PROF. DR. MARLINA, MS, Apt.

Viruses

Viruses, Viroids and Prions

Study of Viruses - Virology5 Kingdoms

1. Plantae2. Animalia3. Fungi4. Protista5. Monera

5 Characteristics of Life1. Cells2. Grow and maintain their structure by

taking up chemicals and energy from the environment

3. Respond to their external environment4. Reproduce and pass on their

organization to their offspring5. Evolve and Adapt to their environment

Viruses are:1. Acellular2. Obligate intracellular parasites3. No ATP generating system4. No Ribosomes or means of Protein

Synthesis

Typical Virus 2 Parts1. Nucleic Acid

DNA or RNA (But never both)2. Capsid (Coat Protein)

Some Viruses:A. EnvelopeB. Enzymes

Host range

Spectrum of host cells that a virus can infect

Some viruses only infect:plantsinvertebratesprotistsfungibacteria (Bacteriophages)

Host rangeMost viruses have a narrow host range

Polio virus - nerve cellsAdenovirus - cells in upper Respiratory

Tract

Host range is determined by Viruses ability to interact with its host cell

Binding Sites match Receptor Sites

Binding Sites - on viral capsid or envelopeReceptor Sites - on host cell membrane

Viral Size

20 nm to 1,000 nm

.02 u to 1 u

Viral Structure

1. Nucleic Acid2. Capsid (Coat Protein)

Nucleic AcidDNA or RNA (But never both)

ssDNA ds DNAss RNAds RNA

Viral StructureCapsid (Coat Protein)

protects viral genome from host endonucleases

capsomeresBinding Sites

Envelopederived from the host cellBinding Sites

Viral Morphology

1. Helical

Viral Morphology

2. Polyhedral

icosahedral

Viral Morphology

3. Enveloped

A. Enveloped Helical

B. Enveloped Polyhedral

Viral Morphology

4. Complex

Viral Classification1. Nucleic Acid

2. Morphology

3. Strategy for replication

Growing Viruses1. Bacteriophages

Lawn of Bacteria on a Spread PlateAdd BacteriophagesInfection will result in “Plaques”

Clear zones on plate

Growing VirusesAnimal Viruses

A. Living Animalsmice, rabbits, guinea pigs

B. Chicken Embryos (Eggs)used to be most common method to grow virusesStill used to produce many vaccines (Flu Vaccine)

C. Cell CulturesMost common method to grow viruses today

Cell Cultures1. Primary Cell Lines

die out after a few generationsB. Diploid Cell Lines

derived from human embryosmaintained for up to 100 generations

C. Continuous Cell LinesTransformed Cells (Cancerous Cells)may be maintained indefinitlyHeLa Cells

Henrietta Lax 1951 (Cervical Cancer)

Viroids and PrionsViroids

Naked RNA (no capsid)300 – 400 nucleotides longClosed, folded, 3-dimensional shape (protect

against endonucleases ?)Plant pathogensBase sequence similar to introns

Prions

Proteinaceous infectious particle1982Diseases

Scrapie (sheep)Creutzfeldt-Jacob disease (CJD)Kuru (Tribes in New Guinea)Bovine Spongiform Encephalopathy (BSE)

Mad Cow Disease

Viral ReplicationBacteriophage

1. Lytic Cycle2. Lysogenic Cycle

Lytic Cycle1. Attachment- binding sites must match

receptor sites on host cell2. Penetration - viral DNA is injected into

bacterial cell3. Biosynthesis

Genome replicationTranscriptionTranslation

Virus uses Host Cells enzymes and machinery

Lytic Cycle

4. Assembly (Maturation) viral particles are assembled

5. Release Lysis

Lysogenic Cycle1. Attachment2. Penetration3. Integration

Viral Genome is integrated into Host Cell Genome

Virus is “Latent”Prophage

Lysogenic Cycle

4. Biosynthesis - Viral Genome is Turned On Genome replicationTranscriptionTranslation

5. Assembly6. Release

Lysis

Lysogenic Convergence1. Corynebacterium diphtheriae2. Streptococcus pyogenes

Scarlet Fever3. Clostridium botulinum

Animal Virus Replication(non-enveloped virus)1. Attachment

Binding Sites must match receptor sites on host cell

2. PenetrationEndocytosis (phagocytosis)

3. Uncoatingseparation of the Viral Genome from the

capsid

Animal Virus Replication(non-enveloped virus)

4. BiosynthesisGenome ReplicationTranscriptionTranslation

5. AssemblyVirus particles are assembled

6. ReleaseLysis

Enveloped Virus Replication1. Attachment2. Penetration3. Uncoating4. Biosynthesis5. Assembly6. Release

Budding

Retro Viruses (1975)

DNA ---------> mRNA ------------> Protein

Central Dogma of Molecular Genetics

RNA -------> DNA --------> mRNA -------> Protein

Normal Virus

Retro Virus

Reverse Transcriptase (Retro)

Retro Viruses1. Many Cancer causing viruses2. HIV

Human Immunodeficiency VirusAIDS

Acquired Immunodeficiency Syndrome

HIV (Human Immunodeficiency Virus)AIDS

Acquired Immune Deficiency Syndromeresults in failure of the immune systemDeath usually results from an Opportunistic Infection

HIV discovered in 1984By who ?Luc Montagneir - Pasteur Institute

HIV Structure

Retro Virus

Nucleic acid - RNA (2 strands)

envelope (gp 120 binding sites)

Reverse Transcriptase

HIV Infection (Cellular Level)

1. Attachment

HIV gp 120 binding sites must match CD4 receptor sites

CD4 Receptor Sites

1. Macrophages

2. Some cells of CNS

3. T4 Helper Cells (CD4 Cells)

2. Penetration

Viral membrane and host cell membrane merge (fusion)

3. Uncoating

HIV Infection

Capsid is removed and Viral Genome is exposed

HIV Infection

4. Integration

Once Viral Genome is integrated - 2 possibilities:

1. Nothing - Virus is “Latent”

Virus may be latent for days, weeks, months or years

Median latency time = 10 years

Latent HIV provirus

2. HIV Genome can be “expressed” or “Turned On”Once HIV Genome is “turned on” death

usually results within 2 yearsWhat causes the HIV Genome to be

“turned on”?Other infectionsStress or shock to the systemDrug abuseAlcohol abuseNutritionExercise (Lack of or too much?)Sunburn ?

(Herpes Simplex 1)

Once HIV Genome is “turned on”

5. BiosynthesisGenome replicationTranscriptionTranslation

6. AssemblyVirus particles are put together

7. ReleaseBudding

Modes of HIV Transmission

HIV is transmitted by exposure to infected body fluids

4 Body Fluids1. Blood2. Semen3. Vaginal Secretions4. Breast Milk

How are these fluids transferred from one person to another?

1. High Risk Sexual Contactunprotected vaginal sexunprotected oral sexunprotected anal sex

2. Needles Intravenous Drug Abuse (sharing dirty

needles)accidental needle sticks

How are these fluids transferred from one person to another?3. Blood to Blood Contact

open sores or woundsTransfusionsOrgan TransplantsArtificial Insemination

4. Mother to Childplacentaas baby passes thru the birth canalbreast milk

HIV and the Immune System1. Cellular Immune System

cells phagocytize microbes

2. Humoral Immune Systemantibodies to destroy or inactivate microbes

Clinical Stages of an HIV Infection1. Acute Infection

Initial infection of HIV (exposure to infected body fluids)

ViremiaFeverHeadachesWeaknessMuscle and joint aches

May last for a couple of weeks

Normal CD4 cell count 1200mm3

2. Asymptomatic DiseaseCD4 cell count < 1000mm3

Virus is “latent” inside CD4 cellsMedian latency period - 10 yrs.No signs or symptoms of illness

(asymptomatic)HIV Positive - antibodies can be detected in

your bloodSeroconversion

6 to 8 weeks

3. Symptomatic DiseaseCD4 cell count < 600mm3

Viral Genome is “turned on”, Symptoms begin to appear

What causes HIV Genome to be turned on?Other infectionsstressshock to the systemalcoholdrug abusenutritionexercise ?

3. Symptomatic DiseaseSymptoms

chronic fatiguelow-grade fevernight sweatsdiarrheaweight loss

Susceptible to Infectionsbacterial pneumoniameningitisoral and vaginal yeast infectionstuberculosis

4. Advanced Disease (AIDS)CD4 cell count < 200mm3

Severe Opportunistic InfectionsPneumocysitis carinii pneumonia (PCP) FungiKaposi’s Sarcoma ( Cancer - Skin and Blood

vessels)Toxoplasmosis (Brain) ProtozoanCryptosporidiosis (G.I. Tract) ProtozoanOther Bacterial, Fungal and Viral Infections

HIV Infection and Immune Response (Graph)

Blood Test - ELISA

Enzyme Linked Immunosorbant Assaytests for HIV Antibodies

If ELISA is positive, same sample is tested again

If ELISA is positive again, then a Western Blot Test is done.Western Blot - test for Viral antigens

Treatment for HIV InfectionNo CureAZT ( Azidothymidine)

Thymine analoglacks a 3’ OHChain TerminatorInhibits Reverse Transcriptase

AIDS Cocktail (Combination Therapy)AZT3TC ( 2’-deoxy-3’-thiacytidine)Protease Inhibitor

Vaccine for HIV ?HIV mutates too rapidly

Reverse Transcriptase causes at least 1 mutation each time it is used1 million variants during Asymptomatic Disease100 million variants during Advanced Disease (AIDS)