What is the cofactor that is required for the T4 DNA ligase

What is the cofactor that is required for the T4 DNA ligase catalyzed reaction? Why do we carry out ligation reaction at 15 degree C but not 37 degree C? Why do we analyze the digested DNA samples (pUC 19 and GFPuv PCR product) before performing ligation?

Solution

(1) ATP is an essential cofactor for the reaction catalysed by T4 DNA ligase.

(2) The sticky ends of DNA are often too short to form stable duplex at 37C, hence low temperatures are considered for ligation reaction. Preferably 4C