Meaning of 5’–3’ in relation to ORFs
I assume you are referring to the &quot;typical 5’—3’ order of appearance&quot;.
The correct way to pronounce these is with the word &quot;prime&quot;, that is, &quot;Five prime to three prime&quot;. These are not units but refer to the directionality of RNA/DNA and the numbers five and three refer to specific carbon atoms arranged in the sugar molecules that make up the RNA/DNA backbone.
RNA/DNA are typically &quot;read&quot; (and always written) from the 5' to the 3' end of the molecule by molecular machinery in the cell, so that's usually the order we read the sequences as well. &quot;5' to 3'&quot; in molecular biology effectively means &quot;beginning to end&quot;.