Viruses: Living or Nonliving?

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

Shows a to-scale comparison of a virus, bacterium, animal cell, and animal cell nucleus. Shows a zoomed-in view of a virus injecting its DNA into an E. coli cell.

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

Human Organ Systems Parasitized by Viruses

Shows a diagram of the human body, highlighting organ systems parasitized by specified viruses.

Unknown source

Viral Structure

Shows the correspondences between a nonenveloped virus and an enveloped virus. Shows several different viral structures, including varied uses of capsids and envelopes.

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

Viral Genome Diversity

Viral Class* Examples/Diseases
  • Papiloma (human warts, cervical cancer)
  • Polyoma (tumors in certain animals)
  • Respiratory diseases
  • Some cause tumors in certain animals
  • Herpes simplex I (cold sores)
  • Herpes simplex II (genital sores)
  • Varicella zoster (chicken pox, shingles)
  • Epstein–Barr virus (mononucleosis, Burkitt's lymphoma)
  • 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
  • Poliovirus
  • Rhinovirus (common cold)
  • Enteric (intestinal) viruses
  • Rubella virus
  • Yellow fever virus
  • Encephalitis viruses
ssRNA that is a template for mRNA
Rhabdovirus Rabies
  • 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.

Viral Reproduction

Shows the general process of viral reproduction outlined above. Shows a virus with only a capsid exiting a lysed host cell. Shows a virus with an envelope budding off of a host cell.

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

Phage Virus Reproduction

The Lytic Cycle

Illustrates the process of the lytic cycle, which takes place in phage viruses, for the phage T4.

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

The Lysogenic Cycle

Illustrates the process of the lysogenic cycle, which takes place in phage viruses, and its interaction with the lytic cycle, for the phage λ.

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

Lytic Cycle vs. Lysogenic Cycle

RNA as Genetic Material

Shows the difference between the standard pathway of information flow and that exhibited by retroviruses.

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