Not sure if this has been posted before, but it's a great write up and explanation on B12 and folate deficiencies. Perhaps helpful if you are trying to understand the methylation cycle.
http://acb.sagepub.com/content/early/2013/05/04/0004563212473279.full.pdf
http://acb.sagepub.com/content/early/2013/05/04/0004563212473279.full.pdf