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VirusesViruses

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ObjectivesObjectives

• DescribeDescribe why viruses are not considered as living organisms. why viruses are not considered as living organisms.

• DescribeDescribe the basic structure of viruses. the basic structure of viruses.

• CompareCompare the lytic and lysogenic cycles of virus replication. the lytic and lysogenic cycles of virus replication.

• SummarizeSummarize the origin of viruses. the origin of viruses.

• At the boundary of life, between the macromolecules (which At the boundary of life, between the macromolecules (which are not alive) and the prokaryotic cells (which are alive), lie are not alive) and the prokaryotic cells (which are alive), lie the viruses and the viruses and bacteriophagesbacteriophages (phages). (phages).

• These creatures are These creatures are parasitesparasites responsible for causing many responsible for causing many diseases in living things (HIV in humans, for example).diseases in living things (HIV in humans, for example).

• Viruses are found everywhere.Viruses are found everywhere.

• Viruses consist of a core of nucleic acid, either Viruses consist of a core of nucleic acid, either DNADNA or or RNARNA, a , a protective coat of protein molecules and sometimes lipids.protective coat of protein molecules and sometimes lipids.

• Viruses and bacteriophages show none of the expected signs Viruses and bacteriophages show none of the expected signs of life.of life.

• Viruses do not respond to stimuli, do not grow, do not do any Viruses do not respond to stimuli, do not grow, do not do any of the things we normally associate with life. of the things we normally associate with life.

• Viruses are not be considered "living" organisms. However, Viruses are not be considered "living" organisms. However, they do show one of the most important signs of life: they do show one of the most important signs of life: the the ability to reproduce in a host cellability to reproduce in a host cell. .

1- Viruses are much smaller than bacteria

4- A virus is a genomeنوو يحامض enclosed in a protective coat واق يغطاء

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

bacteriophagesbacteriophages

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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 rod-shaped, helical, polyhedral or more complex.It is rod-shaped, helical, polyhedral or more complex.• CapsomeresCapsomeres: Are the protein units that form capsid. : Are the protein units that form capsid.

A- CapsidA- Capsid الفيروس الفيروس الجدار ييالجدار

Capsomeres Capsomeres ((proteinsproteins))

CapsidCapsid

Sometimes further Sometimes further wrappedwrapped َغ�َّل�ف� �َغ�َّل�فُي in a membranous in a membranous ُيenvelope (envelope (Viral envelopeViral envelope الفيروس الفيروس الغالف ييالغالف ), eg. Influenza ), eg. Influenza

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.

B- EnvelopB- Envelop الغطاء الغطاءييالفيروسالفيروس

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 the

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

hundred geneshundred genes.

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

1-Enters the cell and releases its 1-Enters the cell and releases its genomegenome2- Replicates using host 2- Replicates using host nucleotides and enzymesnucleotides and enzymes

3- Produce a new capsid unit using 3- Produce a new capsid unit 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

Living Living CellCell

Viral RNAViral RNA

Capsid Proteins

Obligate parasitism إجبار يتطفل

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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.

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

• A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The viral particle includes: The viral particle includes:

1)1) an an envelopeenvelope with glycoproteins, with glycoproteins,

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.

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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 (( دورة مميتــــةمميتــــة )) and/or and/or Lysogenic Lysogenic cycle cycle (( أحيانا مميتة أحيانا دورة مميتة . . ((دورة

Bacteria infecting VirusesBacteria infecting Viruses

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

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

BacteriaBacteriaBacteriaBacteriaVirusVirusVirusVirus

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

سغير مميت

2)- lysogenic cycle 2)- lysogenic cycle (( غيرغيرالدورة التحـلليةالتحـللية ))

Phage lambda (Phage lambda (גג))

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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 فيروسغيرأحيانا

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

&

- 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

Summary of Key Summary of Key CharactersCharacters of VirusesViruses

- Reproduce only within a living host cell - Reproduce only within a living host cell (obligate parasitism إجبار يتطفل ).

- 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).

Viruses are host specific Viruses are host specific – a protein on the – a protein on the surface of the virus has surface of the virus has a shape that matches a a shape that matches a molecule in the plasma molecule in the plasma membrane of it’s host, membrane of it’s host, allowing the virus to allowing the virus to lock onto the host cell.lock onto the host cell.