1

Virus, Lytic Cycle and Lysogenic Cycle

2

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

Viruses are non livingThey have some properties of

life but not othersThey can’t maintain a constant

internal state (homeostasis) or reproduce without a host cell.

3

What are What are Viruses?Viruses?

A virus is a non-cellular particle made up of a nucleic acid and protein coat (capsid) that can invade living cells.

4

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

5

Viewing VirusesViewing Viruses

Viruses are Viruses are smaller than the smaller than the smallest cellsmallest cell

Viruses Viruses couldn’t be seen couldn’t be seen until the until the electron electron microscopemicroscope was was invented in the invented in the 2020thth century century

What is a Nanometer?

A nanometer is a unit of measure. Just like inches, feet and miles. By definition a nanometer is one-billionth of a meter. A meter is about 39 inches long. That is a big number and when you divide a meter into one billion pieces, well that is very small.

6

7

Size of VirusesSize of Viruses

8

Viral Viral StructureStructure

9

CharacteristicsCharacteristicsNon living NoncellularContain: 1. a protein coat called the

capsid 2. nucleic acid core

containing DNA or RNACapable of reproducing only

when inside a HOST cell

10

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

11

CharacteristicsCharacteristics

Outside of host cells, viruses are inactive

Lack ribosomes and enzymes needed for metabolism

To be able to reproduce they use the raw materials and enzymes of the host.

EBOLA VIRUS

HIV VIRUS

12

CharacteristicsCharacteristics

Some viruses cause disease

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

Some viruses may cause some cancers like leukemia MEASLES

13

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

14

Helical VirusesHelical Viruses

15

Polyhedral VirusesPolyhedral Viruses

16

Complex VirusesComplex Viruses

17

Herpes VirusHerpes Virus

SIMPLEX I and II

18

AdenovirusAdenovirus

COMMON COLD

19

Influenza VirusInfluenza Virus

20

Chickenpox VirusChickenpox Virus

21

Papillomavirus – Papillomavirus – Warts!Warts!

22

Bacteriophages

23

PhagesPhages

Viruses that attack bacteria are called bacteriophage or just phage

24

BacteriophageBacteriophage

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

Viral DNA is injected into the host cell

25

Escherichia coli Bacterium

T - EVEN PHAGES ATTACK THIS BACTERIUM

26

Bacteriophages

27

Retrovirus Retrovirus

28

Characteristics of Characteristics of Retroviruses/LatentRetroviruses/Latent

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 and it is able to make DNA

29

Retroviruses Retroviruses HIV, the

AIDS virus and human T cell leukemia virus are retroviruses

30

Viral Viral ReplicationReplication

31

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

32

BacteriophageBacteriophage ReplicationReplication

Bacteriophage inject their nucleic acid

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

33

Attachment Phage attaches by tail fibers to

host cell Entry Tail sheath contracts to

force tail core and DNA into cellReplication Production of phage DNA

and proteinsAssembly Assembly of phage particlesLysis/Release Phage breaks cell wall and releases new viruses

Lytic Cycle ReviewLytic Cycle Review

34

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

35

Lysogenic CycleLysogenic CyclePhage DNA or

genome injected into host cell

Viral DNA joins host DNA forming a provirus

When an activation signal occurs, the phage DNA starts replicating

36

Lysogenic CycleLysogenic CycleViral DNA (part of

provirus) may stay inactive in host cell for long periods of time

Provirus Replicates during each binary fission in bacteria and Mitosis in Eukaryotic Cells

Over time, many cells result which contain the provirus

Lytic and Lysogenic Cycle

37

38

Viral LatencyViral LatencyOnce a provirus cell is activated, host cell enters

the lytic cycle

•New viruses form and the cell lyses (bursts)

39

Latency in Latency in EukaryotesEukaryotesSome 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

SHINGLES

40

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

41

Interferon are naturally occurring proteins made by cells to fight viruses

42

The End