Eukaryotes and Viruses

Chapters 12 and 13

Viral Characteristics and Structure

Why Viruses aren’t Alive

General Characteristics of Viruses

• Name derives from the Latin for “poison”

• Obligatory intracellular parasites• Referred to as filterable• Contain a single type of nucleic

material• The nucleic material is covered in a

protein coat.• Use the synthesis machinery of the

host to multiply.

Why are they not Alive?

• No independent metabolism or reproduction

• No single phylogenetic origin• No cellular structure• No ribosomes

• Though they DO evolve and reproduce.

Host Range

• Viruses have a specific subset of cell types they will infect, referred to as Host Range.

• Most viruses can only infect a single species

• Some viruses can cross species barriers• Numerous factors influence host range

• Viruses that infect bacteria are referred to as bacteriophage or simply phage.

Viral Particle Size

Staphylococcus Bacteria1 μm in diameter

Poxviridae

Lentiviruses

Picornaviridae

Bacteriophage

Viral Structure

• A Virion is a complete, infectious viral particle and is composed of…– Nucleic Acid– Capsid and Envelope

Nucleic Acid

• Only a single type of nucleic acid (RNA or DNA) is present in any species of virus.

• Unlike cellular life, viral nucleic acid can be either single or double-stranded (again only a single type per species)

• Size of the genetic structure can range from a few thousand base pairs to a quarter of a million

Capsid and Envelope

• Capsids are regular repeating protein structures composed of capsomeres.

• Some viral species also have a host-derived envelope surrounding the capsid

• Some viral species have protein/ carbohydrate “spikes” rising from the surface that can be used for identification

Viral Morphology

Viral Taxonomy

How do you do a taxonomy of something that isn’t alive?

Viral Taxonomy

• Without a shared phylogeny there is no use for the higher taxons (Domain, Kingdom, Phylum, and Class)

• Typically viral species are referred to by Order, Family, Genus and a descriptive common name (in place of a species epithet)

• Based on – Nucleic Acid type– Strategy of replication– Morphology– Host range

Viral Replication

No, not 1 becomes 2, more like 1 becomes 1000.

Growing Bacteriophage

Growing Animal Viruses

Viral Identification

• Polyphasic Identification–Morphology– Detection of Antibodies–Western Blotting of known viral proteins– Nucleic Methodologies• PCR• RFLP• RNA PCR

Lytic Bacteriophage Cycle

Attachment

Penetration

Biosynthesis Maturation

Release

Lysogenic Bacteriophage Cycle

Lytic Cycle

Animal Unenveloped DNA Viral Replication

DNA vs. RNA Viral Replication

• There are various types of RNA viruses.• Replication of the Genetic Material can be

simple or a multistep process.– +RNA, direct translation and replication by

viral protein– -RNA, indirect translation and replication by

viral protein– dsRNA, direct translation and relication by

viral protein– Retroviruses, conversion of RNA to DNA,

integration and then production by host.

Comparison

Bacteriophage• Attachment to Cell

Wall proteins.• Viral DNA is injected

into cell• No removal of capsid

required• Biosynthesis in

cytoplasm• Lysogeny• Host cell lysed for

release

Animal• Attachment to plasma

membrane proteins and glycoproteins.

• Capsid enters cells• Capsid removed by

enzymes• Biosynthesis in nucleus or

cytoplasm• Latency• Enveloped viruses bud and

nonenveloped rupture.

Viruses and Cancer

One of many factors

Oncogenic Viruses

• Some viruses and known to help trigger cancers, called oncogenic viruses.

• These viruses affect oncogenes, natural parts of our genetic structure that can cause cancer.

• The process of becoming cancerous is termed transformation.

• Oncogenic Viruses integrate into the host genetic material.

Known Oncogenic Viruses

Type of Virus Viral Species Associated Cancer

DNA Human Papillomavirus Cervical Cancer

Epstein-Barr Virus Burkitt’s Lymphoma

Hepatitis B Virus Liver Cancer

Kaposi Sarcoma-Associated Hepervirus

Kaposi Sarcoma

Merkel Cell Polyomavirus Merkel Cell Carcinoma

RNA Human T-Lymphotropic Virus 1 Leukemia

Hepatitis C Virus Liver Cancer

Latency, Persistence and the Prions

Acute, Latent, and Persistent Viral Infections

• Acute Infections are those that cause immediate proliferation.

• Latent infections can occur by itself or after an acute infection, where the viral load remains undetected for a long period of time before reemerging quickly.

• Persistent Infections are ones where the viral load build over a long period of time.

Graph of Acute, Latent and Persistent Viral Infections

Prions

• Prions are infectious protein particles• Prions are altered forms of a normal

protein in the host that can catalyze the alteration of other “normal” protein particles to the “prion” state

• They cause neurological degradation and death with no known treatment.

• Since each prion protein is infectious, they are extremely resistant to control measures.

Prion Reaction

PrPC + PrPSc 2 PrPSc