(17) Viruses Power Point CP Used

8/4/2019 (17) Viruses Power Point CP Used

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Intro Questions

1. What are viruses?

2. What is special about viruses?

3. What are some common viruses?


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Chapter 19 Viruses


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What Is a Virus?

* particles ofnucleic acid, protein, and occasionally lipid

(DNA or RNA core surrounded by a protein coat)

capsid a viruss outer protein coat (allows the virus to enterthe host cell like a spy)

* reproduce only inside living cells

* using the resources of the host cell to produce

more viruses

Section 19-3

Section Outline

Go toSection:


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How do viruses infect their hosts?

*viruses are usually host specific and onlywork on one type of cell in the host (see pages

484-485 in text)


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T4 BacteriophageTobacco Mosaic

VirusInfluenza

VirusHead

Tail

sheath

DNA

Tail

fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsid

proteins

Section 19-3

Figure 19-13 Virus Structures

Go toSection:


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Types of Viruses

Depends upon whether it has DNA orRNA,double stranded or single stranded

1. double-stranded DNA ex: Chicken Pox

2. single-stranded DNA ex: Roseola3. double-stranded RNA ex: Reovirus,plant viruses

4. single-stranded RNA ex: HIV, Ebola

Called a DNA-like or RNA-like virusdepending on the nucleic acid found


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Viral Replication

Viruses- obligate intracellular parasites must have ahost cell!

Viruses lack the enzymes for metabolism and dont haveribosomes

Viruses can not make their own proteins


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Viral reproductive cycle - Overview

Virus uses hosts machinery

to replicate viral genome

Hosts machinery used to

make capsid proteins

Use transcription and

translation.


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5 Basic Steps of Virus Replication

1. Attachment Virus attaches to host

2. Penetration Injects DNA or RNA

3. Replication and Synthesis hijacks host machinery tomake new DNA/RNA

4. Assembly puts viral DNA/RNA into capsids,

envelopes

5. Release New Viruses Lyses (break out of) cell,

releasing more viruses to infect new cells


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*2 types of pathways to infection:

1. lytic cycle

(quick takeover, destroys some cells inorganism)

2. lysogenic cycle

(delayed takeover, reproduces itself and

waits)


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Lytic InfectionQuick like the common cold

1. Virus attaches to host cell2. Virus enters host cell (injects genome into host)

3. Makes copies of itself and new protein coats by

hijacking host cells metabolic machinery

4. Causes the host cell to burst (lyse) and die5. Host cells death releases the new virus

particles which can infect other cells


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Bacteriophage enzyme lyses the

bacteriums cell wall, releasing

new bacteriophage particles that

can attack other cells.

Bacteriophage proteins

and nucleic acids

assemble intocomplete

bacteriophage particlesBacteriophage takes over

bacteriums metabolism,

causing synthesis of new

bacteriophage proteins and

nucleic acids

Bacteriophage injects

DNA into bacterium

Bacteriophage attaches to

bacteriums cell wall

Bacteriophage

Bacteriophage DNA

Bacteriophage protein

Bacteriophage

protein coat

Bacteriophage DNA

Bacterial

chromosome

Lytic Cycle

Section 19-3

Figure 19-14 The Lytic Cycle

Go toSection:


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1. Virus attaches to host cell2. Virus enters host cell

3. Joins its DNA to the host DNA (this viral DNA nowembedded in hosts DNA is called a prophage)

4.Each time host cell DNA replicates, viral DNA is also

replicated; thus, viral DNA will be inside EVERY cellthat was produced after the viral DNA was inserted

5. When organisms immune system is low, viral DNAtakes over the cells machinery and makes copies ofitself and new protein coats by hijacking host cells

metabolic machinery6. Causes the host cell to burst (lyse) and die

7. Host cells death releases the new virus particles whichcan infect other cells

Lysogenic Cycle Virus Hides


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LysogenicCycle

Slower!

Section 19-3

Figure 19-14 The Lytic vs. LysogenicCycle

Go toSection:

Viral genome hides out until conditions

are right then lyses (exits) the cell

Viral genome

entering host

incorporated as a

prophage in to

host genome

This way, viral

genome is

replicated with eachhost cell division;

more viruses

released


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Animal Viruses

Many animal viruseshave a membrane like

envelope present.

This envelope is a

lipid bilayer with

glycoproteins for

attachment.

Viral envelope comes

from the host cells

plasma membrane, sohost cell may not be

killed upon viral exit

Ex: Herpes


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Viruses and Disease

Viral diseases cannot be cured once caught must suffer through symptoms until body can fightoff infection

a. Most symptoms are caused by the toxinsreleased when host cell lyses (bursts)

b. Prevention is best medicine:vaccines to stimulate immunity


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So why arent all cells wiped out?

Natural Selection favors mutations with receptor sites

that viruses cannot attach to

Cellular enzymes of cell may break down virus before it

can replicate and take over. Restriction Nucleases actscissors for foreign DNA

BUT, Natural Selection also favors viral mutants that

are resistant to destruction

So virus-host relationship is in constant evolutionary

combat Red Queen Hypothesis
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Prions

-- Protein Infectious Particles

-- Contain no DNA or RNA, yetcause disease like mad cow

Ex. Mad cow,

Jacob-Crutzfeld


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Are Viruses Alive?NO!

They do not have all the characteristics of life(cellular structure, ability to reproduce independently),so they are not alive.

They can do what they do because they share thegenetic code with living things and affect living

things, but they are not considered to be alive.

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