BIO 205

Chapter 13

Viruses

• Some capsid proteins are spikes that help the virus attach to and penetrate the host cell

• The nucleocapsid is the capsid with its enclosed genome

• The capsid is the protein coat, made up of capsomeres

• The viral genome contains either DNA or RNA, but not both

• They need a host eukaryote or prokaryote to replicate

• They lack the machinery for generating energy and large molecules

• Viruses are small, obligate intracellular parasites

Viruses Are Tiny Infectious Agents

What are Viruses?

– A virion is a completely assembled, infectious virus outside its host cell

– Viruses surrounded by an envelope are enveloped viruses

– Naked viruses are composed only of a nucleocapsid

Viruses Are Very SmallSize Relationships Among Microorganisms and Viruses

• Viruses that have both helical and isocahedral symmetry have complex symmetry

• Isocahedral viruses have isocahedral symmetry

• Helical viruses have helical symmetry

Viruses Are Grouped by Their Shape

Rabies

Helical Virus Herpes, Icosahedral Virus

Smallpox

Complex Virus

• Many viruses infect certain cell or tissue types within the host (tissue tropism)

• Host range depends on capsid structure

• A host range refers to what organisms the virus can infect

Viruses Have a Host Range and Tissue Specificity

• DNA viruses contain single- or double-stranded DNA genomes

• The International Committee on Taxonomy of Viruses (ICTV) is developing a classification system

• Viruses can be named according to a number of different conventions

Nomenclature and Classification Do Not Use Conventional Taxonomic Groups

The Classification of Viruses

Retroviruses are replicated indirectly through a DNA intermediate

– strand RNA viruses have RNA strands that would be complementary to mRNA

+ strand RNA viruses have mRNA genomes

RNA viruses contain single- or double-stranded RNA genomes

The phage nucleic acid contains only a few of the genes needed for viral synthesis and replication

T-even group bacteriophages are virulent viruses that carry out a lytic cycle of infection

The Replication of Bacteriophages Is a Five-Step Process

Viral Replication and its Control

Bacteriophage

Phase 5: Release is the exit of virions from the bacterium. It is also called the lysis stage when the cell is ruptured

Phase 4: Maturation is the assembly of viral parts into complete virus particles

Phase 3: Biosynthesis is the production of new phage genomes and capsid parts

Phase 2: Penetration occurs when the phage tail releases lysozyme to dissolve a portion of the cell wall. Phage DNA is injected into the bacterial cytoplasm

Phase 1: Attachment occurs when a phage’s tail fibers match with a receptor site on the bacterium’s cell wall

They insert their DNA into the bacterial chromosome as a prophage (lysogenic cycle)

Temperate phages do not lyse the host

Animal viruses attach to host plasma membrane via spikes on the capsid or envelope

Animal Virus Replication Has Similarities to Phage Replication

• This occurs as some animal viruses enter the cell

• Uncoating is the separation of the capsid from the genome

• Animal viruses are usually taken into the cytoplasm as intact nucleocapsids

• Since receptor sites vary from person to person, some people are more susceptible to a certain virus than others

The virus buds, taking the membrane part with it as an envelope

After the new viruses are assembled, envelope proteins are incorporated into a cellular membrane

HIV

It can then be inserted into the host chromosome

Retroviruses use reverse transcriptase to transcribe their RNA to DNA

Some DNA viruses and retroviruses insert their genome into the host chromosome as a provirus

Some Animal Viruses Can Exist as Proviruses

Proviruses

Eventually the provirus will be activated and replicate

They are propagated each time the cell’s chromosome is reproduced

Latent proviruses are immune to the host body’s defenses

It is in a state of latency

The provirus encodes a repressor protein that prevents activation of the viral genes necessary for replication

• maturation/release

• genome replication

• viral penetration/uncoatingSome antivirals exist to affect:

Viruses lack the elements with which antibiotics interfere

Antibiotics do not work against viruses

Antiviral Drugs Can Be Used to Treat a Limited Number of Human Viral Diseases

This prevents the release of new virions into the body

Neuraminidase inhibitors block an enzyme in the spike of influenzaviruses

Protease inhibitors impede the HIV protease that trims viral proteins in capsid construction

Reverse transcriptase inhibitors prevent the synthesis of DNA in retroviruses

• blocking replication of the viral genome

• inserting base analogs in the replicating DNA strand

Most antivirals target the replication enzymes of the virus by:

• Cells in an antiviral state can inhibit viral replication by preventing protein synthesis

• Some IFNs have anti-cancer properties

• Interferon (IFN) is a group of naturally-produced proteins that alert cells to a viral infection

Interferon Puts Cells in an Antiviral State

Interferon

IFNs bind to receptors on cells, triggering them to produce antiviral proteins

Viruses and Virus-Like Agents

• A malignant tumor

Tumor cells can break free from the capsule and spread to other tissues of the body (metastasis)

• A benign tumor

Normally, the body surrounds a tumor with a capsule of connective tissue

A tumor is a clone of abnormal cells

Cancer Is an Uncontrolled Growth and Spread of Cells

Cancer and Viruses

• 60-90% of human cancers are associated with carcinogens.

• Viruses Are Responsible for up to 20 Percent of human Tumors.

• There is now a vaccine against the 2 most common strains of HPV

• Human papilloma virus (HPV) is associated with cervical cancer

• Epstein-Barr virus is linked to Burkitt Lymphoma, a tumor of the jaw

Oncogenic viruses include:

• viruses

• DNA damage

• chemical carcinogens

• radiation

They can be transformed to oncogenes by:

The oncogene theory suggests that protooncogenes normally reside in the chromosomal DNA of a cell

Oncogenic Viruses Transform Infected Cells

The Oncogene Theory

•The v-oncogene can then code of growth factors stimulating uncontrolled cell proliferation

•When the oncogenic viruses infect another cell, the v-oncogene is under the virus’ control not the cell’s control

•V-oncogenes are protooncogenes captured in the viral genome

•When virus replication is triggered, the provirus replicates its only DNA as well as a few adjacent host genes

•Sometimes a virus inserts its DNA (as a provirus) into a cell’s chromosome next to a protooncogene

Oncogenic Virus

• Mutation can occasionally be advantageous and create a new or new strain of virus

• Genetic recombination can lead to “new” viruses

• Emerging viruses may spread to new populations, or may expand host range

• Emerging Viruses Usually Arise Through Natural Phenomena

Emerging Viruses and Viral Evolution

• One hypothesis suggests they originated as introns

• The replication cycle and disease causation process of viroids are not understood

• Viroids are tiny fragments of RNA that cause diseases in crop plants

Viroids Are Infectious RNA particles

Virus-Like Agents

Stanley Prusiner discovered the proteinaceous infectious particle (prion)

Originally, scientists believed TSEs were caused by a virus

TSEs are neurologic degenerative diseases that can be transmitted within or between species

• For example, mad cow disease

Transmissible spongiform ecephalopathies (TSEs) can occur in humans and other animals

Prions Are Infectious Proteins

Prions

Brain tissue showing effects of a prion disease

• Abnormal prions do not trigger an immune response

• This causes normal prions to change shape and become abnormal

• TSEs may spread when infectious prions bind to normal prions.

• Normal cellular prions have a different shape than abnormal prions, the latter of which cause TSEs

• The protein-only hypothesis predicts that prions are composed only of protein and contain no nucleic acids

This results from insoluble aggregates of abnormal prions in the brain

• loss of balance

• weakened muscles

• dementia

Symptoms include:

Death of the host occurs from nerve cell death leading to sponge-like holes in brain tissue

End of Chapter 13