Today, you will learn about DNA and the ways that research scientists can study DNA to identify people.

What is DNA?

What is DNA? Our DNA is our genetic code – instructions on how to create ‘us’! The genetic code is written in DNA bases – there are four DNA bases, A, C, G and T. But what exactly can our DNA tell us about us? For example, how similar is our DNA to the DNA of family members, friends, humans around the world? To start with, watch this brief video about DNA, to discover the structure of DNA, and how it ‘codes’ us.

Less than 1 % (most sources estimate as little as 0.1 %) of our DNA is unique to us (unless you are an identical sibling)!  However, even 0.1 % of your DNA equates to 3 million bases of DNA. Scientists can exploit these unique DNA bases, to determine whose DNA they have. This is the basis for Forensic DNA Profiling (also known as DNA Fingerprinting).

DNA profiling is probably most famous for its uses in forensic science. DNA evidence can be used to determine if biological evidence matches the DNA of a known suspect, or victim. Watch the video below to tour the Forensic Biology and DNA Unit at the Denver Police Department Crime Lab.

Forensic DNA Profiling

So how do scientists carry out DNA profiling? DNA fingerprints are based on special areas in our DNA, which contain repeated patterns of DNA. For example, in one area of our DNA, we might have the repeat ‘AT’. In this area, a person may have one repeat (AT), two repeats (ATAT), three repeats (ATATAT), and so on. 

In each area where repeats are found, scientists examine the area to see how many repeats there are. This will build up a DNA profile, such as: area 1, 3 repeats; area 2, 5 repeats; area 3, 2 repeats… and so on. When we examine enough DNA areas (typically 13 or 20 areas), this DNA profile becomes unique – by chance, we expect that no two people (expect identical siblings) will have the same number of repeats in all the locations! The unique set of repeat numbers a person has is their DNA fingerprint.

Remember, humans have two copies of every part of their DNA – one from each biological parent. Therefore, for each repeat area, you will have two copies of that area, which might have the same number of repeats, or they might not. Therefore, your DNA fingerprint will actually be more like this: area 1, 3/5 repeats; area 2, 5/5 repeats; area 3, 2/7 repeats… and so on.

In the video below, Megan Rokop will demonstrate how we can visualise a person’s DNA fingerprint. Megan also explains why it is useful for us to find a DNA fingerprint, and how she uses DNA fingerprints in her scientific research, examining human disease.

Megan will explain the following steps:

1. Extract DNA from human cells – this leaves us with only the DNA molecules! This has been done before the video starts.
2. Use a process called Polymerase Chain Reaction (PCR) to amplify (make more molecules of) a particular area of DNA where a repeat is.
3. Separate the DNA molecules by size, by moving the DNA through a gel matrix (this is a process called gel electrophoresis). By looking at how far the DNA molecule moves through the gel matrix, scientists can work out how big the DNA molecule is (i.e. how many repeats it has!). Bigger (longer) DNA molecules have more repeats, and smaller (shorter) DNA molecules have less repeats.

Watch the video below, from the start to 37:35. The end of the video is a guide for teachers, which is not part of today’s activities. While watching the video, pause when the video tells you to, and consider the questions which have been asked. All the questions, and the times at which they are asked, are also copied out for you below this video.

1. 02:16 – How many bases of the DNA shown on screen would be different (on average) between Megan, and another person?
2. 06:25 – Which situations can you think of, where it would be useful to identify someone using their DNA?
3. 10:58 – How could you make DNA (which is negatively charged) move towards a point, such as the end of the room? Also, how might a SMALL molecule of DNA move differently to a LARGE molecule of DNA?
4. 17:14 – Why do we need to amplify DNA by PCR?
5. 23:24 – Lanes 6,7,8 and 9 show the DNA bands made by DNA from 4 different people. A band of DNA is seen where the DNA molecules have ended up after using electricity to move the DNA in gel electrophoresis. Long DNA molecules find it harder to travel through the gel matrix from the wells (holes) where the DNA starts – the wells are at the top of the image in the video. Conversely, shorter DNA molecules can move more quickly through the gel, and so move further down the gel, making a band lower down.
   1. People 6, 8 and 9 each have 1 DNA band, while Person 7 has 2 DNA bands. Why does Person 7 have 2 bands of DNA?
   1. Person 6 has a DNA band at a different height to the DNA band of Person 8. Is Person 6’s DNA band longer or shorter than Person 8’s DNA band?
6. 26:41 – Based on the gel shown, could the ‘Mom’ and ‘Dad’ characters be the biological parents of the ‘Me character? Hint: where could ‘Me’ have inherited their DNA bands (i.e. their DNA repeat numbers for this area of DNA) from?
7. 30:20 – For which Baby (A, B or C) can you determine their parents, based on this DNA gel picture? Why can you not definitely say who the parents of the other two babies are?
8. 33:00 – Using this second DNA area, can you now say which are the correct parents for all three Babies? What extra information did you use?
9. 35:37 – Does suspect D, E or F have DNA which matches the DNA found at the crime scene?

Going Further: If you enjoyed learning about DNA Profiling with Megan, you might want to also view this more detailed series of videos by Jeffry Petracca at the DNA Learning Center, which will guide you through each step that a research scientist would undertake in the lab, in real time!

The future of DNA Profiling

In future, rather than amplifying DNA at each location to see the number of repeats, it might be quicker and cheaper to sequence (read) the DNA in its entirety!

DNA sequencing tells us the entire genetic code of the DNA – the letter of each base pair – which gives more information than the unique repeat region pattern seen in DNA fingerprinting. Read this short article on the future of DNA in the justice system, to learn how scientists are developing models that can not only match DNA from a crime scene to that of a suspect, but may be able to use DNA to predict features about an unknown suspect in the future!

What do you think about the idea of using DNA to ‘predict’ the facial features of a suspect? Can you think of any problems with this approach to solving crime?

Models such as that described in the article are only possible due to advances in DNA sequencing. Dr Heather Jeffery is a scientist at Oxford Nanopore Technologies Ltd, a spin-out from the University of Oxford which was founded in 2005. Oxford Nanopore have developed new generation of DNA sequencing technology, which is contributing to making DNA sequencing easier and cheaper to do. For a brief insight into how Oxford Nanopore are sequencing DNA, and why this is so revolutionary, watch the video below.

Resource written by Dr Ana Wallis for the St John’s Inspire Programme Summer School.

Dr Ana wallis

After attending the University of Nottingham for her undergraduate degree in Genetics, Dr Ana Wallis moved to Oxford to study her DPhil (PhD), examining how our genetic material (DNA) is copied when a cell divides. During Dr Wallis’ DPhil, she designed and delivered science outreach materials. Having greatly enjoyed this outreach experience, Dr Wallis went on to work at St John’s College, Oxford and the Oxford University Museum of Natural History, creating STEM outreach programmes for families and pupils. Dr Wallis is currently the Project Lead for the Post-GCSE Inspire Programme.