Genetic structure and function / P.F. Smith Keary.

  • Smith-Keary, P. F.
Date:
1975
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic structure and function / P.F. Smith Keary. Source: Wellcome Collection.

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    THE CHROMOSOMES OF PROCARYOTES 31 The Chromosome of Escherichia coli There are two fundamental differences between procaryotic and eucaryotic cells; firstly, procaryotic cells lack certain organelles such as mitochondria and chloroplasts that are typical of eucaryotic cells, and secondly, they do not have a nuclear membrane and so no discrete nucleus. In bacteria the bulk of the cellular DNA, the chromosome, is concentrated in a diffuse body called the chromatin body which can be observed in living bacteria by phase contrast microscopy. Electron micrographs have shown that the chromosomal DNA within this chromatin body is attached to an invaginated part of the cell membrane (figure 3.1) and it is beHeved that this membrane plays an important part in the regular segregation of the two daughter chromosomes whenever the cell divides, achieving the same purpose as the mitotic spindle in higher organisms. membrane Figure 3.1 The bacterial chromosome is attached to the cell membrane. A non-dividing cell (1) has a single chromosome attached to an invaginated part of the cell membrane. When the chromosome replicates (2) the membrane grows between the separate points of attachment of the daughter chromosomes (3), ensuring that one goes into each daughter cell. Eventually, two daughter bacteria are produced (4) each with a single daughter chromosome attached to its own membrane. Autoradiography Initially, the evidence from electron microscopy suggested that all DNA molecules were linear and had two free ends, but as more refined methods were developed for extracting DNA from cells, it became apparent that many DNA molecules, including the chromosome of E coli, were circular. The first convincing demonstration that the E. coli chromosome was circular was made by John Cairns in 1963, using an autoradiographic technique. With this technique cells oíE. coli are grown in a medium containing tritiated thymidine (that is, thymine labelled with radioactive ^ H instead of normal ^ H)