Your DNA Profile: The Identity You Can't Escape

The Scene of the Crime

A call from Mrs. George to police headquarters reports an unfamiliar car that has been parked for two days on an isolated road near her home. Police Officer Adams responds immediately. He pulls up behind the car, and cautiously approaches it.

A woman's limp body is stretched across the front seat. Her head hangs off the passenger's side, its long hair trailing from the partly open door. The officer's eyes quickly focus on the dark bruise around the victim's neck. “Looks like a homicide,” he thinks, as he calls the station for help.

After pulling on the protective clothing investigators wear to prevent contamination—a Tyvek suit, latex gloves, booties, and a mask—Officer Adams checks the body for a pulse and finds none. Then the officer cordons off the crime scene to protect the evidence.

Inspector Tate and Ms. Clark, a forensic photographer, arrive within minutes. After they suit up, they inspect the body. Officer Adams enters their observations into a laptop: trampled grass near the car, a woman's body squeezed between the back of the seat and the gearshift, crumpled newspapers, a baseball cap lying in the grass several yards away.

Inspector Tate puts the baseball cap into a paper bag and ropes off the additional area. When Ms. Clark finishes photographing the body, the team carefully removes it for transport to the chief medical examiner's office.

Inside the car, the team dusts for fingerprints, inspects for hairs, removes the newspapers that have some small brownish spots on them, and sprays a chemical that will reveal any traces of blood. They place each piece of evidence in a separate paper container and carefully label it. Everything is loaded into an unmarked van and driven to police headquarters, where it is locked in the main evidence area.

A forensics expert will soon log out the evidence to extract samples of DNA that might identify the mystery murderer.

The Investigation

Biochemist Dr. Polly Merase is assigned to handle DNA analysis for the murder case. Her job is to look for samples of DNA in the evidence and use them to create a DNA profile of the murderer. At the main storage area, she signs out the newspapers, the baseball cap, and fingernail cuttings from the victim—all likely sources of DNA. Each item has a sample number and bar code and is logged into the computer system so that it can be constantly tracked.

Returning to the lab, Dr. Merase puts on latex gloves, a mask, and a full-length smock. She swabs down the big evidence table with alcohol and lays out a large piece of clean paper. She selects the newspaper with the brown stains from the collection of evidence. A simple color test will reveal whether the stains are blood.

Dr. Merase places a small cutting of stained newspaper and some ethanol (a type of alcohol) in a test tube labeled with the sample number. If the stains are blood, the ethanol will break open the red blood cells and release their hemoglobin, a protein that carries oxygen. When Dr. Merase adds indicator chemicals, they react with the hemoglobin and turn the sample first pink and then green, proving that the stain on the newspaper is blood!

That was easy. Getting a DNA profile from the blood sample will be complicated.

Dr. Merase first extracts DNA from the white blood cells also present in the blood sample (red blood cells don't contain DNA). Then she makes millions of copies of certain parts of the DNA. Finally, she analyzes the copies to reveal the DNA profile of the person whose blood is on the newspaper. Let's look more closely at the process.

Extracting the DNA

In a test tube, Dr. Merase mixes another cutting of the stained newspaper with two chemicals that will break open the white blood cells and release the DNA. She spins the tube in a machine called a microcentrifuge, and then incubates it overnight at 56 degrees C (113 degrees F).

The next morning, she adds another chemical and spins the tube again. The sample separates into two layers: a watery one containing the DNA, and another, heavier layer that sinks to the bottom of the tube. Dr. Merase removes the watery layer and filters out the DNA. The result is a small amount of liquid (equal to one-quarter of a drop from a medicine dropper). Dr. Merase tests the liquid and finds that it contains enough DNA to continue the analysis.

Copying the DNA

DNA molecules also contain sequences of base pairs that are not genes. These sequences used to be called “junk” DNA, but now scientists have learned that some of them can be used to determine identity and are key to DNA profiling. The sequences contain base pairs in repeating patterns; the number of repeating patterns in a sequence varies from person to person. The repeating patterns are called STRs (short tandem repeats) and can have from 20 to 100 base pairs.

In a new test tube, Dr. Merase mixes a small amount of the extracted DNA with a solution containing DNA “probes.” The probes attach to the starting points of the STRs to be copied. She also adds some other chemicals, including several fluorescent ones that will become part of the STR copies.

Dr. Merase places the test tube containing the sample in the PCR (polymerase chain reaction) machine. It makes copies of 15 STRs, plus amelogenin, another DNA sequence that tells the sex of the person. The DNA profile will be based on these 16 sequences. The machine goes through several cycles of heating and cooling over the next three hours. The amelogenin and the STR sequences split apart during heating and copy themselves during cooling.

Profiling the DNA

Dr. Merase puts the solution containing amelogenin and the STR copies into the genetic analyzer. The solution travels down a long tube that is as thin as a hair. An electric current passes through the tube and causes the shorter STRs to move faster than the longer ones. A clear “window” in the tube is lined up with a laser beam. As each group of STRs passes through the window, the laser beam causes the fluorescent chemicals they contain to flash. A detector, connected to a computer, records the flashes and prints out a graph. Each of the 15 STRs, plus the amelogenin, is represented by one or two peaks on the graph. This ordinary-looking graph, called an electropherogram, is the DNA profile.

The most important information that an electropherogram shows is the number of repeating base sequences at each STR location. To determine this number, the computer first measures the amount of time needed for one of the STR groups to pass in front of the laser. Then the computer compares that measurement to the amount of time needed for control DNA sequences, of known lengths, to move past the laser.

There can be one or two peaks for each STR, depending on whether both parents of the individual being tested have the same number of repeats at a particular location. For example, if a man inherits 10 repeats on one chromosome of the pair from his father, and 12 repeats on the other chromosome from his mother, there will be two peaks at that location. If both his mother's and his father's chromosomes have 10 repeats, there will be one peak. For the amelogenin location, a single peak indicates a woman, and two peaks, a man.

The more STR locations analyzed, the more likely it is that a match (between the unknown DNA profile and a suspect's profile) is meaningful. (There are a huge number of variations at each STR location in the population as a whole.) Using 15 STRs means that each person's electropherogram is unique. Only identical twins will have identical electropherograms. If Dr. Merase finds an exact match, she can testify that the probability of randomly choosing another individual with a DNA profile matching that of the suspect is one in more than 6 billion persons (the entire world population).

Dr. Merase sees two amelogenin peaks and notes that the blood sample comes from a male. Now she will analyze and compare the other DNA evidence from the case to determine who that male is and if he is the murderer.

The Wrap-Up

Dr. Merase analyzes a sample of the victim's blood, as well as other DNA samples taken from the car that could belong to anyone else who had been in it. None of the profiles matches the one from the bloodstained newspaper. She then tests the other evidence from the crime scene. DNA profiles from the sweat stains on the baseball cap and the scrapings of skin from beneath the victim's fingernails are identical to the profile from the bloodstained newspaper.

Finally, she compares the unknown profile with the ones stored in CODIS (Combined DNA Index System). CODIS is a national computerized database established by the FBI (Federal Bureau of Investigation) to help solve violent crimes. Crimes can be linked to each other and to convicted criminals by matching DNA profiles from crime scene evidence with DNA profiles of known offenders.

Dr. Merase enters the profile of the unknown man. It is an exact match with the profile of a convicted criminal recently released from prison. She writes a report summarizing her findings. That report, along with other evidence, will be used in court to convict the murderer.


DNA Basics

The 46 chromosomes in the nucleus of every human cell are made up of DNA (deoxyribonucleic acid). Chromosomes occur in pairs, one coming from each parent.

Each chromosome is made of a long DNA molecule shaped like a sprial ladder, or double helix. The “rungs” of the ladder are made of four chemicals called bases, and known as A, T, C, and G. They bond together in pairs: A bonds only with T and C bonds only with G. A gene is a string, or sequence, of base pairs on a chromosome. The base pairs in a gene act like a code that tells the cell how to manufacture a particular protein.

When a DNA molecule copies itself, the base pairs split apart and the two sides of the double helix separate. Each side rebuilds its partner from chemicals available in the cell.

there is an illustration of a dna strand

DNA Profiling: An Imperfect Science

When DNA evidence is used in a trial, the prosecutor must be able to show that the DNA hasn't been contaminated. (In the highly publicized murder trial of former professional football star O.J. Simpson, defense attorneys implied that the blood samples from the crime scene might have been tampered with, damaging the prosecution's case.) DNA must be handled with care: Crime scenes must be secured, people handling DNA evidence must wear protective clothing, and labs must follow strict procedures. A detailed record must clearly show where DNA evidence has been kept and who has handled it. DNA profiling can identify people who commit crimes, and it can also help acquit suspects and free innocent people from prison. It's a powerful tool—someone's life may depend on it.

Vocabulary

  • forensic: Relating to the use of science or technology in the investigation and establishment of facts or evidence in a court of law: a forensic lab, for example.
  • incubate: To maintain (eggs, organisms, or living tissue) at optimal environmental conditions for growth and development.

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Activity

  1. What do the letters “STR” stand for, and what is it?
  2. What are STRs used for in a forensic lab?
  3. What does the amelogenin sequence reveal?
  4. The article states that the odds of two different people having the same DNA profile match are about one in six billion. Do you think these odds are low enough to use DNA evidence when convicting someone of a crime? Why or why not? Write a few sentences explaining your answer.