Viruses, Viroids, and Prions

18-2

1

Are Viruses Living or Are Viruses Living or Nonliving?Nonliving?

• Viruses are both and neither• They have some properties of

life but not others• For example, viruses can be

killed, even crystallized like table salt

• However, they can’t maintain a constant internal state (homeostasis).

2

What are Viruses?What are Viruses?

•A virus is a noncellular particle made up of genetic material (DNA) and protein that can invade living cells.

3

Discovery of VirusesDiscovery of Viruses•Beijerinck (1897) coined the Latin name “virus” meaning poison•He studied filtered plant juices & found they caused healthy plants to become sick 4

Tobacco Mosaic VirusTobacco Mosaic Virus

•Wendell Stanley (1935) crystallized sap from sick tobacco plants•He discovered viruses were made of nucleic acid and protein

5

SmallpoxSmallpox•Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses•Deadly viruses are said to be virulent•Smallpox had been eradicated in the world today, but is now coming back 6

Viewing VirusesViewing Viruses•Viruses are smaller Viruses are smaller than the smallest cellthan the smallest cell•Measured in Measured in nanometers nanometers•Viruses couldn’t be Viruses couldn’t be seen until the seen until the electron microscope electron microscope was invented in the was invented in the 2020thth century century

7

Size of VirusesSize of Viruses

8

Viral Viral StructureStructure

9

CharacteristicsCharacteristics

• Non living structures• Noncellular• Have a protein coat called the

capsid• Have a nucleic acid core

containing DNA or RNA• Capable of reproducing only

when inside a HOST cell10

CharacteristicsCharacteristics• Some viruses are

enclosed in a protective envelope

• Some viruses may have spikes to help attach to the host cell

• Most viruses infect only SPECIFIC host cells

11

CAPSID

ENVELOPE

DNA

SPIKES

CharacteristicsCharacteristics•Capsids (coats) are made of individual protein subunits called capsomeres

12

CAPSOMERES

CharacteristicsCharacteristics

•Outside of host cells, viruses are inactive•Lack ribosomes and enzymes needed for metabolism•Use the raw materials and enzymes of the host cell to be able to reproduce

13

EBOLA VIRUS

HIV VIRUS

CharacteristicsCharacteristics

•Some viruses cause disease such as:•Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola•Some viruses may cause some cancers like leukemia•Virus-free cells are rare

14MEASLES

Viral ShapesViral Shapes

•Viruses come in a variety of shapes•Some may be helical shape like the Ebola virus•Some may be polyhedral shapes like the influenza virus•Others have more complex shapes like bacteriophages

15

Helical VirusesHelical Viruses

16

Polyhedral VirusesPolyhedral Viruses

17

Complex VirusesComplex Viruses

18

Taxonomy of Taxonomy of VirusesViruses

19

Herpes VirusHerpes Virus

20

SIMPLEX I and II

AdenovirusAdenovirus

21COMMON COLD

Influenza VirusInfluenza Virus

22

Chickenpox VirusChickenpox Virus

23

Papillomavirus – Papillomavirus – Warts!Warts!

24

Used for Virus Used for Virus IdentificationIdentification

• RNA or DNA Virus• Do or do NOT have an

envelope• Capsid shape• HOST they infect

25

Bacteriophages

26

PhagesPhages•Viruses that attack bacteria are a bacteriophage (phage)•T-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails

27

T-phagesT-phages•The most commonly studied T-phages are T4 and T7•They infect E. coli , an intestinal bacteria•Six small spikes at the base of a contractile tail are used to attach to the host cell •Inject viral DNA into cell

28

Escherichia coli Bacterium

29T - EVEN PHAGES ATTACK THIS BACTERIUM

T-Even Bacteriophages

30

Diagram of T-4 Diagram of T-4 BacteriophageBacteriophage

•Head with 20 triangular surfaces•Capsid contains DNA•Head & tail fibers made of protein

31

32

Clicker Question - PhagesClicker Question - Phages•A Bacteriophage or a phage is:A virus that infects a __________ cell.

A.PlantB.AnimalC.BacteriaD.ProtistaE.Fungi

RetrovirusesRetroviruses

33

Characteristics of Characteristics of RetrovirusesRetroviruses

•Contain RNA, not DNA•Family Retroviridae•Contain enzyme called Reverse Transcriptase•When a retrovirus infects a cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell

34

35

ENZYME

RetrovirusesRetroviruses

•The enzyme reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template

36RTase

RetrovirusesRetroviruses

•HIV, the AIDS virus, is a retrovirus•Feline Leukemia Virus is also a retrovirus

37

Clicker Question!•Retroviruses contain:

A. tRNAB. DNAC. RNAD. rRNAE. mRNA

Viral Viral ReplicationReplication

39

Viral AttackViral Attack

• HOST specific• Humans rarely share viral

diseases with other animals• Eukaryotic viruses usually have

protective envelopes made from the host cell membrane

40

5 Steps of Lytic 5 Steps of Lytic CycleCycle

• 1. Attachment to the cell• 2. Penetration (injection) of viral

DNA or RNA• 3. Replication (Biosynthesis) of new

viral proteins and nucleic acids• 4. Assembly (Maturation) of the

new viruses• 5. Release of the new viruses into

the environment (cell lyses)41

One-step Growth One-step Growth CurveCurve

42

Viral LatencyViral Latency•Some viruses have the ability to become dormant inside the cell•Called latent viruses•They may remain inactive for long periods of time (years)•Later, they activate to produce new viruses in response to some external signal•HIV and Herpes viruses are examples 43

Viral LatencyViral Latency•Once a prophage cell is activated, host cell enters the lytic cell•New viruses form & the cell lyses (bursts)

44

INACTIVE STAGE

ACTIVESTAGE

Latency in Latency in EukaryotesEukaryotes

•Some eukaryotic viruses remain dormant for many years in the nervous system tissues• Chickenpox (caused by the virus Varicella zoster) is a childhood infection•It can reappear later in life as shingles, a painful itching rash limited to small areas of the body

45

SHINGLES

Latency in EukaryotesLatency in Eukaryotes•Herpes viruses also becomes 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) 46

SKIN TO SKIN CONTACT

PASSED AT BIRTH TO BABY

The Lysogenic CycleThe Lysogenic Cycle

47

Virulent VirusesVirulent Viruses

48

HOST CELL

LYSES & DIES

Clicker Question!

• Viruses replicate:

A. On their ownB. With other viruses

by joining piliC. When ever they

want toD. In a host cell only

50

• All Viruses ONLY contain RNA.

A. TrueB. False

Clicker Question!Clicker Question!

Treatment for Treatment for Viral DiseaseViral Disease

51

VaccinesVaccines • An attenuated virus is a weakened,

less vigorous virus

• Attenuated virus is capable of stimulating an immune response and creating immunity, but not causing illness

52

Other Viral TreatmentsOther Viral Treatments•Interferon are naturally occurring proteins made by cells to fight viruses•Antiviral drugs (AZT)•Protease inhibitors – prevents capsid formation

53

Viroids & PrionsViroids & Prions

54

ViroidsViroids•Small, circular RNA molecules without a protein coat•Infect plants•Potato famine in Ireland•Resemble introns

55

PrionsPrions•Prions are “infectious proteins”• They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins• They have no DNA or RNA•The main protein involved in human and mammalian prion diseases is called “PrP” 56

Prion Prion DiseasesDiseases•Causes neurons to Causes neurons to

rapidly degenerate.rapidly degenerate.•Examples:Examples:•Mad cow disease Mad cow disease (bovine spongiform (bovine spongiform encephalitis: BSE)encephalitis: BSE)•People in New Guinea People in New Guinea used to suffer from used to suffer from kuru, which they got kuru, which they got from eating the brains from eating the brains of their enemiesof their enemies

57

Clicker QuestionPrions can be contracted by…

A: Eating leftovers from the fridge.

B: Eating the brain of an infected animal.

C: Eating meat that has come into contact with the brain or spinal cord of an infected animal.

D: Both B and C

Clicker Question!•What infectious element

causes Mad Cow Disease?A: PhagesB: PrionsC: ViroidsD: PlasmaE: Virus