When they are done, have each student swill two teaspoons of the saltwater solution in their mouths for 30 seconds. Make sure that students swish the solution around for the full 30 seconds. Students should cap the test tube and gently rock it on its side for 2-3 minutes. It is important that students are not too vigorous while mixing. DNA is an extremely long molecule.
Physical abuse can break it into smaller fragments, a process known as shearing.
Again, it is very important that the students take care in adding the ethanol so that the alcohol floats above the soapy solution already in the tube. Tell students to allow the tube to stand for one minute. Then, have them use a thin acrylic or glass rod to slowly move some of the ethanol into the soap layer. Have students twirl the rod to spool the DNA strands around it. If too much shearing has occurred, the DNA fragments may be too short to wind up, and they may form clumps instead.