Screening and ordering cloned fragments requires very large DNA fragments over 100 kb in size.

While conventional electrophoresis can resolve DNA fragments up to 20,000 nucleotide bases in size, very large DNA fragments are fragile, so they must be separated by more refined techniques, such as:

1.  Pulsed-field electrophoresis (PFE)- fragments are separated between several pairs
     of opposing electrodes.  Pulsing causes DNA to change orientation in the gel

2.  Orthagonal-field-alternation gel electrophoresis (OFAGE)- also called
     field-inversion electrophoresis, this technique utilizes a long forward currect,
     alternated with a short reverse current.  OFAGE causes orientation changes
     which allows DNA to migrate on a size-dependent basis.

Without alternation of current, DNA molecules move through a gel in a size-independent manner, alligning parallel with the current and moving end-on through the agarose or polyacrylamide matrix (reptation).