diff --git a/mrdna/segmentmodel.py b/mrdna/segmentmodel.py
index fd78e543c6778d06a440c9949a1af7abff7d0eee..650e5a53ac7a1669fada692f35485aa03f9c781a 100644
--- a/mrdna/segmentmodel.py
+++ b/mrdna/segmentmodel.py
@@ -2835,3 +2835,16 @@ proc calcforces {} {
             ## material
             tclFile.write("graphics top materials on \n")
             tclFile.write("graphics top material AOEdgy \n")
+            
+            ## iterate through the model segments
+            for s in self.segments:
+                if isinstance(s,DoubleStrandedSegment):
+                    print("## dsDNA! \n")
+                    draw_cylinder(s,10,"cyan")
+                elif isinstance(s,SingleStrandedSegment):
+                    print("## ssDNA! \n")
+                    draw_cylinder(s,3,"orange")
+                else:
+                    raise Exception ("your model includes beads that are neither ssDNA nor dsDNA")
+            ## tclFile complete
+            tclFile.close()