Chromosomal Ultraviolet light

In contrast to the results with SAB, when cells were exposed to the monofunctional alkylating agent, N-methyl-N -nitro-N-nitrosoguanidine (MNNG), SCE frequency remained low in those cells exposed in mid to late S phase and was highest in cells exposed in Gi or early S phase. Thus, as has been shown by others with ultraviolet light or 8-methoxypsoralen-induced DNA lesions (12, IS), MNNG exposure resulted in lesions that lead to SCE formation only when the lesions were produced prior to the replication of the chromosome, i.e., ahead of the repUcation foik. 

Although they absorb ultraviolet light, the nucleic acids of the nucleoli are not stained with the Feulgen reagent and disappear after treatment with ribonuclease. Small amounts of ribonucleic acid are also associated with the chromosome and, finally, after chromosomal nucleic acid is digested, the presence of the basic proteins can be revealed by adequate staining methods. 

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