DNA Profiling

DNA Profiling

  • DNA profiling, also known as DNA fingerprinting or typing, is a process used to identify individuals based on their unique DNA patterns.

  • Each individual (except identical twins) has a unique DNA sequence. While around 99.9% of the DNA sequence in humans is identical between individuals, 0.1% varies and it’s this variation which is exploited in DNA profiling.

  • DNA is isolated from a sample (e.g., blood, saliva, hair follicle), cleaned, and then cut into fragments using restriction enzymes. These enzymes cut DNA at specific sites, producing a range of different sized fragments.

  • The DNA fragments are separated by gel electrophoresis, a process which uses an electric field to separate fragments based on their size and charge. Small fragments move faster and therefore further than larger fragments.

  • The pattern of fragments (the DNA fingerprint) is visualised using a dye that binds to DNA. This can take several forms, but commonly used method is called Southern blotting.

Short Tandem Repeats (STRs)

  • STRs are sections of repetitive DNA where a short (2-6 base pair) sequence is repeated multiple times. The number of repeats varies between individuals and can be used to create a unique DNA profile.

  • To analyse STRs, specific sites within the DNA are targeted using polymerase chain reaction (PCR), a technique that allows for the targeted amplification of specific DNA regions.

  • In forensic science, a standard set of STRs are analysed to give a specific individual’s DNA profile. This profile can be compared with others to determine relatedness or identity.

In Practice

  • DNA profiling plays a crucial role in forensic science, primarily being used to tie suspects to crime scenes. It can also be used to identify human remains or to establish family relationships in paternity and immigration disputes.

  • Limitations include the need for relatively large amounts of undegraded DNA, potential for contamination, and complications in interpreting mixed DNA samples from multiple individuals.

Key Takeaways

  • Recognise the steps of DNA profiling: DNA extraction, fragmentation, separation through gel electrophoresis, and visualisation.

  • Understand the role and analysis of Short Tandem Repeats (STRs) in generating a unique DNA profile.

  • Appreciate the practical applications of DNA profiling in forensic science, along with its inherent limitations.