Transcription of Ribosomal RNA Gene Clusters

Electron micrograph (26,500x) showing transcription of the DNA encoding ribosomal RNA (rRNA) molecules in the nucleolus of a developing egg cell of the spotted newt.

Eukaryotes have several hundred identical genes encoding ribosomal RNA.

The long filaments (green arrow) are DNA molecules coated with proteins. The fibers extending in clusters from the main axes are molecules of ribosomal RNA which will be used in the construction of the

Genetic Recombination in Bacteria

Index to this page

Bacteria have no sexual reproduction in the sense that eukaryotes do. The have

But the essence of sex is genetic recombination, and bacteria do have three mechanisms to accomplish that:

The Hershey and Chase Experiments

In 1952 (7 years after Avery's demonstration that genes were DNA), two geneticists:

  • A. D. Hershey and
  • Martha Chase

provided further proof. They worked with a DNA virus, called T2, which infects E. coli (and so is a bacteriophage).

The figure shows the essential elements of the infective cycle of DNA bacteriophages like T2.

  • The virions attach to the surface of their host cell (a).
  • The proteins of the capsid inject the DNA core into the cell