Some biologists refuse to dignify the virus as a form of life.

• It has no cell wall.
• It cannot reproduce without a host.

Scientists think that every kind of living organism is probably host to at least one virus!

First described in 1892, a pathogen infecting tobacco plants. Since then, 6000 viruses have been described. The virus is the most numerous type of biological entity.

biology > microbiology > virology

We use the term infect and host. Implying the virus is a parasite.

We could say viruses are found inside every form of life: plants, animals, bacteria,

The virus is not included in the list of life forms.

Yet, viruses are found within every life form. Viruses

We say the virus infects a host, implying that the virus is always a parasitic infection, despite the fact that every known organism carries at least one type of virus within.

Submicroscopic. One-hundredth the size of most bacteria.

Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such as cell structure, that are generally considered necessary criteria for life. Because they possess some but not all such qualities, viruses have been described as “organisms at the edge of life”,[9] and as self-replicators.

Is the virus alive?

Yes, because it

• carries genetic material
• reproduces
• evolves through natural selection

No, because it

• has no cell wall
• cannot reproduce without a host

When not infecting a cell, the virus exists as a virion, an independent particle, consisting of:

• genetic material, DNA or RNA
• a protein coat, the capsid
• (optional) an outside envelope of lipids

Shape can be one of:

• helical
• icosahedral
• more complex

Size: one-hundredth the size of most bacteria.

Replication

horizontal gene transfer

Virology Lectures 2021, Youtube Vincent Racaniello, Ph.D. Columbia University

Lecture 1. What is a virus?

24:06 map, spread of man from africa outward, populations traced by variations in the polyomavirus, which is found in 90% of humans

25:20 viruses shape host populations, and vice versa

38:45 scale of size, viruses smaller than bacteria, larger than ribosomes

54:00 replication, only within cell, eclipse period building components, then burst of new virus particles created

56:25 https://talk.ictvonline.org/taxonomy/

Lecture 2. The Infectious Cycle

Lecture 3. Genomes and Genetics

“Nucleic acid is the genetic code.”

nucleic acid

• deoxyribonucleic acid (DNA)
• ribonucleic acid (RNA)