Tris, or tris(hydroxymethyl) aminomethane, is a common biological buffer, used throughout the DNA extraction process. During extraction from any number of sources, DNA is pH sensitive. During cell lysis, removal of unwanted cellular components and precipitation, tris is used to maintain a stable pH.

Application

The quick answer is that tris is a basic buffer, whereas tris HCl is the acidic buffer. Keep in mind, buffers are used to resist changes to pH. Even small concentrations of a strong acid or base, without a buffer, could significantly change environmental pH.