Viruses, viroids and prions

What are viruses?• Very small• Obligatory intracellular parasites

– Difficult to isolate, detect, cultivate– Somewhat like Rickettsia…

What are viruses?• Contain genetic material

– DNA or RNA• Protein coat

– Sometimes encased in lipids,carbs and proteins

• Reproduction inside living cells• No metabolic enzymes

– Use host enzyme– Problem for drug creators!

Helical viruses

Enveloped viruses

Polyhedral Viruses

Complex viruses

What is a host range?• Host cells a virus can infect

– Very narrow– Useful for treating diseases?

• Viral therapy• Oncolytic viruses

• Range determined by cell receptor sites

What is a virion?• One, complete,

infectious viral particle– Contains

• Nucleic acid– DNA or RNA– Both can be

double- or single-stranded

• Protein coat (capsid)

– Classification based on type of capsid

– Capsomeres: protein subunits

What is a virion?– Contains envelope (not all)

• Covers capsid• Lipids, carbs, proteins• Used to fuse with host PM• May have spikes

What types of viruses are there?

• Helical– Rabies, ebola

What types of viruses are there?

• Polyhedral– Animal, plant, phages

What types of viruses are there?

• Enveloped– Influenzavirus

What types of viruses are there?

• Complex– Many

phages– Addition

al structures

What are some viruses I should know?

• No specific epithet

• DNA viruses– Adenoviridae– Poxviridae– Herpesviridae

• Human herpes virus 1, HHV 2, HHV 3

– Papovaviridae– Hepadnaviridae

What are some viruses I should know?

• RNA viruses• Picornaviridae• Retroviridae

– Lentivirus– Human Immunodeficiency Virus 1, HIV 2

Virus Identification• Cytopathic effects• Serological tests

– Detect antibodies against viruses in a patient.– Use antibodies to identify viruses in

neutralization tests, viral hemagglutination, and Western blot.

• Nucleic acids– RFLPs– PCR

Viral Replication

How do viruses multiply?

• Virus only has a few genes and proteins– All other

proteins come from host cell

• E.g. ribosomes, tRNA, etc.

– Must take over host metabolism

How do phages multiply?• Two possibilities

– Lytic cycle– Lysogenic cycle

• T-4– About 100 genes– Multiplication in 5 stages

• Attachment• Penetration• Biosynthesis• Maturation• Release

– animation

What’s the difference between lytic and lysogenic cycles?

Lytic cycle: Phage causes lysis and death of host cell.Lysogenic cycle: Prophage DNA incorporated in host DNA.

What else should I know about the lysogenic phase?

• When latent (temperate phase)– Can’t be infected

with the same virus again

– Phage conversion can happen

• C. diptheriae: produces toxin only in latent phase

• C. botulinum, C. cholerae and some streptococci also

What else should I know about the lysogenic phase?

• When latent (temperate phase)– Specialized

transduction is possible

How do viruses multiply in animals like us?

• Attachment: Viruses attach to cell membrane.• Penetration by endocytosis or fusion.• Uncoating by viral or host enzymes.• Biosynthesis: Production of nucleic acid and

proteins.• Maturation: Nucleic acid and capsid proteins

assemble.• Release by budding (enveloped viruses) or rupture.

What’s different about entry?

• Attachment first– Sites vary from person to

person• Penetration

– Q: how does it happen in phages?

– Pinocytosis OR– Fusion (animation)

• HIV• Uncoating

– by viral or host enzymes

Figure 13.14a

What are the final stages for multiplication in animal viruses?

• Maturation– Nucleic acid – Capsid proteins

• Release– budding (enveloped viruses) or

• animation– Rupture

Multiplication of DNA Virus

Figure 13.15

Pathways of Multiplication for RNA-Containing Viruses

Figure 13.17

Multiplication of a Retrovirus

PLAY Animation: Viral Replication

Figure 13.19

DNA and RNA transcriptase

• DNA, reverse transcriptase: Cellular enzyme transcribes viral DNA in nucleus; reverse transcriptase copies mRNA to make viral DNA.

• RNA, reverse transcriptase: Viral enzyme copes viral RNA to make DNA in cytoplasm.

Viruses and disease

Is there a connection between viruses and cancer?

• Yes!• Oncogenic

viruses– 10% of all

cancers– DNA integrates

into host cell– Tumor-specific

transplantation antigen (TSTA)

– Cytopathic changes

Can you give me some examples?

• HPV—cervical cancer• Adenoviridae—adenocarcinoma • Herpesviridae

– Epstein-Barr (EB) virus—Burkitt’s lymphoma

– HHV-8—Kaposi’s sarcoma

• Hep-B—liver cancer

Do all viral infections causes symptoms immediately?

• No• Latent period for many

– EB may be latently carried in 9 out 10 people– Herpesviruses can stay for lifetime

• Cold sores

– Chickenpox—shingles (zoster)

• Persistent infections for some– Usually fatal– Gradual appearance and worsening of symptoms– Subacute sclerosing panecephalitis (from

measles)

What are prions?

• Not viruses• Infectious proteins• Inherited and

transmissible by – Ingestion– Transplant– surgical instruments

• Mad Cow Disease– Creutzfeldt-Jacob disease