Statistical and image analysis of sister chromatid exchange in maize

摘要

Sister chromatid exchanges occur among the DNA strands of each chromatid during replication and can be detected at cytological level with adequate preparations of metaphase chromosomes. This type of exchange apparently does not have genetic consequences and it has been observed that mutagenic agents increase the number of SCE (Latt 1979).SCE was discovered by McClintock (1938) and visualized in the classic experiments by Taylor et al. (1957), who demonstrated the semi-conservative pattern of DNA duplication. There was a remarkable advance in SCE studies in the 1970s, when the use of thymidine analog BrdU showed color differentiation between sister chromatids (Schvartzman and Cortés 1977).When the cell passes through the first replication cycle in the presence of BrdU, the chromosomes show chromatids containing one strand of DNA with incorporated BrdU (TB chromatids). After the second consecutive replication cycle, one chromatid shows BrdU incorporated in only one DNA strand and the sister chromatid shows BrdU incorporated in both DNA strands (BB-chromatids). The BB-regions of chromatids stain even less than the TB-regions, hence allowing their differentiation (Perry and Wolff 1974; Latt 1977).After the establishment of SCE protocol in plants, one main interest was to compare these events in different species. Considering the chromosome number and genome size differences it has been suggested that the SCE frequency in plants should be expressed per DNA content (Schvartzman 1987).In the present study with maize, the FPG technique, image analysis and statistical methods were used to assess the SCE frequency at the breaking points per chromosome arm, per centromeric region and expressed per micrometer and per DNA picogram.