1

Viruses

2

Are Viruses Living or Are Viruses Living or Non-living?Non-living?

Viruses are both and neitherThey have some properties

of life but not othersFor example, viruses can be

killed, even crystallized like table saltHowever, they can’t

maintain a constant internal state (homeostasis).

3

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.

4

Viral Viral HistoryHistory

5

Discovery of VirusesDiscovery of VirusesBeijerinck (1897)

coined the Latin name “virus” meaning poison

He studied filtered plant juices & found they caused healthy plants to become sick

6

Tobacco Mosaic VirusTobacco Mosaic Virus

Wendell Stanley (1935) crystallized sap from sick tobacco plants

He discovered viruses were made of nucleic acid and protein

7

SmallpoxSmallpoxEdward 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

8

Viewing VirusesViewing VirusesViruses 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

9

Size of VirusesSize of Viruses

10

Viral Viral StructureStructure

11

CharacteristicsCharacteristics

Non living structuresNoncellularContain a protein coat called

the capsidHave a nucleic acid core

containing DNA or RNACapable of reproducing only

when inside a HOST cell

12

CharacteristicsCharacteristics

Some viruses are enclosed in an protective envelopeSome viruses may

have spikes to help attach to the host cellMost viruses infect

only SPECIFIC host cells

CAPSID

ENVELOPE

DNA

SPIKES

13

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

EBOLA VIRUS

HIV VIRUS

14

CharacteristicsCharacteristics

Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola

Some viruses may cause some cancers like leukemia

Virus-free cells are rare

MEASLES

15

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

16

Helical VirusesHelical Viruses

17

Polyhedral VirusesPolyhedral Viruses

18

Complex VirusesComplex Viruses

19

Herpes VirusHerpes Virus

SIMPLEX I and II

20

AdenovirusAdenovirus

COMMON COLD

21

Influenza VirusInfluenza Virus

22

Chickenpox VirusChickenpox Virus

23

Papillomavirus – Papillomavirus – Warts!Warts!

24

RNA or DNA VirusDo or do NOT have an

envelopeCapsid shapeHOST they infect

Used for Virus Used for Virus IdentificationIdentification

25

Bacteriophages

26

PhagesPhages

Viruses that attack bacteria are called bacteriophage or just phage

T-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails

27

Escherichia Coli Bacterium

T - EVEN PHAGES ATTACK THIS BACTERIUM

28

T-Even Bacteriophages

29

Viral Viral ReplicationReplication

30

Viral AttackViral Attack

Viruses are very specific as to which species they attackHOST specificHumans rarely share viral

diseases with other animalsEukaryotic viruses usually

have protective envelopes made from the host cell membrane

31

5 Steps of Lytic 5 Steps of Lytic CycleCycle

1. Attachment to the cell2. Penetration (injection) of

viral DNA or RNA3. Replication (Biosynthesis) of

new viral proteins and nucleic acids4. Assembly (Maturation) of the

new viruses 5. Release of the new viruses

into the environment (cell lyses)

32

BacteriophageBacteriophage ReplicationReplication

Bacteriophage inject their nucleic acid

They lyse (break open) the bacterial cell when replication is finished

33

Attachment Phage attaches by tail fibers to

host cellPenetration Phage lysozyme opens cell wall,

tail sheath contracts to force tail core and DNA into cellBiosynthesis Production of phage DNA

and proteinsMaturation Assembly of phage particlesRelease Phage lysozyme breaks cell wall

Lytic Cycle ReviewLytic Cycle Review

34

Attachment:Phage attaches to host cell.

Penetration:Phage pnetrates host cell and injects its DNA.

Merozoites released into bloodsteam from liver may infect new red blood cells

1

2

3

Bacterial cell wall

Bacterial chromosome

Capsid

DNA

Capsid

SheathTail fiberBase platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

35

4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

36

Viral LatencyViral LatencySome viruses have the ability

to become dormant inside the cellCalled latent virusesThey 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

37

Lysogenic CycleLysogenic CyclePhage DNA

injected into host cell

Viral DNA joins host DNA forming a prophage

When an activation signal occurs, the phage DNA starts replicating

38

Lysogenic CycleLysogenic Cycle

Viral DNA (part of prophage) may stay inactive in host cell for long periods of time

Replicated during each binary fission

Over time, many cells form containing the prophages

39

Viral LatencyViral LatencyOnce a prophage cell is activated, host cell enters the lytic cellNew viruses form a & the cell lyses (bursts)Virus said to be virulent (deadly)

INACTIVE STAGEACTIVESTAGE

40

Virulent VirusesVirulent Viruses

HOST CELL

LYSES & DIES

41

The Lysogenic CycleThe Lysogenic Cycle

42

Latency in EukaryotesLatency in EukaryotesHerpes 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

43

VirulenceVirulence

VIRUS DESTROYING HOST CELL

44

Lytic and Lysogenic Lytic and Lysogenic CyclesCycles

45