AbstractsBiology & Animal Science

A quantitative analysis of sister chromatid exchanges and labelling patterns in H 3 thymidine labelled second division (post isotope incorporation) chromosomes of Vicia faba has demonstrated that: (1) The frequency of sister chromatid exchanges per chromosome is proportional to chromosome length. (2) The time of availability of colchicine may have a slight effect on the prospects of sister chromatid exchange events. However, overall, colchicine has no significant effect on the frequency of sister chromatid exchanges; on the incidence of isolabelling or on the type of iso-labelling (3) The distribution of sister chromatid exchanges per chromosome follows a Poissonian form. The prospects of sister chromatid exchange events at either the first or second interphases are equal and independent. (4) The ratio of single to twin sister chromatid exchange is consistent with the expectation that; sister chromatid exchanges occur independently of the tritium dose, and that chromosomal sub-units rejoin non-randomly, i.e. they exhibit polarity and are dissimilar in an analagous manner to the DNA double helix. (5) A high frequency of chromosomes show isolabelling, the probability of which increases with chromosome length and/or exchange frequency. (6) Twin and single isolabelling segments have been noted, however the ratios obtained do not fit those for sister chromatid exchanges and the origin of isolabelling remains unexplained. Mitotic cycle determinations using the labelled mitosis wave method and the double labelling method have been carried out for Vicia faba and Spironema fragrans root tips. The variability of such methods has been commented on.