DNA Profiling


DNA profiling (a.k.a. DNA fingerprinting) describes the process by which individuals can be identified and compared based on their DNA sequence

When DNA fragments are cut with restriction endonucleases, fragments of different lengths can be resolved using gel electrophoresis

Gel Electrophoresis

Gel electrophoresis is a technique which is used to separate fragments of DNA according to size

  • Samples of fragmented DNA are placed in the wells of an agarose gel
  • The gel is placed in a buffering solution and an electrical current is passed across the gel
  • DNA, being negatively charged (due to phosphate), moves to the positive terminus (anode)
  • Smaller fragments are less impeded by the gel matrix and move faster through the gel
  • The fragments are thus separated according to size 
  • Size can be calculated (in kilobases) by comparing against a known industry standard

DNA Profiling

  • DNA profiling is a technique by which individuals are identified on the basis of their respective DNA profiles
  • Within the non-coding region of an individual's genome, there exists satellite DNA - long stretches of DNA made up of repeating elements called short tandem repeats (STRs)
  • These repeating sequences can be excised to form fragments, by cutting with a variety of restriction endonucleases (which cut DNA at specific sites)
  • As individuals all have a different number of repeats in a given sequence of satellite DNA, they will all generate unique fragment profiles
  • These different profiles can be compared using gel electrophoresis

DNA Profiling Using STR Analysis

Applications of DNA Profiling

  • A DNA sample is collected (blood, saliva, semen, etc.) and amplified using PCR
  • Satellite DNA (non-coding) is cut with specific restriction enzymes to generate fragments
  • Individuals will have unique fragment lengths due to the variable length of their short tandem repeats (STR)
  • The fragments are separated with gel electrophoresis (smaller fragments move quicker through the gel)
  • The DNA profile can then be analysed according to need

Two applications of DNA profiling are:

  • Paternity testing (comparing DNA of offspring against potential fathers)
  • Forensic investigations (identifying suspects or victims based on crime-scene DNA)  

                                                 Paternity Testing                                                                                                        Forensic Investigations

Paternity Testing:  Children inherit half of their alleles from each parent and thus should possess a combination of their parents alleles

Forensic Investigation:  Suspect DNA should be a complete match with the sample taken from a crime scene if a conviction is to occur

  • Individuals can also be tested for genotype provided the two alleles are of different sizes (two bands = heterozygous ; one band = homozygous)