Learning objectivesi. Describe the history of virus discovery.ii. Contrast the characteristic of a virus as compared to a living cell.iii. Understand the origin of virus.iv. Explain virus component and shape.v. Characterize a bacteriophage.vi. Contrast bacteriophage lytic and lysogenic life cycle.vii. Describe the life cycle of an enveloped DNA and RNA virus.viii.State the characteristic of viriods and prions.
History of virus discoveryIn the late 1800s,botanists had been tryingto find the cause oftobacco mosaic disease.In 1892, D. IWANOWSKItried to filter the sap ofinfected tobacco plants(Filter capable ofremoving particles thesize of all knownbacteria).
History of virus discovery The filtrate was FULLY CAPABLE of producing the ORIGINAL DISEASE in new hosts. Nothing could be seen in the filtrates using the most powerful microscopes, nor could anything be cultivated from the filtrates. Iwanowski concluded that the bacteria was so small / or they made a filterable toxin.
History of virus discoveryA Dutch botanist named MartinusBeijerink ruled out the filterable toxinconclusion because the filtered sapare capable of causing undilutedinfection.The agent cannot be cultivated onnutrient media (need a host)In 1935, Stanleydiscovered this agentafter crystallization
Virus characteristicsViruses are not classified into any of thebiological classification system.They lie in the threshold of life andnonlife.
Virus characteristics Non-Life LifeThey are acellular, They could onlywith no cell nucleus, reproduce within theorganelles or living cells that theycytoplasm. Therefore, infect. They use theirthey do not have the genetic information tocomponents necessary force the host cell toto carry metabolic replicate themselvesactivities (obligate intracellularindependently. parasite) .Viruses cannot moveand reproduce ontheir own.
Virus size Viruses are smaller than bacteria. Viruses are too small even to be seen by a light microscope. The biggest size virus is about 240-300nm (1/10 of red blood cells/ size of the smallest bacteria) The tiniest virus is 20nm – smaller than a ribosome
Virus origin According to a hypothesis, viruses are bits of nucleic acid that ‘escaped’ from cellular organism. Some traces are from animal cells, plant cells and bacterial cells. Their multiple origins explain why viruses are species-specific. However, some other have broader range of host cells
Virus component Virus consist of only I) Nucleic acid (DNA or RNA) The DNA /RNA could be single or double stranded. II) A capsid or a protein coat which functions in protecting the genetic material during the viral infection process. III) In some viruses, there is an outer envelope that encloses the coat, and is made of parts of the previously infected cells.(A complete virus that consist of the genetic material,the protein coat and an envelope is called the virion)
Virus shapeVirus shape can be based on the capsidi. Helical (rod-shaped) e.g. tobacco mosaic virusii. Polyhedral / Icosahedral (many- sided shaped) e.g. adenovirus.iii. Complex combination of both by having structures like tail (helical and polyhedral) e.g. bacteriophageiv. Most enveloped virus have spherical shape e.g. influenza virus
Virus classificationBefore, viruses are classified according to the type ofhost that they infected.The current system reflect phenotypiccharacteristics.The Baltimore classification system distinguishviruses based on their- Method of replication- Genome type (DNA or RNA)The International Committee on Taxonomy ofViruses devised and implemented several rules onthe naming and classification of viruses early in the1990s.It started at the level of order and ends at genus
BacteriophageMuch of the knowledge comes fromstudying bacteriophage, because they canbe cultured easily within living bacteria.Bacteriophage possess dsDNA inside theircapsid (protein head). The capsid functionsas protection of their genetic material.Their tail fibers are the base used to attachthemselves to bacterial host cellThe tail is the channel for their geneticmaterial to be injected to the host cell.
Bacteriophage replicationThere are two types of bacteriophagereplication, LYTIC and LYSOGENICcycles.In a lytic cycle, the virus destroys thehost cell. It is a rapid process wherethe host cell undergoes lysis.In a lysogenic cycle, the viral genomeusually becomes integrated into thehost cell.
Virus lytic cycleThere are five steps in a typical bacteriophage lytic reproduction,i. Attachment-A virus will attach to a suitable host cellii. Penetration- The whole virus or only the genetic material (nucleic acid) will penetrate the cell’s cytoplasm. A bacteriophage capsid remains on the outside of the bacterial cell whereas many viruses that infect animal cell enter a host cell intact.
Virus lytic cycleiii.Replication and synthesis - The viral DNA/RNA directs the host cell to produce many copies of viral nucleic acids and proteins necessary for its replication.iv. Assembly - The viral nucleic acids and proteins are assembled together to form new infectious particles.v. Release - Newly generated viral particles are released from the host cell.
Virus lysogenic cycleThe infection will enter a latent period. Thehost cell is not killed in this process, butthe viral nucleic acid will undergo geneticrecombination with the host cell’schromosome. This integrated structure iscalled a prophage.When the bacterial DNA replicates, theprophage also replicates.Certain external condition such as UV lightand x-rays cause viruses to revert to a lyticcycle and then destroy their hosts.
Replication of an enveloped DNA virusEnveloped virus has a different way of infectingeukaryotic cells.After attachment to a host-cell receptor, someenveloped viruses fuse with the animal cell’splasma membrane. The viral capsid and nucleicacid will then be released into the animal cell.
Enveloped virus penetration stepSome virus enter the cell through endocytosis.In this process, the plasma membrane of theanimal cell invaginates to form a membrane-bounded vesicle that contains a virus.
Replication of an enveloped DNA virusThe viral DNA will be replicated andtranscribed by the host cell.After the viral genes are transcribed, the viralstructural proteins are synthesized throughtranslation outside nucleus.The new virus particles are then assembled.Enveloped viruses obtain their glycoproteinspikes on the envelopes by picking up afragment of the host plasma membrane asthey leave the infected cell.
Replication of an enveloped RNA virusThe viral genome (single stranded RNA)function as a template for synthesis ofcomplementary RNA strandSome complementary strands becamemRNA that will translatedViral genome RNA are made usingcomplementary strandsAfter translation, assembly and release stepbe done in proper sequence
Replication of an RNA virus (retrovirus) Virus attach through specific glycoprotein and enter through endocytosis Digestion of capsid through cellular enzyme Viral RNA ia a template for complementary DNA sense by reverse transcriptase Second DNA strand will be synthesized by reverse transcriptase Ds DNA incorporated with the cell’s DNA as a provirus The genes are replicated, transcribed and translated to build the components for the RNA virus assembly before being released
H1N1 virusH1N1 virus isolatedfrom patients foundthat it is made up ofgenetic elementsfrom four differentflu viruses – NorthAmerican Mexicaninfluenza, NorthAmerican avianinfluenza, humaninfluenza, and swineinfluenza virus
Viruslike agentsViruses is considered as thesmallest living / nonliving microbe.However, there are even smallerinfectious agents found – viroidsand prions.
ViroidsIn 1961, an infective agent in potatoes hasbeen discovered. The agent is called viroidand it is smaller than viruses with noprotein coats.Viroids are infectious RNA particle thatmay cause plant diseases by interferingwith mRNA processing.
PrionsPrions are infectious particles made of protein.Research indicates that prions are normalproteins that become folded incorrectly.Prions could cause neurological degenerativediseases such as mad cow disease and Scrapie.