In molecular biology, upstream and downstream both refer to a relative position in DNA or RNA. Each strand of DNA or RNA has a 5' end and a 3' end, so named for the carbons on the deoxyribose (or ribose) ring. Relative to the position on the strand, downstream is the region towards the 3' end of the strand. Since DNA strands run in opposite directions, downstream on one strand is upstream on the other strand.
Examples
Transcription and translation of DNA and mRNA, respectively, have their direction defined by the newly synthesized strand, that is, in downstream direction (5' --> 3'). However, this is in upstream direction (3' --> 5') for the copied template strand.
See also
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