DNA molecules are flexible and occupy an effective volume pores in the gel matrix sieve the dna molecules according to this volume; large molecules migrate more slowly through the gel because they have a larger effective volume than do smaller dNas and thus have more difficulty passing through the interstices of the gel Different sizes are separated because they moved different distances through the gelDNA molecules are flexible and occupy an effective volume pores in the gel matrix sieve the DNA molecules according to this volume;large molecules migrate more slowly through the gel because they have a larger effective volume than do smaller DNAs,and thus have more difficulty passing through the interstices of the gel. Different sizes are separated because they moved different distances through the gel