Viruses: Living or Nonliving?

Viruses are not considered, by the current thinking, to be living organisms.
They cannot reproduce or do much of anything on their own—they need the help of living cells.
- Some books call them obligate intracellular parasites.

Characteristics	Viruses	Living Organisms
Hereditary material	DNA or RNA; can be single-stranded or double-stranded	DNA; always double-stranded
Cell membrane present?	No	Yes
Genetic recombination?	Yes	Yes
Can carry out transcription independently?	No—even if a polymerase is present, transcription of the viral genome requires the use of ATP and nucleotides provided by the host cell	Yes
Can carry out translation independently?	No	Yes
Metabolic capabilities	Virtually none	Extensive—synthesis of ATP, reduced carbon compounds, vitamins, lipids, nucleic acids, etc.

Size of Viruses

Figure 18.1, page 230, and unlabeled figure, page 229, Campbell's Biology, 5th Edition

Unknown source

Inner genome: stores genetic information
Capsid: protein coat
- Made of protein subunits called capsomeres.
- All viruses have a capsid.
Envelope: membrane cloaking the capsid
- Made from the membrane of the host cell.
- Not all viruses have this structure.
- Two famous viruses with an envelope:
  - Influenza virus
  - Human Immunodeficiency Virus (HIV)

Unknown source; part of figure 18.2, page 231, Campbell's Biology, 5th Edition

Viral Class*	Examples/Diseases
dsDNA
Papovavirus	Papiloma (human warts, cervical cancer) Polyoma (tumors in certain animals)
Adenovirus	Respiratory diseases Some cause tumors in certain animals
Herpesvirus	Herpes simplex I (cold sores) Herpes simplex II (genital sores) Varicella zoster (chicken pox, shingles) Epstein–Barr virus (mononucleosis, Burkitt's lymphoma)
Poxvirus	Smallpox Vaccinia Cowpox
ssDNA (parvovirus)
	Roseola Most parvoviruses depend on coinfection with adenoviruses for growth
dsDNA (reovirus)
	Diarrhea Mild respiratory diseases
ssRNA that can serve as mRNA
Picornavirus	Poliovirus Rhinovirus (common cold) Enteric (intestinal) viruses
Togavirus	Rubella virus Yellow fever virus Encephalitis viruses
ssRNA that is a template for mRNA
Rhabdovirus	Rabies
Paramyxovirus	Measles Mumps
Orthomyxovirus	Influenza viruses
ssRNA that is a template for DNA synthesis (retrovirus)
	RNA tumor viruses (e.g., leukemia viruses) HIV (AIDS virus)

* The subclasses within each class differ mainly in capsid structure and in the presence or absence of a membraneous envelope.

A virus is an obligate intracellular parasite; it needs a host cell to reproduce!
After entering the host cell, the virus:
1. Uses the host's nucleotides and enzymes to replicate its own genetic material.
2. Uses the host's enzymes and ribosomes to transcribe and translate the viral genes to make the capsid proteins.
3. The capsid proteins and viral DNA self assemble into new viral particles and leave the cell.

Figure 18.6, page 325, Campbell's Biology, 5th Edition; unknown sources

Figure 18.4, page 323, Campbell's Biology, 5th Edition

Figure 13.2, Purves's Life: The Science of Biology, 7th Edition

Lytic cycle
- Kills the host bacterium.
- Virulent virus: a virus that only reproduces using the lytic cycle
Lysogenic cycle:
- Does not kill the host bacterium.
- Viral DNA becomes incorporated into the host DNA.
  - Once it is integrated it is called a prophage.
  - The prophage will remain silent and be faithfully reproduced with the host DNA until some point it exits the bacterial chromosome and goes through the lytic cycle.
- Temperate virus: a virus that uses both modes of reproduction

Retroviruses (e.g. HIV) use RNA to make DNA, hence the name—“retro.”
An important enzyme called reverse transcriptase allows them to do this.
- It transcribes DNA from an RNA template.
- This is a violation of the standard pathway of information flow (DNA → RNA → protein), and hence a modification, some would say, violation, of the central dogma.

Modified figure 12.2, Purves's Life: The Science of Biology, 7th Edition