The marked advantage of the Nicholson models is clear in the construction of nucleic acid models. Details of internal structure such as the all important twist and tilt of the base- pairs which is frequently ignored or impossible in other models, is easily reproduced by allowing rotation of the H-bonds and of the O and NH2 units relative to the purine or pyrimidine ring (see Cat. No.421 K ) .The H-bonds used for nucleic acids are 0.093" diameter stainless steel and therefore rigid. Different colours for each of the four bases makes the base pairs easily identifiable so that in DNA for example, the genetic significance is easily explained. Red oxygen and blue nitrogen (except ring nitrogen) show the H-bonding mechdnism clearly, whilst the sugar phosphate backbone in red and white clearly emphasises and contrasts its polar nature over the pastel coloured base pairs in the hydrophobic interior .

The same units moulded in different colours serves as the nucleus for all three pyrimidine bases accurately based on the dimensions of the cytosine ring.

Where complementary bases are to be hydrogen bonded in the Watson Crick fashion, e.g. CG, AT, AU, these are best ordered as the base pairs. The bases are linked through stainless steel inserts supplied between the two NI atoms. A second unit consisting of an H-bond between a red keto O and a blue NH2 unit to be inserted in the two C6 carbons is supplied. In the case of the GC base pair, a similar bond to the latter is inserted in the two C2 carbons. All H-bonds are capable of being removed and reinserted in their sockets but force is required. For Hoogsteen pairs individual units may be ordered. Purine units can be drilled at N7 to receive an H-bond pin.

The correct twist angle (rotation of the plane one base relative to the other about an axis approximately parallel to the N1-N1 line) is easily established with the aid of a, protractor.

The sugars are made out of tetrahedral carbon units utilising the double socket carbon and narrow arm oxygen so that the completed ring has a free socket. The four conformations of the furanose ring e.g. C2 endo in B and C DNA and C3 endo in A DNA and RNA, which have such important consequences for secondary structure, can easily be reproduced and inter-converted. The phosphorus and oxygen atoms can be added and their conformation for accurate work may be set by adjusting the torsional angles with the keyed protractor. This enables the backbone conformation of short lengths to be conveniently and reasonably accurately established. These short lengths can be joined together after addition of the bases. Alternatively since many nucleic acids are symmetrical and hence repetitive in character, the bases may be fixed in position on a central support rod, the sugar ring in its correct conformation added, and the phosphate orientated to link successive sugar rings.

Construction time for 1 1/2 turn DNA should not exceed 8 hours.

Two DNA kits are available:-

410K 1 TURN DNA (11 Base pairs)

411K 1,/2 TURN DNA (15 Base pairs)

One turn of DNA has 10 base pairs, so that the eleventh pair is superimposed on the first. The one and a half turn of DNA contains sufficient units to construct a double helical model containing 30 deoxynucleotides (15 base pairs).

The models are approximately 25 cm. diameter and 34 cms. high per turn. They are supported by means of short lengths of aluminium rod attached horizontally by nylon T clamps to a central aluminium rod mounted in an afromosia baseboard. Alternatively, a small hole may be drilled at the top of the rod so that the structure may be suspended from the same vertical rod. They may be added to indefinitely by ordering additional base pairs

The existence of the wide and shallow groove of the double helix is immediately apparent with the 1 1/2 turn DNA kit. The models may be extended by the use of the magnetic units so that H-bonding of the top few necleosides may be disrupted and pairing with incoming nucleoside triphosphates demonstrated, and the double helix reformed. Instructions for assembly are supplied.