Matching DNA

­Law enforcement officials have used a variety of methods to examine DNA. T­he exact steps in preparing and analyzing the DNA can vary based on which method the­ investigators use. But, in general, the tests examine non-coding portions of DNA strands. Genes, which serve as templates for making proteins in your cells, make up only five percent of a DNA strand. The remainder of your DNA is non-coding and includes lots of repeating base pairs. Different types of tests look for and analyze different base pair repetition patterns.

Mitochondrial DNA Analysis
Most forensic DNA tests use material from the nucleus of a cell. Sometimes, especially in older samples of tissue like hair and teeth, there is no nucleus remaining in the sample. In these cases, investigators often use mitochondrial DNA analysis, which uses DNA from a cell's mitochondria.

­Restriction Fragment Length Polymorphism (RFLP) analysis was one of the first forensic methods used to analyze DNA. It analyzes the length of strands of DNA that include repeating base pairs. These repetitions are known as variable number tandem repeats (VNTRs) because they can repeat themselves anywhere from one to thirty times.

RFLP analysis requires investigators to dissolve DNA in an enzyme that breaks the strand at specific points. The number of repeats affects the length of each resulting strand of DNA. Investigators compare samples by comparing the lengths of the strands. RFLP analysis requires a fairly large sample of DNA that hasn't been contaminated with dirt.

Polymerase Chain Reaction (PCR) analysis is a newer technique that can amplify the DNA in a much smaller sample. It does this by making lots of identical copies of a small amount of DNA. It's often used as a preliminary step in Short Tandem Repeat (STR) analysis, which is the most commonly-used type of forensic analysis today.

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­STR analysis examines how often base pairs repeat in specific loci, or locations, on a DNA strand. These can be dinucleotide, trinucleotide, tetranucleotide or pentanucleotide repeats -- that is, repetitions of two, three, four or five base pairs. Investigators often look for tetranucleotide or pentanucleotide repeats in samples that have been through PCR amplification since these are the most likely to be accurate.

In STR Analysis, examiners have to:

  • Extract the DNA from the cells in the sample
  • Quantify the DNA
  • Amplify the DNA using PCR
  • Use capillary electrophoresis to extract the amplified DNA
Several of these steps are fairly labor-intensive, but many of them can now be performed by robots and machines.

The FBI's CODIS database uses samples that have undergone STR analysis examining 13 loci. The odds of two people having identical 13-loci STR profiles are about one in a billion.