In this article we will discuss about the Nucleus-Cytoplasm Interaction with Hammerling’s Experiment.
The cytoplasm is the centre of metabolic activities in the cell and the nucleus is the ultimate controlling centre. The cytoplasm provides the suitable environment in which the cytoplasmic as well as nuclear genes operate. Many enzymes operate through the cytoplasm and particularly through certain living cytoplasmic structures. Hence it can be expected that the cytoplasm exerts an influence on genie action.
In fact, the cytoplasm contains several hereditary units which are termed plasmogenes or cytogenes. These units which contain their own genetic material are believed to be self-duplicating and are capable of determining hereditary characters. The totality of hereditary factors transmitted through cytoplasm is called plasmone.
The cytoplasmic inheritance is not subject to the same general rules as nuclear inheritance (i.e., it does not follow Mendelian rules), but in some cases it seems to be of importance.
The control a nucleus exercises over the cytoplasm is easily demonstrable. If the nucleus of a cell is removed, the cytoplasm may survive for some time but normal function soon ceases. The mammalian red blood cell (RBC) is enucleated in its terminal stage of differentiation and as such it is extremely short-lived.
The Hammerling’s experiments involving interspecific nuclear transplant in the marine green alga acetabularia can be cited as an excellent example to show functional dependence of the cytoplasm upon the nucleus. The genus acetabularia is unicellular alga which consists of basal rhizoids, a long stalk and a cap.
The nucleus is confined to the rhizoidal region. The cap characteristics are of great significance in the species identification. Hammerling took two species of acetabularia, namely (1) A. mediterranea (2) A. cranulata.
In A. mediterranea, the cap has an average of 81 rays with rounded tips (Fig. 18.1).
In A. cranulata, average number of rays per cap is 30 and the tips of the rays are pointed.
Hammerling conducted the following interspecific grafting experiment:
1. He took immature plants (in which the cap has not developed) of each species and grafted them together by cutting the tops of the stalks and bringing the cut surfaces into contact. A new stalk soon developed at the point of grafting and this gave rise to a cap which was intermediate in appearance between the caps of the parents.
A number of such caps had an average of 42 rays each and hence approached A. cranulata but the tips of the rays were usually rounded like those in A. mediterranea. The intermediate character of this graft hybrid is to be expected because it contains two nuclei, one from each species.
In control experiments, two plants of A. mediterranea were grafted together which produced caps resembling the normal caps of species. Similar results were obtained withv4. cranulata.
2. If rhizoids of A. mediterranea (nucleated part) were grafted with the decapitated rhizoidless stalk (enucleated part) of A. cranulata and vice versa, the caps showed characteristics of the species from which the nucleus was derived. Such grafts contained only one nucleus.
The type of cap was found to depend on the length of the stalk grafted. Sometimes, the caps were found to be intermediate in appearance if the enucleate stalk was 1 to 2 cm in length, whereas if the piece of the grafted stalk was about .5 cm. long or less the caps usually resembled that of rhizoid parent.
Since the rhizoids represented only a very small proportion of the total cytoplasm, the major changes in the Sanction of the cytoplasm had to be based upon the mode of control exercised by the transplanted nucleus. Thus for the question at hand the answer must be that neither the nucleus nor the cytoplasm exercises exclusive control over the other.