7.2: Locating hereditary material within the cell
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Further evidence suggesting that hereditary information was probably localized in the nucleus emerged from transplantation experiments carried out in the 1930’s by Joachim Hammerling using the giant unicellular green alga Acetabularia, known as the mermaid's wineglass. Hammerling’s experiments (video: http://youtu.be/tl5KkUnH6y0) illustrate two important themes in the biological sciences. The idiosyncrasies of specific organisms can be exploited to carry out useful studies that are simply impossible to perform elsewhere. At the same time, the underlying evolutionary homology of organisms makes it possible to draw broadly relevant conclusions from such studies. In this case, Hammerling exploited three unique features of Acetabularia. The first is the fact that each individual is a single cell, with a single nucleus. Through microdissection, it is possible to isolate nuclear and anucleate (not containing a nucleus) regions of the organism. Second, these cells are very large (1 to 10 cm in height), which makes it possible to carry out various microsurgical operations. You can remove and transplant regions of one organism (cell) to another. Finally, different species of Acetabularia have mophologically distinct “caps” that regrow faithfully following amputation. In his experiments, he removed the head and stalk regions from one individual, leaving a “holdfast” region that was much smaller but, importantly, contained the nucleus. He then transplanted large regions of anuclear stalk derived from an organism of another species, with a distinctively different cap morphology, onto the nucleus-containing holdfast region. When the cap regrew it had the morphology characteristic of the species that provided the nucleus - no matter that this region was much smaller than the transplanted (anucleate) stalk region. The conclusion was that the information needed to determine the cap’s morphology was located within the region of the cell that contained the nucleus, rather than dispersed throughout the cytoplasm. Its just a short step from these experimental results to the conjecture that all genetic information is located within the nucleus.
Contributors and Attributions
Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.