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VirusesViruses

Page 328Page 328

CHAPTER 18CHAPTER 18

2Fig. 18.1, Page 329Fig. 18.1, Page 329

1- Viruses are much smaller than bacteria

4- A virus is a genomeنووى enclosed in a حامض protective coat واقى غطاء

3- Viruses are not cells

2- Virus is about 20nm in diameter

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Genome Genome ((DNA/RNADNA/RNA))

Protein coat (Protein coat (capsid))

Virus is a genome enclosed in a protective coatVirus is a genome enclosed in a protective coat

Fig. 18.2a & b, Page 330

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Viral Viral CapsidCapsid and and EnvelopeEnvelope

• A protein shell that encloses the viral genome.A protein shell that encloses the viral genome.• It is rode-shaped, helical, polyhedral or more complex.It is rode-shaped, helical, polyhedral or more complex.• CapsomeresCapsomeres: Are the protein units that form capsid. : Are the protein units that form capsid.

A- CapsidA- Capsid الفيروسى الفيروسى الجدار الجدار

Page 329, 330

Capsomeres Capsomeres ((proteinsproteins))

CapsidCapsid

Sometimes further Sometimes further rappedrapped َّفيُغَلَّف in a membranous in a membranous يُغَلenvelope (envelope (Viral envelopeViral envelope الفيروسى الفيروسى الغالف eg. Influenza ), eg. Influenza ,( الغالف

virus.virus.

Membranous envelop (viral envelopeviral envelope)

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• Some viruses have viral viral envelopesenvelopes, membranes cloaking their capsids.

• These envelopes are derived from the membrane of the host cell.

Fig. 18.2c, Page 330Fig. 18.2c, Page 330

B- EnvelopB- Envelop الغطاء الغطاءالفيروسىالفيروسى

6Fig. 18.1, Page 328

Viral Genome:Viral Genome: ((Hereditary Hereditary materialmaterial الوراثية الوراثية المادة ((المادة

Viral genomes may consist of: - double-stranded DNA (dsDNA), - single-stranded DNA (ssDNA), - double-stranded RNA (dsRNA), - single-stranded RNA (ssRNA). depending on the specific type of

virus.

The viral genome is usually organized as a single linear or circular molecule of nucleic acid.

The smallest viruses have only fourfour genesgenes, while the largest have severalseveral hundredhundred.

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- DNA enclosed in a protein coat (- DNA enclosed in a protein coat (sometimes, membranous envelop alsosometimes, membranous envelop also) )

- Can be crystallised - Can be crystallised يتبلوريتبلور

Key Key CharactersCharacters VirusesViruses

- Reproduced only within a living host cell - Reproduced only within a living host cell (obligate parasitism تطفل.(إجبارى

- They lack - They lack يفتقديفتقد enzymes for metabolism enzymes for metabolism

- Have - Have no ribosomesno ribosomes for making their own proteins for making their own proteins

- Each type of virus infects a limited range of host cells- Each type of virus infects a limited range of host cells ( (host range host range اإلصابة اإلصابة مدى ( (مدى

Most viruses of eukaryotes attack specific tissues. eg. Human coldHuman cold viruses infect only the cells lining the upper respiratory tract, and AIDSAIDS virus binds only to certain white blood cells (Immune system).

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Virus-Reproduction within a living host cell Virus-Reproduction within a living host cell

1-Inters the cell and releases its genome1-Inters the cell and releases its genome

2- Replicates using host 2- Replicates using host nucleotides and enzymesnucleotides and enzymes

3- Produce a new capsid units using 3- Produce a new capsid units using host cell resources.host cell resources.

4- The new viral DNA and proteins 4- The new viral DNA and proteins assemble to form new viruses assemble to form new viruses

Page 331 (Fig. 18.3)

Living Living CellCell

Viral RNAViral RNA

Capsid Proteins

obligate parasitism إجبارى تطفل

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• A viral infection begins when the genome of the virus enters the host cell.

• Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome.

• The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell.

Fig. 18.3, Page 331

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• Viruses with an outer envelope Viruses with an outer envelope use the envelope to enter the use the envelope to enter the host cell.host cell.

– Glycoproteins on the envelope bind to Glycoproteins on the envelope bind to specific receptors on the host’s specific receptors on the host’s membrane.membrane.

– The envelope fuses with the host’s The envelope fuses with the host’s membrane, transporting the capsid and membrane, transporting the capsid and viral genome inside.viral genome inside.

– The viral genome duplicates and directs The viral genome duplicates and directs the host’s protein synthesis machinery to the host’s protein synthesis machinery to synthesize capsomeres with free synthesize capsomeres with free ribosomes and glycoproteins with bound ribosomes and glycoproteins with bound ribosomes.ribosomes.

– After the capsid and viral genome self-After the capsid and viral genome self-assemble, they bud from the host cell assemble, they bud from the host cell covered with an envelope derived from the covered with an envelope derived from the host’s plasma membrane, including viral host’s plasma membrane, including viral glycoproteins.glycoproteins.

• These enveloped viruses do not These enveloped viruses do not necessarily kill the host cell.necessarily kill the host cell.

Fig. 18.6, Page 334Fig. 18.6, Page 334

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• HHuman uman IImmunodeficiency mmunodeficiency VVirus (irus (HIVHIV), the virus that ), the virus that causes causes AIDSAIDS ( (AAcquired cquired IImmuno-mmuno-DDeficiency eficiency SSyndrome) is yndrome) is a retrovirusa retrovirus.

The viral particle includes:The viral particle includes:

1)1) an an envelopeenvelope with glyco- with glyco-proteins, proteins,

2)2) a capsid containinga capsid containingtwo identical RNA strandstwo identical RNA strandsas its genomeas its genome

3)3) Two copies of Two copies of reversereversetranscriptase.transcriptase.

Fig. 18.7a, Page 336

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• Viruses that infect bacteria, are called bacteriophagesbacteriophages or phagesphages.

• It has a 20-sided capsid-head that encloses their DNA and protein tail piece that attaches the phage to the host and injects the phage DNA inside.

• Phages reproduce by Phages reproduce by LyticLytic Cycle Cycle (( مميتــــة ــللية مميتــــة تحـ ــللية دورة تحـ )) and/or and/or Lysogenic Lysogenic cycle cycle (( غيرغيردورة دورة تحـلليةتحـللية )) . .

Fig. 18.2d Page 330Fig. 18.2d Page 330

Bacteria infecting VirusesBacteria infecting Viruses

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Phages reproductive cycles within bacteria:Phages reproductive cycles within bacteria:

Bacteriophages Bacteriophages ((on E. colion E. coli))

BacteriaBacteriaVirusVirus

1)- lytic cycle 1)- lytic cycle (( الدورة التحـلليةالتحـللية )) , The phage reproductive cycle results in the death of the host.

– In the last stage, the bacterium lyseslyses (breaks open) and releases the phages produced within the cell to infect others.

• Virulent phages قاتلة reproduce only by a lytic فيروساتcycle.

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Phage T4Phage T4((virulent virusesvirulent viruses))

فيروس مميت

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The phage genome replicates without destroying the host cell.

• Temperate phages, like phage lambda (phage lambda (גג)), may use both lytic lytic andand lysogenic lysogenic cyclescycles.

• Within the host, the virus’ circular DNA engages in either the lytic or lysogenic cycle.

• During a lytic cycle, the viral gene immediately turn the host cell into a virus-producing factory, and the cell soon lyses and releases its viral products.

TemperateTemperatevirusvirusفـيــرو

غير سمميت

2)- lysogenic cycle 2)- lysogenic cycle (( غيرغيرالدورة التحـلليةالتحـللية ))Phage lambda (Phage lambda (גג))

16Fig. 18.5, Page 333

2)- Lysogenic cycle 2)- Lysogenic cycle (( غيرغيرالدورة التحـلليةالتحـللية ))

Unit 3Unit 3

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Only 1 tail fibreOnly 1 tail fibreTemperate virusTemperate virus

مميت غير فيروسأحيانا

Phage lambda (Phage lambda (גג))Phage T4Phage T4

((virulent virusesvirulent viruses))فيروس مميت

Many tail fibreMany tail fibre

lytic cycle lytic cycle (( الدورة التحـلليةالتحـللية ))

Lysogenic cycleLysogenic cycle (( غيرغيرالدورة التحـلليةالتحـللية ))

lytic cycle lytic cycle التحـلليةالتحـللية))((الدورة

&