Frequencies of exchange were determined in C-bands of chromosomes 1, 9 and 16 in six normal males, and related to relative C-band area. Comparing these different chromosomes, more exchanges occurred on average in 9 than in 1 although their mean C-band sizes were similar. Chromosome 16 exchanges were fewer, both overall and relative to C-band area. Comparing the same chromosome between individuals, there was a positive correlation between relative frequency and band size in both 1-1 and 9-9 exchanges. No clear trend was observed for other exchange events. If homology is required for interchange, it cannot be dependent solely on overall C-band size. Perhaps certain DNA sequences, sensitive to mitomycin C damage, are located in part of each C-band, with less per unit area in chromosome 1 than in 9 and still less in chromosome 16. X- and U-type exchanges between chromosome 9s occurred in near equal frequencies in all individuals. If synapsis of specific, affected sequences is a pre-requisite for interchange, this observation suggests that the affected sequence in chromosome 9 is arranged in both orientations relative to the centromere.
In XY cells, exchanges in the Y C-band were rare (less than 0.8%). However, in XYY cells we observed 58 Y chromosomes involved in exchange out of 208 (28%). Most were Y-Y interchanges. Therefore mitomycin C does affect Y C-bands, but subsequent interchange requires the presence of a homologue. This may be the consequence of interphase association of the homologues, or by homologous interaction of an affected DNA sequence in the two Ys. If the latter, the proposed sequence cannot be localized to proximal or distal parts of the Y C-band, as exchange points were observed in both. Also, it may be oriented in one direction in the Y, as the Y-Y exchanges which could be unambiguously classified were X-type events.
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