B.Restriction Enzyme Analysis

The purified ct-DNA is digested with various restriction enzymes，as directed by the supplier.It is known that ct-DNA contains a variable amount of restriction sites for different enzymes ranging from 10—40［6］.These subsets of fragments can be conveniently separated from one another according to size by gel electrophoresis.

Since the large fragments migrate very slowly in the agarose gel，a low percentage gel should be used.It is a general guide that if there are less than 15 fragments generated from a higher plant chloroplast genome by a given restriction enzyme，then a 0.7%—0.8%gel is adequate.If the number of fragments are over 25，then a 1.0%—1.5%gel is suitable.The intensity of some bands are higher than others because of the existence of multiple copies having similar or identical sizes.The exact number of copies in such bands can be estimated from densitometer tracings.(https://www.daowen.com)