Every summer I look forward to attending some of the several Summer School workshops organised by the Royal Institution; this year I particularly loved the LYSC Forensics workshop (01.08.18) which focused on the DNA fingerprinting process to identify and compare DNA samples.
Understanding the basics of DNA can help us get a better understanding of forensics and how techniques can be used to identify criminals from only a few drops of blood at a crime scene.
DNA (deoxyribonucleic acid) is a long molecule which consists of billions of smaller units known as nucleotides. Just as there are differences in our appearances, there are also differences in our DNA which are known as genomic variations. The genomic variations between people tend to be very minor and the more closely related two people are, the smaller the variation between their genomes. Two random, non-related people would have 99.9% of their DNA in common and yet there are still more than three million differences between your genome and anyone else’s.
These variations arise due to mutations – changes that occasionally occur in a DNA sequence during replication. When a mutation occurs in a in a sperm cell or an egg cell, it will be passed along to the offspring. Your genome contains around 100 mutations which make it unique.
Most genome variations are relatively minor; to help put this into perspective, I like to visualise the human genome as a book. If you’ve ever been to the Wellcome Collection, you might have discovered the Library of the Human Genome – an expansive bookcase containing a human genome typed out in a series of 109 books. With that in mind, the books of your genome and mine would essentially tell the same story but yours might have a typo on page 17 of book 56 whilst mine might be missing a few letters on the penultimate page of book 34.
(Credit: Wikipedia)
The differences seem so small that it is remarkable that they can be detected through the use of forensic techniques.
The workshop explained and guided us through the restriction fragment length polymorphism (RFLP) technique of DNA fingerprinting; this process was the first DNA profiling technique which was inexpensive enough to see widespread application.
In the workshop, we were presented with (isolated) DNA found at a hypothetical crime scene and samples of DNA from five ‘suspects’. By adding a restriction endonuclease (a type of digestive enzyme produced by bacteria), we chemically ‘cut’ the DNA into fragments. My work experience at the Centre for Liver Research proved extremely helpful during the workshop as I was already comfortable with using a micropipette to transfer and mix substances.
The next step in the process involved separating the fragments through the process of gel electrophoresis. Gel electrophoresis takes advantage of the fact that DNA is negatively charged and that, due to genomic variation, the isolated fragments of DNA will be of different sizes.
First, we prepared the gel by pouring agarose into a casting tray and pressing a well comb into the molten agarose. When the agarose set, the comb was removed to create a series of wells in the gel.
The fragments are separated by passing an electric current through the gel. This causes the negatively-charged DNA to move from the wells to the positively-charged electrode through the agarose. Shorter fragments can move easily through the pores in the agarose and so will move faster and migrate farther than longer fragments in a given time. This separates the fragments of DNA into bands. The unique pattern of bands of the suspects’ DNA can then be compared to that of the DNA found at the crime scene.
(A blue loading dye was added to the DNA samples before pipetting them into the gel. The dye is co-migratory and so will separate and migrate at the same rate as the DNA fragments. )
After the gel was run, we used a transilluminator to better see the bands of DNA fragments.
From this, we could conclude that the DNA found at the crime scene belonged to Suspect 3 – the experiment was a success!
Curiosity Killed The Cation 