The Sanger dideoxy method for determining DNA sequence is limited in that a stretch of only 1000 or fewer bases can be analyzed in one reaction. Suppose you wish to sequence a newly isolated double-strand DNA tumor virus that contains 5000 base pairs. You decide to use the Sanger method on restriction fragments of the DNA for sequencing. You use an enzyme that makes a significant number of cuts to give, on average, fragments of 300 nucleotides or less.
Why might it be a good idea also to sequence a second set of fragments cleaved by another restriction enzyme?