Spring 2011

Know How

Illustration by Betsy True

EVEN THOUGH 99.9 PERCENT OF HUMAN DNA is exactly the same in all people, a single droplet of blood or stray eyelash collected at a crime scene still carries all the genetic information needed to convict a criminal. Back at the lab, forensic scientists simply probe the remaining 0.1 percent of the genome—3 million nucleotide bases—for telltale variations. This process, known as DNA profiling or genetic fingerprinting, reveals a suite of variations in the genetic code that, taken together, constitute an individual’s unique DNA profile. Here’s how it works:

1. Collect a sample and extract its DNA. Scientists only need a tiny amount of DNA—around 100 micrograms—to construct a DNA profile from a crime scene sample. That’s so little, a few cells from saliva on a straw will do.

2. Amplify the telltale regions. Scientists use a powerful technique called Polymerase Chain Reaction (PCR) to make millions of copies of the sample’s telltale DNA regions. In particular, they home in on regions known as Short Tandem Repeats, or STRs, which are composed of short units of DNA—just four or five bases long—that are repeated numerous times in a row. What makes these regions telltale is that the number of repeats they contain varies widely from person to person. In criminal investigations, 13 such STR regions, all located in the non-coding DNA between our genes, are analyzed for the number of repeated units they contain.

3. Count the repeats. During PCR, fluorescent dyes are attached to all the STR copies that get made—one type of dye for each STR region—so that all of the DNA copies from a given region can be distinguished from the others in the mix. Scientists run the mixture through a capillary electophoresis machine, which separates the various DNA fragments by size. From there, it’s a fairly easy thing to calculate the length of each STR region, and, therefore, the number of repetitive units at each site.

4. Look for a match. To convict a suspect, his or her STR repeats must match those in the crime scene sample—at all 13 STR regions. According to the FBI, when all 13 STR sites match perfectly, it’s virtually guaranteed you’ve got your culprit; the odds of fingering the wrong person are about one in 1 billion. A single STR mismatch, however, is enough to exonerate a suspect and spur investigators to search CODIS, the nation’s database of DNA profiles, in hopes of solving the crime.

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