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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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