Only one side of the DNA ladder is copied (the sense side). The sense side starts with a 3’ end. This means the corresponding mRNA will have to assemble starting from the 5’ end. This is my initial thought, but can someone expand on it? Also, is this explained by why replication is performed in the 5' to 3' direction as suggested by this thread: Why is DNA replication performed in the 5' to 3' direction?