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A.Viruses
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Are Viruses Living or Are Viruses Living or Non-living?Non-living?
Viruses are NOT living They have some properties
of life but not othersFor example, viruses can be
killed, even crystallized like table salt
However, they can’t maintain a constant internal state (homeostasis).
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What are What are Viruses?Viruses?
A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.
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B.Viral B.Viral HistoryHistory
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Discovery of VirusesDiscovery of Viruses Beijerinck (1897) coined the Latin name “virus” meaning poison
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Tobacco Mosaic VirusTobacco Mosaic Virus
Wendell Stanley (1935) crystallized sap from sick tobacco plants He discovered viruses were made of nucleic acid and protein
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SmallpoxSmallpox Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses Deadly viruses are said to be virulent Smallpox has been eradicated in the world today
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Viewing VirusesViewing Viruses Viruses are Viruses are smaller than the smaller than the smallest cellsmallest cell
Measured in Measured in nanometersnanometers
Viruses couldn’t Viruses couldn’t be seen until the be seen until the electron microscopeelectron microscope was invented in the was invented in the 2020thth century century
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Size of VirusesSize of Viruses
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C.Viral C.Viral StructureStructure
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CharacteristicsCharacteristics
Contain a protein coat called the capsid
Have a nucleic acid core containing DNA or RNA
Capable of reproducing only when inside a HOST cell
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CharacteristicsCharacteristics
Some viruses are enclosed in an protective envelope
Some viruses may have spikes to help attach to the host cell
Most viruses infect only SPECIFIC host cells
CAPSID
ENVELOPE
DNA
SPIKES
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CharacteristicsCharacteristics
Some viruses cause disease Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola Some viruses may cause some cancers like leukemia
MEASLES
Head
Tail sheath
DNA
T4 Bacteriophage Tobacco Mosaic Virus
Influenza Virus
RNA
Membrane envelope
Tail fiber
RNA
Capsid proteins
Capsid
Surface proteins
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Herpes VirusHerpes Virus
SIMPLEX I and II
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AdenovirusAdenovirus
COMMON COLD
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Influenza VirusInfluenza Virus
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D.Bacteriophages
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PhagesPhages
Viruses that attack bacteria are called bacteriophage
Capsid contains DNA Head & tail fibers made of protein
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phagesphages
They infect E. coli , an intestinal bacteriaSix small spikes at the base of a contractile tail are used to attach to the host cell Inject viral DNA into cell
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E. Viral E. Viral ReplicatioReplicatio
nn
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Viral AttackViral Attack
Viruses are very specific as to which species they attack
HOST specific Humans rarely share viral
diseases with other animals Eukaryotic viruses usually
have protective envelopes made from the host cell membrane
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5 Steps of Lytic 5 Steps of Lytic CycleCycle
1. Attachment to the cell2. Entry (injection) of viral DNA
or RNA3. Replication of new viral
proteins and nucleic acids4. Assembly of the new viruses 5. Release/Lysis of the new
viruses into the environment (cell lysis)
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Attachment:Phage attaches to host cell.
Entry:Phage pnetrates host cell and injects its DNA.
Merozoites released into bloodsteam from liver may infect new red blood cells
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2
3
Bacterial cell wall
Bacterial chromosome
Capsid
DNA
Capsid
SheathTail fiberBase platePin
Cell wall
Tail
Plasma membrane
Sheath contracted
Tail core
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4 Assembly/Maturation:Viral components are assembled into virions.
Tail
5 Release/Lysis:Host cell lyses and new virions are released.
DNA
Capsid
Tail fibers
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Viral LatencyViral Latency Some viruses have the ability to become dormant inside the cellThey may remain inactive for long periods of time (years)Later, they activate to produce new viruses in response to some external signalHIV and Herpes viruses are examples
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Lysogenic CycleLysogenic Cycle1.Phage DNA injected into host cell 2.Viral DNA joins host DNA forming a prophage3.When an activation signal occurs, the phage DNA starts replicating
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Lysogenic Cycle, cont’dLysogenic Cycle, cont’d
4.Viral DNA (part of prophage) may stay inactive in host cell for long periods of time5.Replicated during each binary fission6.Over time, many cells form containing the prophages
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Lysogenic Cycle, Lysogenic Cycle, cont’dcont’d
7.Once a prophage cell is activated, host cell enters the lytic cell8.New viruses form a & the cell lyses (bursts)
INACTIVE STAGEACTIVESTAGE
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The Lysogenic CycleThe Lysogenic Cycle
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Lysogenic virus examples…Lysogenic virus examples…Some eukaryotic viruses remain dormant for many years in the nervous system tissues Chickenpox (caused by the virus Varicella zoster) is a childhood infectionIt can reappear later in life as shingles, a painful itching rash limited to small areas of the body
SHINGLES
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……Lysogenic virus Lysogenic virus examplesexamplesHerpes viruses also
become latent in the nervous system
A herpes infection lasts for a person’s lifetime
Genital herpes (Herpes Simplex 2)
Cold sores or fever blisters (Herpes Simplex1)
SKIN TO SKIN CONTACT
PASSED AT BIRTH TO BABY
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F. RetrovirusesF. Retroviruses
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Characteristics of Characteristics of RetrovirusesRetroviruses
Contain RNA, not DNAContain enzyme called Reverse TranscriptaseWhen a retrovirus infects a cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell
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ENZYME
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