After reading this question, I realized that the time Madara acquired Izanagi, was the same time he got the DNA of Senju Clan from Hashirama. However, after an extensive review on their fight (Episode 626), I can't see where and when Madara bit Hashirama. It said in the wiki that:
"...To use Izanagi to its full potential, users must also have the genetic traits of the Senju, who are also descended from the Sage.."
Now, the question is when and where did Madara bite Hashirama? How come Madara activated Izanagi (I assume on its full potential) just right after he acquired the flesh/blood from Senju?