Transcription and RNA Processing

ID #2239

During transcription of DNA to RNA, why does the sigma subunit of the holoenzyme break off once the RNA strand starts to be produced?

This is beyond the scope of MCB 150, but there are different types of sigma factors in organisms like E. coli for various conditions. Since the rest of the polymerase has the same function regardless of the sigma factor, having "exchangeable" parts which can be detached/attached is an advantage. 

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