Sequence-Detecting Nanoscale Sensor

Makarand writes "A nanoscale sensor made of a single molecule - just 20 nanometers long - capable of detecting a specific short sequence in a mix of DNA or RNA molecules has been created by physicists at UCLA. This nanoscale sensor could be used to detect the early stages of cancers for which genetic markers are well known or extremely minute traces of biological weapons. When a target molecule binds to the probe molecule in the sensor, the probe molecule changes shape and pulls on the sensor. The motion of the sensor is detected by an optical technique to measure conformational changes in the probe molecule at the nanometer scale."

Re:A step closer to immortality

[schlyne]

As far as I know, cancerous cells just never stop growing. The cell doesn't go through the interphase step of Mitosis. They just reproduce continuously (with no delay, unlike normal cells).

I wonder how well they could isolate the sequences between a cell dividing normally and a cancerous one.

How long does it take?

[Cruel Angel]

I didn't see anything that indicates how long it take to bind to the appropriate sequence. Or how many false positives or compelete misses there are. (though I would guess false positives are very low).

It's a good start, but clearly there's a long way to go before it is more than just a 'lab' tool.

Re:How long does it take?

[Bowling Moses]

Or anything about sequence specificity. For example, can they make a probe bind only the sequence ATGCTGACCTT, or can they make a probe specific for only AxxCTGxCCTT? The concentration is important too--the higher the concentration of particles the easier (generally) it will be for something similar to bind and make a false positive.

Still, the ability to bind and report the binding of a single molecule of DNA of a (presumably) highly specific sequence is quite the accomplishment.

Re:How long does it take?

[Sgt York]

r can they make a probe specific for only AxxCTGxCCTT?

Yes. The probe would be either a cocktail of all the permutations of the listed sequence, or it could be AIICTGICCTT, where I is inosine. Inosine is a nitrogenous base derived from adenosine which can pair up with any of the four standard nucleotides.

First ...

[Anonymous Coward]

... you have to get the molecule to the sensor. I would think you would need a huge array of the nanoscale devices to cover a decent area. I didn't see mention of how to "unbind" the molecule so the device could be reused?

Might be useful for detecting contagious diseases

[Animats]

This could lead to walk-through portals that detect contagious diseases like SARS. Detection is now too slow for use in airports, but something like this could change all that.

Distributed computing to the rescue?

[KingArthur10]

There are a million ways molecule can attach to another depending on a number of variables such as placement of electrons and atoms in the molecule. I'm thinking, with all those possibilities, it would seem that the best way to simulate all those possibilities and pick out which molecules to use to bind with certain parts of DNA would be to develop a distributed computing project such as folding and Seti. I don't entirely understand HOW they are able to detect such deformities in the DNA with a single molecule, but given they can develop a method to accurately sense them, I'd imagine that it would take a heck of a lot of computing power to match the deformity up with a molecule. Just my tired two cents worth.

dependent upon DNA hybridization

[Bowling Moses]

I just skimmed the article late this evening (early this morning? Whatever.). Anyway, it looked like what they'd done was to attach a single-stranded DNA sequence at one end to a slide, the other end is attached to a 1-micrometer diameter bead. Charge repulsion between the bead and the slide stretches the DNA strand, keeping it under tension. DNA with various sequences then can be introduced into the system, if they match the opposite strand of the fixed DNA strand well, then it will hybridize forming a double stranded DNA. Double stranded DNA forms a double helix structure which is more "fixed" structure than single stranded DNA, which can range from nearly linear to a random coil depending in part in the amount of tension its under and the sequence. Regardless, if there is a hit then the distance between the bead and the slide will change as the DNA is hybridized into a double strand, forming the double helix that we've all seen in biology textbooks. One problem is that multiple different DNA strands can hybridize nearly as tightly as an exact match, for example if we have the sequence 5' ACTGACTGACTG 3' then 5' CAGTCAGTCAGT 3' will bing to it, but so will 5' CAGTCAATCAGT 3', which differs by only one position. I hope I did that right, it's late, but anyway you can still get hybridization of DNA molecules that are only very similar but are not quite identical. This study used DNA strands 10's of nucleotides long so being off by one or even a couple of positions will still result in tight binding, although this can be tweaked a bit by messing with the DNA concentration; lower concentrations will favor more exact matches in general. But still, cool idea.

Re:dependent upon DNA hybridization

[sam_handelman]

Lower concentrations will also favor no match whatsoever, in general.

What matters is the relative concentration of the target DNA sequence, and of all other remotely similar sequences. Roughly speaking, for each base pair that is different between your ideal target and your actual target, you get a difference of a few kcal/mole in binding energy.

Every 1.36 kcal/mole (roughly) corresponds to a ten-fold decrease in binding affinity.

So, roughly speaking, a single-nucleotide mis-match is going to have 1/1000 times the binding affinity of a perfect match to the probe. This means, that under IDEAL circumstances, you can detect your target against a background of 1000-times its own concentration in single-base substitutions. Of course, under circumstances where your probe is long enough that random DNA will tend to bind indiscriminately, this won't work.

Contamination with single-stranded binding proteins, which do exist, might also be a confounding factor, either giving you a false positive or fouling up your probe.

Anyway, this may or may not be good enough for any particular application. I suspect that this technique will never actually be as sensitive as PCR, wherein the binding-affinity experiment is effectively "repeated" each replication cycle. If you choose a sequence carefully enough - and use a longer probe so that close matches are not so likely to appear at random (a ten nucleotide probe appears one in every 2^20 ~= 1/billion times, at random. The human genome is likely to include one instance of every decamer,) you might get performance good enough for the applications they describe.

Re:dependent upon DNA hybridization

[Sgt York]

I suspect that this technique will never actually be as sensitive as PCR

I disagree. While PCR can detect single copies of sequences in theory, it rarely (if ever) does so in common practice. There are tweaks you can do that will help amplify the system, and use of specialized detection equipment can get it down pretty low, but in my experience, anything less than ~100 copies of target is unreliably detectable, at best.

As for targets, a decamer is good in theory, but in practice you run into several problem

Re:dependent upon DNA hybridization

[Sgt York]

Also, according to the paper in PNAS, it would seem that a SNP(*) would alter the perturbation of the bead differently than a 100% match. You wouldn't get a pure helix, so the stretch/shrink that results from the hybridization would differ if there was a SNP or two. I don't know the physics well enough to say if that difference would be detectable by the insturmentation they describe, but if they can detect the kind of deflection they discuss in the paper, I imagine it wouldn't be too dificult.

*SNP - Si

A leap forward

[Sgt York]

So, who's buying this tech? This could easily replace many of the current tools used to analyze gene expression at the bench. It may be years or decades brfore there is actual treatment based on this tech, but it may be used at the bench in the next few years. People (like my lab, for instance) spend huge ammounts of money to assay changes in RNA transcription under certain circumstances. If this could be measured in real time...hell, even if it could be measured quickly.

For those in the field, imagine being able to assay the ammount of your transcript of interest in an RNA sample as easily as you are able to measure total RNA. Pop a cuvette in a specialized spec and get a reading? You could have your answer in seconds as opposed to hours. Granted, the tech is not at that point yet, but it could easily get there in a few years.

Again I ask....what company is buying this? I want stock in them NOW.

Biochip

[xluap]

How about a chip with thousands of those sensors? It could detect all known hereditary illnesses.

However, would you want to know?