virus characteristics

what are viruses?

• acellular particles with either DNA or RNA but NEVER both

• lacking in ribosomes, enzymes,+ ability to make energy

• can’t grow, divide, or synthesize proteins independently

• viral replication depends on HOST CELL’s machinery

• bc they rely on a host to reproduce they are known as OBILGATE INTRACELLUAR PARASITES

• viruses also infect every type of cell (bacteria, algae, fungi , protozoa, plants, animals)

what happened in 1892?

Dmitri ivanovsky found that the infectious agent causing tobacco mosaic disease passed through filters that removed bacteria

what happened in 1898?

Martinis Beijerinck described this agent as a “contagious living fluid” coning the concept of viruses (virus=poison)

what happened in 1930s?

the first viruses were visualized using the electron microscope

what did all these discoveries lead to?

the identification of hundred of viral species and the brith of virology as a field

Viruses: Life or Nonlife?

• exact origin of viruses is unknown

• theories explain how they may have evolved

• likely Arose multiple times in history from different ancestral sources

• classical + modern scenarios explain how they became depend on host cells

what are the 3 classical theories of viral origin?

• regressive (reduction) hypothesis

• progressive (escape) hypothesis

• virus first hypothesis

what is the regressive hypothesis?

• virus may have evolved from once free living cells that lost genes and structure over time becoming parasites

• ex: proxvirus retain many cell like genes supporting a reduction from free living ancestors

what is progressive hypothesis?

• viruses may have originated from genetic material (plasmids or transposons) that “escaped” from cells + gained the ability to move bw hosts

• ex: retroviruses may have evolved from mobile genetic elements that escaped from host genomes

what are virus first hypothesis?

• viruses may predate cells evolving from self replicating molecules in the pre cellular RNA world

• ex: simple RNA viruses may trace back to early self replicating molecules in the RNA world

virus are better described ___ or ____ rather than dead or alive

active + inactive

modern perspectives of viral evolution:

• modern molecular evidence shows viruses have multiple independent origins + evolve alongside their hosts

• horizontal gene transfer bw virus + cells is frequent

• - abt 8% of human genome comes from ancient viral DNA

• - ex: bacterial toxins (diphtheria toxin) are carried by bacteriophages

• - ex: ancient viral DNA in human genome contributes to embryo development

• - a viral new gene help form the placenta in mammals by fusing cells together process that originally evolved from a viral envelope gene

• viral genomes are mosaics of ancient viral genes + host derived sequences

• viruses act as part of the “mobile gene pool” that drives genetic exchange + evolution across all life

Smallest virus:

parvovirus sound 20 nm in diameter

viral architecture is best observed with….

special stains and electron microscopy