DNA Science
Generating DNA profiles
DNA profiling is used to identify an individual based on the unique characteristics of that person's DNA. Most of our DNA is identical to DNA of others. However regions of non-coding DNA ("junk DNA"), show great variability from person to person. These variations between individuals are called polymorphisms. Sequences with the highest degree of polymorphism are very useful for DNA analysis in forensic cases and paternity testing.
Short tandem repeats (STRs) are short sequences of DNA, normally 2-5 base pairs long, which are repeated numerous times. STR analysis has largely replaced the original "restriction fragment length polymorphism" (RFLP) analysis developed by Sir Alec Jeffreys in 1985. Modern STR analysis using a polymerase chain reaction to "amplify" or multiply the DNA has several advantages.
- Discrete alleles from STR systems may be obtained due to their smaller size, so DNA fragments differing by a single basepair in size may be differentiated.
- Smaller quantities of DNA, including degraded DNA, may be typed using STRs.
- The quantity and integrity of the DNA sample is less of an issue with PCR-based typing methods than with conventional RFLP methods.
After the DNA has been extracted from the sample, each region of interest is copied with a technique known as the polymerase chain reaction (PCR). PCR produces millions of copies for each starting DNA molecule, thus allowing very minute amounts of DNA to be examined. Multiple STR regions can be amplified simultaneously to increase the informativeness of the DNA profiling method.
We examine 10 STRs and Amelogenin, which can be distinguished from each other due to their colour and length. The colour results from a fluorescent dye that is attached during the PCR stage. One of the primers for each of the loci amplified by the SGM plus kit, is labelled with either 5-Fam, Joe or Ned which are detected as blue, green and yellow respectively, on the 3100 genetic analyser. The three different coloured dyes allow loci with overlapping size range to be co-amplified and analysed at the same time to provide highly informative results with minimal sample consumption.
