There are a couple of them :
P.O. Box 299
Wenham, MA 01984
" you can create models of DS and SS DNA and RNA up to 1200
atoms starting with a character string listing of the bases"
Mac version $199
This program will generate coordinate output files for further
use with programs that use cartesian or z-matrix coordinates.
" you can build-up a structure in 3 dimensions, using z-matrix
(angles & distances) or cartesian coordinates. Once you have your
structure, you can do "what ifs" by changing atoms, groups, etc"
Mac Version $249
Molecular Applications Group
880 Lathrop Drive
Stanford, CA 94305
Once you have the molecular coordinates of a molecule, this program
will allow you to manipulate the image.
Runs on Mac
contact the Molecular Applications Group at
MAG at CELLBIO.STANFORD.EDU