Biology Reference
In-Depth Information
208
essential parts: duality and helicity. Duality was also suggested by
Chargaff's rule of [A] = [T] and [G] = [C]. Many molecular biological
processes, e.g. DNA-DNA hybridization, semi-conservative replication,
transcription, reverse transcription, DNA-RNA hybridization, etc., depend
on this duality. On the other hand, helicity has resulted in numerous
topological problems, as well as problems of energy requirement for
unwinding and re-winding during DNA semi-conservative replication. One
of the physicists turning molecular biologist once commented that in the
highly viscous medium of the cell nucleus, our long DNA molecule of about
3.2 billion base-pairs had to unwind and re-wind during one cell cycles. The
amount of energy lost due to viscous drag and then dissipated as heat would
be very large.
To overcome the unwinding and re-winding problems, biochemists and
molecular biologists have isolated numerous enzymes known as gyrases,
helicases, topo-isomerases, etc. The scientific literature is filled with papers
explaining how the problem of helicity can be resolved. Some of the topo-
isomerases have been considered as “magicians' magician”, since they can
cut one of the DNA backbone strands, move it over to the other side of the
other strand, and ligate the cut strand back without leaving a trace. This
process will reduce the double helix by one turn. It is a fascinating story to
read these articles. Many of the three-dimensional structures of these
enzymes have been determined by X-ray crystallography.
Helicity causes another problem. Many of the biological active DNA
molecules, e.g. that of plasmids,
E. coli
, etc., are double stranded intact
circles. How can these molecules unwind and re-wind? Are cutting and
pasting absolutely required? Why should evolution give rise to such DNA
molecules with so many difficulties for them to replicate? Or, does
evolution actually provide a vital advantage to such molecules? This has
been an interesting problem to molecular biologists since 1953 when the
DNA double helix was proposed.
All of such findings strongly suggest that we may need to look for another
possible secondary structure of DNA. Interestingly enough, that structure
was originally suggested by Wilkins, Stokes and Wilson in 1953 as:
“two similar coaxial helices of twice the above size and relatively displaced
along the axis equal to half of the pitch”.
