4.1 Virology: Structure and Classification

Learning objectives

•Describe why viruses are not considered living organisms

•Be able to describe and explain the basic structure of a virus, and additionally know the various types of these structures/classifications

Characteristics of life

Organization, Homeostasis, Growth and Metabolism, Response to stimuli, Heredity, Reproduction, Evolution and Adaptation

Properties viruses share with life

Exhibit structural organization

Contain genetic material

Contain a replicative life cycle Evolve over time

Properties of life that viruses lack

Are not comprised of cells

Lack homeostasis Have no growth or metabolism

Do not respond to stimuli

Cannot reproduce without the use of host cell machinery

virus

a non-living obligate intracellular parasite

virion

a virus that is extracellular

viral genome

DNA or RNA, varies considerably in size and organization

Sense DNA

DNA or RNA, varies considerably in size and organization

antisense DNA

Template-strand of DNA that gets transcribed

Sense RNA (+RNA)

mRNA that is translated into a protein

Antisense RNA (-RNA)

RNA that is a complimentary strand to translated mRNA

Central Dogma

Antisense DNA is transcribed into Sense RNA (which is translated into protein)

Mutation rate

Fast mutation rates lead to new strains of the same virus

capsid

protein coat protecting the genome, may carry accessory proteins, composed of capsomeres which are made up of protomers

Helical viruses

Repeated capsomeres that form a filamentous capsid surrounding the genomic material; may be referred to as filamentous, may not always appear strictly helical (ex. Tobacco mosaic virus, Mumps virus, Ebola virus)

spherical

Has a spherical capsid/envelope; not always its own category, it is the capsid and not the presence of a spherical phospholipid membrane that make a virus spherical (ex. Influenza virus)

polyhedral

Capsomeres that form 20 faces, 12 vertices, and 30 edges; may be referred to as icosahedral, cubic, regular, or isometric (ex. Adenovirus, Herpesvirus)

complex

The “catch-all” group with no real pattern, often may have a combination of several shapes but may not necessarily be classified as such (ex. T4 Bacteriophage, Variola virus, Rabies virus)

Envelope

host-derived lipid membrane, contains host and viral proteins

Attachment proteins

glycoproteins that recognize one or more specific host cell receptors (Controls viral tropism: the spectrum of cells of a host that a virus may infect)

HIV

HIV GP120 binds to CD4 as well as CCR5 or CXCR4 found on T cells