Classical theory asserts that radiation-induced chromosomal exchanges result from the interaction of lesions on both chromosomes involved, a notion supported by substantial indirect evidence, but more recently questioned on biophysical and molecular grounds. When mitotic HeLa cells were irradiated with60 Co γ rays, and fused together with Sendai virus, numerous chromosome exchanges were observed between the genomes of different cells at the next mitosis. However, when irradiated and unirradiated cells were fused together, the frequency of intergenomic exchange was 40-fold lower, suggesting that the vast majority of radiation-induced exchanges do, in fact, require damage to both chromosomes.

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