When the cells are broken apart, their DNA and contents spill into the buffer. Additionally, RNase A (destroys RNA), proteases (destroys proteins), and SDS (sodium dodecyl sulfate, solubilizes the membrane fragments) are often included. Taken together, this soup of cellular contents and fragmented RNA and proteins can have a big impact on the pH of the solution. Because DNA is pH sensitive, it is important for tris to buffer the soup and maintain the pH at a steady point.
DNA Percipitation