chapter 19

• Viruses and bacteria are the simplest biological systems—microbial models in which scientists find life’s fundamental molecular mechanisms in their most basic, accessible forms.
• Molecular biology was born in the laboratories of microbiologists studying viruses and bacteria.
  • Microbes such as E. coli and its viruses are called model systems because of their use in studies that reveal broad biological principles.
  • Microbiologists provided most of the evidence that genes are made of DNA, and they worked out most of the major steps in DNA replication, transcription, and translation.
  • Techniques enabling scientists to manipulate genes and transfer them from one organism to another were developed in microbes.
• In addition, viruses and bacteria have unique genetic features with implications for understanding the diseases that they cause.
• Bacteria are prokaryotic organisms, with cells that are much smaller and more simply organized than those of eukaryotes, such as plants and animals.
• Viruses are smaller and simpler still, lacking the structure and metabolic machinery of cells.
  • Most viruses are little more than aggregates of nucleic acids and protein—genes in a protein coat.

In 1971, David Baltimore described a scheme for classifying viruses based on how the virus produces mRNA.

The table below shows the results of testing five viruses for nuclease specificity, the ability of the virus to act as an mRNA, and presence (+) or absence (-) of its own viral polymerase (SEE IMAGE)

34) Given Baltimore's scheme, a positive sense single-stranded RNA virus such as the polio virus would be most closely related to which of the following?

A) T-series bacteriophages
B) retroviruses that require a DNA intermediate
C) single-stranded DNA viruses such as herpes viruses
D) nonenveloped double-stranded RNA viruses
E) linear double-stranded DNA viruses such as adenoviruses

Based on the above table, which virus meets the Baltimore requirements for a retrovirus?

A) A
B) B
C) C
D) D
E) E