General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

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

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    30 General cytogenetics §1.4 be brought to simultaneous division by adding a phytohaemagglu- tinin, usually prepared from certain varieties of kidney bean, Pha- seolus. This substance has been in long use for blood group ana¬ lysis where it intensifies the agglutination reaction. In the absence of haematocytes, it stimulates the leucocytes to simultaneous divi¬ sion, and consequently the cells reach metaphase almost at the same time. No such reactions are known in plants. Here growth substances like lAA, indole acetic acid, can stimulate cell divi¬ sion under certain conditions. Cytokinins (e.g. kinetine) have a somewhat stronger effect. Pronounced synchronization (also in animals) may sometimes be realized by carefully executed temper¬ ature fluctuations. More effective, under favourable conditions equalling the effect of phytohaemagglutinin on leucocytes, is 5-amino-uracil. It blocks DNA synthesis without preventing the preparatory stages. A large number of cells in a meristem can then be brought into the same stage, just preceding DNA synthesis. After the active substance has been thoroughly washed out, DNA synthesis is immediately resumed, and large numbers of cells pass through mitosis simultaneously. Rather soon, however, the cells are desynchronized again. The size of the chromosomes at metaphase may be altered by a factor of two by the combination of the proper concentrations of nitrogen and phosphorus in the culture medium. Quite a different category of factors are those that affect the functioning of the mitotic spindle. By blocking mitosis at meta¬ phase, they may have a synchronizing effect, but for the study of chromosomes their major function is spreading and shortening the chromosomes. Heat shock at least several minutes at 35-40°С for plants of temperate regions, somewhat higher for most animals, in¬ terferes with the proper functioning of the spindle. The prometa¬ phase and anaphase movements fail. After splitting of the centro¬ meres all chromosomes may be brought together in a single nu¬ cleus: this restitution nucleus has twice the original chromosome number. The chromosomes remain in metaphase for a longer time, may become short and straight because of continued condensation and are spread about the cell without clear equator. This would facilitate their study if their structure did not deteriorate as a