$6\mathrm{.}14$ A $3\mathrm{.}1-$kilobase linear fragment of DNA was digested with PstI and produced a $2\mathrm{.}0-kb$ fragment and a $1\mathrm{.}1-kb$ fragment. When the same $3\mathrm{.}1-kb$ fragment was cut with HindIII, it yielded a $1\mathrm{.}5-$kb fragment, a $1\mathrm{.}3-$kb fragment, and a $0\mathrm{.}3-kb$ fragment. When the $3\mathrm{.}1-kb$ molecule was cut with a