File talk:MTHFR metabolism.svg

Arrow directions?
I'm glad to see the MTHFR page getting a fancy diagram, but I don't understand the arrows. Why do DNA and RNA synthesis only have inbound arrows? Surely they are the source of something? And if I assume all the arrows are backwards then that means THF has no origin? And what is the gray line between DHF and THF?

Crag (talk) 10:40, 1 November 2014 (UTC)