CHAPTER 13 VIRUSES, VIROIDS, AND PRIONS

Viruses

Miniscule, acellular, infectious agents having either DNA or RNA

Why are viruses not living?

1. Cannot carry out any metabolic pathway

2. Niether grow nor respond to environment

3. Cannot reporduce independently

Viruses have a __________ and __________ state

extracellular; intracellular

The extracellular state of a cell is called a _______

virion

Capsid

Protein coat surrounding nucleic acid

Nucleocapsid

Nucleic acid and capsid

Intracellular state

Capsid is removed and the virus exists as a nucleic acid

Viruses show _______ variety in their genomes than cells

more

Viruses are primarily categorized and classified by…

genetic material

The genetic material of viruses can be either _____ or _____ but never both

DNA; RNA

What are the different genetic material types a virus can have?

1. dsDNA, ssDNA, dsRNA, ssRNA

2. Linear and segmented or single and circular

3. Smaller than genomes of cells

Why do most viruses infect only particular host’s cells?

Due to affinity of viral surface proteins for complementary proteins on host cell surface; May be so specific they only infect particular kind of cell in a particular host

Generalists

infect many kinds of cells or many different hosts

Capsids

1. Protect viral nucleic acid

2. Means of attachment for host’s cells

3. Composed of proteinaceous subunits called capsomeres

   1. May be composed of single or multiple types of proteins

What are the three basic viral shapes?

Helical, polyhedral, and complex

Viral envelope is acquired…

from host cell during viral replication or release

Viral envelope

1. Portion of membrane system of host

2. Composed of phospholipid bilayer and proteins

3. Some proteins are virally coded glycoproteins (spikes)

____________ viruses are more fragile than ______ viruses

enveloped; naked

What is virus classification based on?

1. Type of nucleic acid

2. Presence of an envelope

3. Shape

4. Size

Viral genera have only been organized into _________

families

Lytic replication

Viral replication that usually results in death and lysis of host cell

What are the 5 stages of lytic replication

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Lysogenic replication of bacteriophages

A modified replication cycle in which infected host cells grow and reproduce normally for generations before they lyse

Lysogenic replication occurs with ____________ phages

temperate

Prophage

inactive stage

Lysogenic conversion

Results when phages carry genes that alter phenotype of a bacterium

Replication of animal viruses

1. Same basic replication pathway as bacteriophages

2. Differences result from

   1. Presence of envelope around some viruses

   2. Eukaryotic nature of animal cells

   3. Lack of cell wall in animal cells

Attachment of animal viruses

Chemical attraction between viral protein and cell receptor; no tails or tail fibers; Have glycoprotein spikes or other attachment molecules that mediate attachment

Entry and uncoating of animal viruses

Enter host cell by direct penetration, membrane fusion, and endocytosis

Synthesis of DNA viruses of animals

Each type of animal virus requires different strategy depending on its nucleic acid

DNA viruses often enter the _________

nucleus

RNA viruses often replicate in the __________

cytoplasm

Synthesis of dsDNA viruses

1. Similar to replication of cellular DNA

   1. Viral genome replicated in the nucleus

   2. Viral proteins made in the cytoplasm

2. Exception: Hep B viruses replicate DNA from an RNA intermediary

Synthesis of ssDNA

1. Cells do not use ssDNA

2. Parvoviruses

   1. DNA strand folds back on itself to form dsD

      NA, which is replicated by cellular DNA

      polymerase

   2. Newly replicated strand is released as ssDNA

Synthesis of RNA viruses of animals

1. Positive-sense (+) viral RNA can act as mRNA

2. Negative-sense (−) viral RNA cannot be directly

   translated

What are the 4 types of RNA viruses?

+ssRNA, retroviruses, -ssRNA, and dsRNA

Retroviruses

Use DNA intermediary (not genomes) transcribed by viral reverse transcriptase as template to produce viral genomes

Where do most DNA viruses assemble?

nucleus

Where do most RNA viruses develop?

cytoplasm

The number of viruses produced depends on…

type of virus and the size and initial health of host cell

How are enveloped viruses released?

via budding (can result in persistent infections)

How are naked viruses released?

exocytosis or lysis

Latency of animal viruses

1. When animal viruses remain dormant in host cells (called provirus)

2. May be prolonged for years with no viral activity

3. Incorporation of provirus into host DNA is

   permanent

Neoplasia

Uncontrolled cell division in multicellular animals

Tumor

mass of neoplastic cells

___________ tumors cause cancer

Malignant

Metastasis occurs when…

tumors spread

Proto-oncogenes

promote cell growth and division

Uncontrolled activation of oncogenes can lead to…

cancer

What are some environmental factors that contribute to the activation of oncogenes?

UV light, radiation, carcinogens, and viruses

What percentage of human cancers are caused by viruses?

20-25%

How do viruses cause cancer?

1. CArry copies of oncogenes as part of their genomes

2. Promote oncogenes already present in host

3. Interfere with tumor repression

What are some examples of specific viruses that are known to cause some human cancers?

Burkitt’s lymphoma, hodgkin’s disease, kaposi’s sarcoma, and cervical cancer

What are the three types of media used for culturing viruses?

1. Media consisting kf mature organisms

2. Embryonated eggs

3. Cell cultures

Lysis of bacteria produces plaques which allow for…

estimation of phage numbers by plaque

assay

What are the concerns of culturing viruss in plants and animals?

Lab animals can be difficult and expensive to maintain + ethical concerns

Culturing Viruses in Embryonated Chicken Eggs

1. Inexpensive

2. Among largest of cells

3. Free of contaminating microbes

4. Contain nourishing yolk

5. Fertilized chicken eggs are often used because the embryonic tissues provide ideal site for growing viruses

Cell cultures

1. Cells isolated from an organism and grown on a

   medium or in a broth

2. Two types

   1. Diploid cell cultures

   2. Continouous cell cultures

Viroids

1. Extremely small, circular pieces of ssRNA that are

   infectious and pathogenic in plants

2. Similar to RNA viruses but lack capsid

3. RNA does not code for proteins

4. Viroid RNA adheres to complementary plant RNA

   1. Plant enzyme degrades the dsRNA

   2. Results in a disease state

Prions

Proteinaceous infectious agents

Cellular PrP

1. Made by all mammals

2. Normal, functional structure has α-helices

Prion PrP

1. Disease-causing form has β-pleated sheets

2. Prion PrP causes cellular PrP to refold into prion PrP

Spongiform encephalopathies

Prion disease in which large vaculoes form in brain

How are prion disease transmitted?

ingestion, transplantation, or contact of mucous membranes with infected

tissues

How are prions destroyed?

incineration or autoclaving in concentrated sodium hydroxide