distemper Virus Virus  Replication of DNA viruses Double stranded DNA virus Single stranded DNA virus Host RNA polymerase transcribes mRNA from viral DNA Viral mRNA feeds into host protein synthesis mechanisms Viral proteins Enzymes (e.g. viral DNA Viral proteins to make polymerase to make new capsids more copies of genome)  Classification of viruses – based on nucleic acids dsDNA DNA viruses Virus name Adenovirus Host Humans Double Single organism stranded DNA stranded viruses DNA viruses Disease Colds, plus name others  Adenovirus infection (in humans) Adenoviruses can cause a wide range of illnesses such as: common cold, sore throat, bronchitis, pneumonia, diarrhoea, conjunctivitis, bladder inflammation or infection, inflammation of stomach and intestines britannica (gastroenteritis).  Adenovirus infection (in humans) Adenoviruses can be spread (transmission routes) by the following means/ mechanisms: close personal contact, such as touching or shaking hands; through the air by coughing and sneezing; touching an object or surface with adenoviruses on it, then touching your mouth, nose, or eyes before washing your hands.  Examples of different viruses, classified by nucleic acid DNA ssDNA viruses Virus name Aveparvo- virus Double Single Host Chickens stranded DNA stranded organism viruses DNA viruses Disease Gastro- name enteritis  Aveparvovirus infections in chickens Infections of young turkeys & broiler chickens were first reported in the 1980s. Disease symptoms include: Impaired growth, poor feathering, diarrhoea, distended intestines & weakened bones. Sometimes referred to as “runting-stunting syndrome” Researchgate  Aveparvovirus infections in chickens Can lead to decreased hatching of eggs, and cerebellar hyperplasia. A recent study (2017) determined that 66% of birds surveyed on Polish turkey farms tested positive for one of four key enteric viruses. Interestingly, Parvovirus was detected in ~27% of healthy birds – What does this signify?  Examples of different viruses, classified by nucleic acid dsRNA RNA viruses Virus name Hypo- viridae Host Fungi Single Double stranded stranded RNA organism RNA viruses viruses Disease (Decreased name virulence)  Replication of dsRNA viruses Double stranded RNA virus viral RNA polymerase transcribes viral mRNA Viral proteins Enzymes (e.g. viral RNA polymerase Viral proteins to make to make more copies of genome) new capsids  Image result for chestnut blight Hypovirus infection of fungi This is an unusual viral infectious disease of fungi. The virus infects a type of fungus that parasites trees. The fungus itself is highly virulent, and is believed to have killed millions of trees once imported to the Americas from Asia. norfolktrees  Hypovirus Research has shown that infection by the virus leads to a hypovirulent form of the fungus. This, in turn, leads to less severe infection of the trees by the fungus. Due to viral infection of the fungus, it is therefore possible for trees to recover from the infection.  Hypovirus Image result for Thyreophagus corticalis The virus has been found to be present in the droppings of mites, which spread between trees. The virus can then infect the fungus, and therefore might be able to be used to control fungal disease in trees. Mite: Thyreophagus corticalis Hypovirus https://www.sciencedirect.com/science/article/pii/S1878614615002056  Hypovirus The disease is unusual because of its life cycle. ScienceDirect Virus: Cryphonectria hypovirus 1 (Hypovirus) Fungal host: Cryphonectria parasitica Plant: Castanea sativa (chestnut tree)  Examples of different viruses, classified by nucleic acid ssRNA RNA viruses Virus name Rubella Single Double Host Humans stranded stranded RNA organism RNA viruses viruses Disease Rubella name Negative Positive sense sense  ssRNA viruses The ssRNA viruses can be divided in to two types: 1. positive sense 2. negative sense Remember- binding nucleic acid strands are complementary, not identical! What this means in that positive sense genomes can be directly translated in to viral proteins. Negative sense cannot be, and must undergo one further step…  Replication of ssRNA viruses Positive sense single Negative sense single stranded RNA virus stranded RNA Viral RNA polymerase Viral genome acts as produces a positive sense mRNA; feeds into host strand; and then it acts as protein synthesis mRNA; feeds into host mechanisms protein synthesis mechanisms Viral proteins Enzymes (e.g. viral RNA Viral proteins to make polymerase to make new capsids more copies of genome)  Rubella Image result for rubella virus UNSW Embryology  Rubella In children, rubella is usually mild, with few noticeable symptoms. For children who do have symptoms, a red rash is typically the first sign. The rash generally first appears on the face and then spreads to the rest of the body, and lasts about three days. cdc & nhs  Rubella Other symptoms that may occur 1 to 5 days before the rash appears include: fever, headache, mild pink eye, general discomfort, cough, runny nose, plus others. Most adults who get rubella usually have a mild illness, with low-grade fever, sore throat, and a rash that starts on the face and spreads to the rest of the body (spread by aerosol droplets). cdc & nhs  And finally…. retroviruses Retroviruses can infect many species of animal, including humans. Examples: bovine leukaemia virus, human immunodeficiency virus, simian immunodeficiency virus  Retroviruses The name Retrovirus is a very broad term, and refers to certain viruses that use RNA as their nucleic acid core. The viruses carry an enzyme known as Reverse Transcriptase (RT). This enzyme transcribes RNA into DNA – a reversal of the usual cellular process. This makes it possible for genetic material from a retrovirus to become permanently incorporated into the genome of an infected cell. Image result for hiv virus structure HIV www.cgl.ucsf.edu  Replication of Retroviruses Positive sense single stranded RNA Viral reverse transcriptase enzyme uses viral RNA as template to make negative sense ssDNA; host enzymes copy this to make ds DNA strand which is integrated into host genome; thus feeds into host protein synthesis mechanisms. Viral proteins Enzymes (e.g. viral reverse Viral proteins to make transcriptase to make more new capsids copies of genome)  Summary Points Viruses are obligate intracellular parasites. Viruses are classified according to whether they possess ds/ssDNA or ds/ssRNA as their nucleic acid, with or without an envelope. They are highly specific to a host species. Their reproductive cycles depend on them being either DNA or RNA- based; each follows a distinct pattern. Learn at least one process of viral replication.  Further Reading Brock Biology of Microorganisms – Chapter 5 Paper copies are available from the library, or an online version is available via the READING LIST tab for this module.