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Chapter 2

VIRUSES

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Introduction

Not plants, animals, or bacteria; parasites

that live in a host.

Not living organisms.Without host, virus dead. The use host to

generate energ.

!lassi"ied b the organisms the in"ect.#irus cannot penetrate plant cell $all, so it

transmitted b insect or other organisms

that "eed on plants.

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WHAT IS A VIRUS?

Viruses may be defined as a cellular organisms whosegenomes consist of nucleic acid, and which obligately replicate

inside host cells using host metabolic machinery to different

extents, to form a pool of components which assemble into

particles called virions.

FViruses cannot be grown on sterile

media, but require the presence of

specific host cells.

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%undamentall then, a virus is&

' pac(age o" genetic in"ormation

protected b a protein shell "or

deliver into a host cell to bee)pressed and replicated

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A virus differs from a cell in three

fundamental ways:

i A virus usually has only a single type of nucleic

acid serving as its genetic material. This can be single

or double stranded DNA or RNA;

ii Viruses contain no enzymes of energy metabolism,

thus cannot make ATP;

iii Viruses do not encode sufficient enzymatic

machinery to synthesize their componentmacromolecules, specifically, no protein synthesis

machinery.

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Structure of the virus

' virus contain nucleic acid, either +N' or N' -but not both, and a protein

coat -capsid; some enclosed b an envelope o" "at or protein.

Capsid

proteinNucleocapsid

Naked

capsid virus

DNA

RNA

or =+

Icosahedral Helical

Naked capsid

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Structure of the virus

0an tpes o" protein have a glcoprotein surrounding the nucleocapsid.

NucleocapsidLipid memrane!

"l#coproteinsEnveloped virus+

Enveloped

Lipid

Glycoprotein

Matrix

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#'5 678+!T8N

2 Tpes & ltic and lsogenic.

5tic& virus ta(e over cell and produce ne$

virus. The host cell then lsis releasingne$ virus to in"ect other cells.

5sogenic& virus : bacteria chromosome

as prophage. bacteriophage.

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5tic viral reproduction

There are 3 steps&

a 'ttachment and penetrationb eplication or snthesis o" viral

component

c 'ssembl and phagematuration

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'ttachment

1 7hage bind to host cell through protein.

2 #iral en

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eplication

1 ?enome ta(e@over the metabolism o" the host,

converting to its o$n ends.

2 #iral nucleic acid is transcribed and translate.

3 #iral protein and en

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'ssembl and 7hage 0aturation

1 #arious viral component accumulate and complete to

ne$ phage/virus.

2 6n

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5sogenic viral reproduction

5sogenic& virus : bacteria chromosome

as prophage. bacteriophage.

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Aacteriophage virus

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6T8#B

Btill other viruses invade animal cells and

replicate $ithout (illing the host cell

immediatel. Ne$ viruses are released b

budding o"" the host cellCs plasmamembrane, turning the host cell "or a time

into a viral "actor. The human

immunode"icienc virus -D#, a retrovirusassociated $ith '+B, replicates in this

$a.

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6T8#B

etroviruses are N' viruses that uses en$#me

reverse transcriptase to produce a +N'

intermediate. 8nce inside the host cell, reverse

transcription -ma(ing +N' "rom N' isaccomplished b the reverse transcriptase,

turning the single@stranded N' into +N'. This

ne$ +N' is incorporated into the host +N',

$here it transcribes ne$ viral N' genomes, as$ell as the N' to snthesi

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RE%R&Virus replication

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Virus "ro'th

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VIRUSES (N) )ISE(SES

#iruses cause a variet o" diseases. #iral diseases

include the "lu, common cold, herpes, measles,

chic(en po), small po), and encephalitis.

0an di""erent viruses in"ect humans and other

animals.

'ntibiotics are not e""ective against viruses.

#accination o""ers protection "or unin"ectedindividuals.

#iruses are usuall Euite speci"ic.

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VIRUSES (N) )ISE(SES

#iral diseases can be spread bet$een plants b

insects such as aphids and lea"hoppers as the

insects "eed on plant tissues.

Bmptoms o" viral in"ection include reduced plant

si

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VIRoid (N) prions

Viroids are even more simpler than viruses. The are small,

circular, single@stranded molecules o" infectious RN( lac(ingeven a protein coat.

#iroids use higher plants such as potatoes, tomatoes and

cucumbers to reproduce, inserting themselves into the nucleus o"

a plant cell to be replicated there.

#iroids are usuall transmitted b seed or pollen.

Bo "ar no human viroids have been discovered, but it is considered

a real possibilit that the e)ist.

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VIRoid (N) prions

*rions are infectious protein particles

t isthought to be responsible "or some neurological diseases such

as a group o" transmissible and/or inherited neurodegenerative

diseases including Creut$feldt,-akodisease, kuru, and

.erstmann,Straussler, s#ndromein humans as $ell as scrapiein

sheep and goats.Furu in"ects its victims $hen the eat the brain tissue o" the victims.

The mad co' disease that $as "irst detected in 6ngland and parts

o" 6urope a "e$ ears ago is in "act a ne$ prion disease and it has

caused the use o" bee" in Aritain to "all precipitousl.

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( H#pothesis on propa"ation

process of prion

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end