How DNA Evidence Works

From the crime scene, a piece of DNA evidence travels to a forensic laboratory. These labs vary quite a bit, both in terms of how they are structured and what kind of analyses they offer. Public laboratories are often associated with a law enforcement entity or the district attorney's office, while others are independent government entities. Private forensic laboratories, some dedicated just to DNA analysis, also exist.

Many labs have the ability to conduct testing on nuclear DNA, which is the copy of DNA that exists in the nucleus of every cell. But only a few labs offer more specialized techniques, such as Y-chromosome or mitochondrial DNA analysis. Let's look at some of these techniques in greater detail.

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 30 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.

Many laboratories are replacing RFLP analysis with short tandem repeat (STR) analysis. This method offers several advantages, but one of the biggest is that it can start with a much smaller sample of DNA. Scientists amplify this small sample through a process known as polymerase chain reaction, or PCR. PCR makes copies of the DNA much like DNA copies itself in a cell, producing almost any desired amount of the genetic material.

Once the DNA in question has been amplified, 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 because these are the most likely to be accurate.

The Federal Bureau of Investigation (FBI) has chosen 20 specific STR loci to serve as the standard for DNA analysis. They expanded that number from 13 to 20 in January 2017.