What does "upstream" refer to in terms of DNA strands?

The term "upstream" in the context of DNA strands refers specifically to the direction toward the 5' end of the DNA molecule. In molecular genetics, this terminology is often used to describe regions of the DNA that are located before the transcription start site of a gene. When discussing transcription, the 5' region of the DNA strand is upstream of the coding sequence that will be transcribed into RNA.

This directionality is critical in understanding how genes are regulated and expressed. Promoter regions, which are crucial for initiating transcription, are typically located upstream of the coding region of a gene. As such, when identifying elements involved in gene regulation such as enhancers or silencers, these features are also described as being upstream when they occur before the transcription start site in the direction towards the 5' end.

In summary, the correct association of "upstream" with the 5' end of the DNA strand underscores the importance of orientation in genetic regulation and gene expression processes.