Because Milli-Q water is water that has been purified to remove extra ions and othr stuff that are found in water and might interfere with experiments. It is actually Triple distilled , so ULTRA PURE..
We generally add milliQ (DNase free) water. For digestion water is added usually. It provides right Hydrogen bonds between enzyme amino acid residues and bases present in the restriction sites. Utpal Roy
preparation of 500 ml 1x TAE buffer,50ml of 10x buffer add to 450ml DI.water. to calculate just use MV=MV so 500mL * 1x= 10x * V then solve for V. add the amount of DI water you need to get the volume you calculated above.