|
The
genes in your cells are made up of deoxyribonucleic acid,
or DNA: a long, stringy chemical made by fastening together
a bunch of small chemical bits like railroad cars in a freight
train. The DNA in your cells is actually two of these strings,
running side by side. Some of the small chemical bits (called
nucleotides) like to stick to certain other bits on the opposite
string. DNA has a rather boring structure, but the stickiness
of the nucleotides can be used to make far more interesting
shapes. In fact, there's a whole field of nanotechnology investigating
this, and it may even lead to an early version of molecular
manufacturing.
Take
a bunch of large wooden beads, some string, some magnets,
and some
small patches of hook-and-loop fastener (called velcro when
the lawyers aren't watching). Divide the beads into four piles.
In the first pile, attach a patch of hooks to each bead. In
the second pile, attach a patch of loops. In the third pile,
attach a magnet to each bead with the north end facing out.
And in the fourth pile, attach a magnet with the south end
exposed. Now string together with a random sequence of
beads--for example
,
1) Hook, Loop, South, Loop, North, North, Hook.If you wanted
to make another sequence stick to it, the best pattern
would be:
2) Loop, Hook, North, Hook, South, South, Loop. Any other
sequence wouldn't stick as well: a pattern of:
3) North, North, North, South, North, Loop, South would
stick to either of the other strands in only two places.
Make
a few dozen strings of each sequence. Now throw them all in
a washing machine and turn it on. Wait a few minutes, and
you should see that strings 1) and 2) are sticking together,
while string 3) doesn't stick to anything. (No, I haven't
tried this; but I suspect it would make a great science fair
project!)
But
we can do more than make the strings stick to each other:
we can
make them fold back on themselves. Make a string of:
N, N, N, L, L, L, L, H, H, H, H, S, S, S and throw it in the
washer on permanent press, and it should double over.
With
a more complex pattern, you could make a cross:NNNN, LLLLHHHH,
LNLNSHSH, SSLLNNHH, SSSS The NNNN and SSSS join, and each
sequence between the commas doubles over. You get the idea:
you can make a lot of different things match up by selecting
a sequence from just four letter choices. Accidental
matches of one or two don't matter, because the agitation
of the water will pull them apart again. But if enough of
them line up, they'll usually stay stuck.
Just
like the beads, there are four different kinds of nucleotides
in the chain or strand of DNA. Instead of North, South, Hook,
and Loop, the nucleotide chemicals are called Adenine, Thiamine,
Guanine, and Cytosine, abbreviated A, T, G, and C. Like the
beads, A will only stick to T, and G will only stick to C.
(You may recognize these letters from the movie GATTACA.)
We have machines that can make DNA strands in any desired
sequence.
If
you tell the machine to make sequences of ACGATCTCGATC andTGCTAGAGCTAG,
and then mix them together in water with a little salt, they
will pair up. If you make one strand of ACGATCTCGATCGATCGAGATCGT--the
first, plus the second backward--it will double over and stick
to itself. And so on. (At the molecular scale, things naturally
vibrate and bump into each other all the time; you
don't need to throw them in a washing machine to mix them
up.)
Chemists
have created a huge menu of chemical tricks to play with DNA.
They can make one batch of DNA, then make one end of it stick
to plastic beads or surfaces. They can attach other molecules
or nanoparticles to either end of a strand. They can cut a
strand at the location of a certain sequence pattern. They
can stir in other DNA sequences in any order they like, letting
them attach to the strands. They can attach
additional chemicals to each nucleotide, making the DNA chain
stiffer and stronger.
A
DNA strand that binds to another but has an end hanging loose
can be peeled away by a matching strand. This is enough to
build molecular tweezers that open and close. We can watch
them work by attaching molecules to the ends that only fluoresce
(glow under UV light) when they're close together. A motor
that goes around in circles in either direction, in controllable
steps, depending on which strands are mixed in next, has also
been built.
(The first URL has a neat animation of how the tweezers work.)
http://news.bbc.co.uk/1/hi/sci/tech/873097.stm
http://www2.nano.physik.uni-muenchen.de/publikationen/Preprints/p-02-10_Simmel_ENN.pdf
Remember
that DNA strands can bind to themselves as well as to each
other. And you can make several strands with many different
sticky sequence patches to make very complex shapes. Just
a few months ago, a very clever team managed to build an octahedron
out of only one long strand and five short ones. The whole
thing is only 22 nanometers wide--about the distance your
fingernails grow in half a minute.
http://www.nanotechweb.org/articles/news/3/2/5/1
So
far, this article has been a review of fact. This next part
is speculation. If we can build a pre-designed structure,
and make it move as we want, we can--in theory, and with enough
engineering work--build a molecular robot. The robot would
not be very strong, or very fast, and certainly not very big.
But it might be able to direct the fabrication of other, more
complex devices--things too complex to be built by pure
self-assembly. And there's one good thing about working with
molecules:
because they are so small, you can make trillions of them
for the price of one. That means that whatever they do can
be done by the trillions--perhaps even fast enough to be useful
for manufacturing large products such as computer chips. The
products would be repetitive, but evenrepetitive chips can
be quite valuable for some applications.
Individual control of adjacent robots would allow even more
complex systems to be built. And with a molecular-scale DNA
robot, it might be possible to guide the fabrication of smaller
and stiffer structures, leading eventually to direct mechanical
control of chemistry--the ultimate goal of molecular manufacturing.
This
has barely scratched the surface of what's being done with
DNA engineering. There's also RNA (ribonucleic acid) and PNA
(peptide nucleic acid) engineering, and the use of RNA as
an enzyme- or antibody-like molecular gripper. Not to mention
the recent discovery of RNA interference which has medical
and research uses: it can fool a cell into stopping the production
of an unwanted protein, by making it think
that that protein's genes came from a virus.
Nucleic
acid engineering looks like a good possibility for building
a primitive variety of nanorobotics. Such products would be
significantly less strong than products built of diamondoid,
but are still likely to be useful for a variety of applications.
If this technology is developed before diamondoid nanotech,
it may provide a gentler
introduction to the power of molecular manufacturing.
|
