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Asked by holmesrj04 to Richard, Amelia on 15 Jun 2011. This question was also asked by kabence1997.
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Richard Badge answered on 14 Jun 2011:
Hi holmesrj04
It certainly possible to exactly copy parts of someones DNA – we do it every day in my lab! The process uses a special enzyme called a DNA polymerase whose job is to take the building blocks of DNA (Chemicals labelled G, A , T and C) and using one of the two strands of a DNA spiral (the famous double helix) make a mirror image copy.
Because of the way DNA is built – the sequence of one strand tells you the sequence of the other we can make millions of copies of short pieces of DNA in a few hours – the technique is called the Polymerase Chain Reaction (or PCR for short), and is the basis for all those CSI stories where they get enough DNA to identify somebody from a smear on a glass…
It is also possible to copy most of the DNA from a person using a similar technique called Whole Genome Amplification (WGA – scientists love acronyms!), but only as a collection of short bits, and it does not work reliably for small numbers of cells…
We sometimes use WGA on our chimpanzee DNA samples to make more of a particularly precious sample…
Richard
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Amelia Markey answered on 15 Jun 2011:
Hi well Richard is totally right and it’s whole genome amplification or WGA that my device does. I use both PCR and WGA in my research but my device only does WGA.
With my work what I hope to achieve is to be able to load some cells onto my device and put them in really tiny droplets (nano sized). In these droplets I will break the cells open to let the DNA out. Then I can add the ingredients for WGA, this will then copy the DNA I have in the droplet then I can store the DNA on the device.
Like Richard said people use WGA to copy the DNA of precious samples they have, this is usually when they can only collect a few cells or a small amount of DNA. Good examples are in forensics where you might only have a single hair or blood spot or in cancer diagnosis where you may only have a few cancer cells in your sample from the patient.
Comments
Sue commented on :
Hello holmesrj04. I’ve snuck over here from the Forensic Science Zone, because this was a very interesting question.
In forensic science we regularly copy parts of people’s DNA using the DNA polymerase enzyme that Richard mentioned, in a chemical reaction called PCR (polymerase chain reaction.) We copy 10 different parts of the DNA called STRs (short tandem repeats.) Like the name suggests, STRs are areas of the DNA that have a repeating pattern. What’s interesting about that for forensic purposes is that we know that the number of repeats that everyone has in their STRs varies, so we can use that information to try to identify someone. It’s very unlikely that anyone will have the same number of repeats at each of their STRs than anyone else, apart from identical twins.
The copying part of the process is only really so that we’ve made enough DNA from that sample to be able to measure the length of the STRs. It only works if there’s enough DNA in a sample to begin with. So, we might start off with a sample from a crime scene – a small spot of blood the size of a pin head would be enough. We take the blood sample and extract the DNA out of it using chemicals that bust open the cells. We then carry out PCR on this DNA to copy the 10 STRs that we’re interested in. At this point we have a tiny tube containing lots of copies of each of the different STRs. To separate and measure them we use another process called electrophoresis. It’s a bit like passing the samples through a sieve: The smaller (shorter length) STRs pass through more quickly because they can fit through the holes more easily than the bigger (longer) STRs. When the STRs pass through the ‘sieve’, a fancy camera records them. This tells us how long each STR was – in other words how many repeats it contained. The lengths of each of the 10 STRs, plus the results of a sex test (i.e. is the sample male of female) make up a DNA profile.
Once we have a DNA profile from the crime stain (such as the blood spot I mentioned), we can compare that to a reference DNA profile from the suspect in the case. The reference profile is made in exactly the same way except we get the DNA from a swab taken from the inside of the person’s cheek. This is called a buccal swab. If the crime stain profile matches the reference profile, then that provides very good evidence that the blood spot came from the suspect.