Sister chromatid exchange, unequal

Not all data favored stepwise linear deletion as the basis for switching. Obata et al. (1981) found that the DNA organization in a particular plasmacytoma could be explained by unequal sister chromatid exchange, but observations inconsistent with this possibility were reported by Wabl et al. (1985). Alt et al. (1982) observed that a subclone of a transformed pre-B cell line had switched from synthesis of a p chain to synthesis of a y chain, with retention of the same VH and JH as well as both copies of the gene these results suggested that the switch in heavy chain synthesis was a transcriptionally controlled event (see discussion below). 

Amplification of genes under selective conditions has been widely observed-for example, in development of pesticide-resistant forms of insects. Such amplified structures could arise either through recombination with unequal sister-chromatid exchange, schematized in Figure 25.41, or by a conservative transposition process. Later, homologous recombination within an amplified region can lead to excision of sequences containing one or more amplified sequences. In order to replicate autonomously, these excised sequences must have a centromere. Such elements probably represent the double-minute chromosomes. 

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