Television replaced radio as America's primary means of home entertainment in the 1950s, and in the half-century since many critics have complained that its great potential as an educational venue has never been fully realized. However, programs marketed as entertainment are sometimes surprisingly educational, none more than the CSI programs shown on the CBS network for the last few years.

The letters CSI stand for Crime Scene Investigation, and that is the focus of the program. Each episode begins with a crime scene, which is investigated and sifted for clues. The science used to carry out the investigation is highlighted, and without particularly intending to do so, a regular viewer soon absorbs quite a lot of information about fingerprints, trace chemicals, fiber analysis, rates of body cooling -- in short, the science of forensic investigation.

Like making a bad-tasting medicine palatable by encasing it within a piece of candy, CSI educates its audience about science painlessly.

In the spirit of this sort of drama-enhanced science education, I offer a CSI investigation, one in which DNA plays a central role. After working our way through some basic DNA science, we will revisit the crime scene, and attempt to apply what we have learned to sort through the clues.

The crime scene

Our episode begins, as many CSI investigations do, with a murder scene: 10-month-old Joey is crying in his playpen, his mother dead on the floor with a bullet in her right temple. A blood-splattered revolver lies by her body.

When the dead woman's sister finds the body, she calls 911 and CSI soon arrives. They ask if the father has been contacted, but the sister informs the CSI team that the dead woman is a single mom.

A suicide note on the table says "Joey is better off without me."

• The kitchen door appears to have been recently forced.
•  The sister tells the investigators that she visits often and is sure the dead mother did not own a revolver.
• The suicide note is written in red ink, but the CSI investigators can find no red ink pen in the house.
•  Finally, and damningly, the dead woman's hands are clean. Spattered with her blood, the revolver could not have been held by her hand without its fingers becoming spattered as well, so clearly she was not holding the gun when it fired a bullet into her brain.

Not suicide, then, but murder.

Check the DNA

The CSI investigators sample the victim's blood, and request a cheek swab from the sister, so that they can compare DNAs to confirm her relationship to the murdered woman. Modern DNA analysis can be performed with very little physical material--the number of cells scraped from the inside cheek of a person's mouth provides plenty of cells. The tiny amount of DNA extracted from the cells can be chemically amplified in much the same way a cell replicates its DNA, using the same enzymes only in a test tube. There is even enough DNA in the scalp cells adhering to the tip of a human hair for this sort of analysis.

Of course, all this DNA analysis wouldn't be of much use if everybody's DNA was the same, but it isn't. In fact, except for identical twins, everybody's DNA is different. However, people who are related contain similar sections of DNA, such that a relationship can be determined through DNA analysis. So your DNA is like a molecular fingerprint, a calling card that reveals not only your true identity but also your relationships with other people.

The sister of the murder victim tells the CSI investigators that the dead woman was naive and very religious. She had met the baby's father at a church social, she said--but when she became pregnant he was never mentioned again.

Because family members are often involved in domestic murders, the CSI investigators wanted very much to talk to the baby's father. Perhaps the child's DNA fingerprint might provide an avenue to pursue, on the off chance the father's DNA profile is registered in a police database. Feeling it a long shot but worth a try, the CSI investigators sample the child's DNA as well.

So far the crime scene has seemed routine, but the medical examiner soon changes that. The autopsy reveals the dead woman is a virgin! Her hymen is still intact, which means she never had intercourse. The scar on her abdomen confirms that she gave birth to Joey by Caesarean section. Was this a miraculous conception? No. The hymen presents no barrier to semen, and she could easily have gotten pregnant from "playing around" without full penetration.

Trace the gun 

The gun found on the floor beside the body offers another potential line of investigation. The bullet in her skull was fired from it, and, as her sister had claimed, she did not own it. A Colt Detective Special, it was purchased in Nevada years ago. After considerable effort, the team traces the weapon through several owners to the winner of a local card game, a man with a big mole between his eyes. Everyone called him "Cyclops" but no one seemed to know his real name. Another dead end.

How about the suicide note? A handwriting expert tells the CSI team that she definitely wrote the note, but probably under considerable stress. The spacing of her writing is erratic, a telltale giveaway. The red ink is from a cheap pen, but if the missing pen were to be found, he says he may be able to match it to the ink flow indicated by the note.

One promising lead opens up. The dead woman's boyfriend turns up -- he just walks into the police station. He has a clear alibi for the time of the murder, as several men working with him at the time could testify, so he is soon eliminated as a potential suspect.

If anything, his appearance only makes the puzzle of the murder more murky. When his DNA is checked to confirm that he is indeed the father of the dead woman's child, there is no match! Whoever fathered the dead woman's child, it wasn't the boyfriend she met at the church social.

Still, you have to admit, this sure is an unusual little family. Mom's a virgin, and daddy isn't the daddy. Time to step back and look at the DNA evidence with a more jaundiced eye. Who haven't the CSI team checked? The dead woman.

When the murder victim's DNA profile is examined, all bets are off. Her DNA doesn't match the DNA of her son! This, finally, is the clue that allows the CSI team to unravel the case.

Embryo bank 

To the surprise of the CSI team, baby Joey is not a DNA match to mom, who is in fact a virgin. It appears then that the dead woman was a surrogate mother, the embryo that was to become baby Joey having been donated by a completely different set of parents.

Could it be that they had expected to be given Joey after birth, and that the birth mother, now bonded to the child, had refused? That might be a motive for murder.

Investigations move more quickly when you know what to look for. Examining every scrap of paper in the house, the CSI team finds a document in the dead woman's desk. It is a registration form for PROJECT SUNFLOWER, an organization that finds mothers for abandoned embryos.

When CSI questions the PROJECT SUNFLOWER staff, they learn that the organization believes that every embryo is a baby from the day it is fertilized, even if in a laboratory dish. Joey, and many like him, began life as a leftover fertilized egg from a fertility clinic that the PROJECT SUNFLOWER staff had placed in a new home. His birth mother had "adopted" him nine months before he was born.

Why then the murder? Its hard to see a motive, but it can't hurt to get more information. How about the biological parents? It took a court order, but PROJECT SUNFLOWER records lead to the address of Joey's biological parents.

They tell the CSI team that they had tried three times to have a baby at the fertility clinic, without success, and had eventually donated their frozen embryos to PROJECT SUNFLOWERS.

As a routine matter, the CSI team collects the clothing they had worn recently, to check for bloodstains. They are deeply offended to be considered suspects. They wanted a child, they said, but not enough to commit murder. Now, of course, they will take custody of Joey as his biological parents.

The final clue 

Examined in the lab, the woman's blouse shows no blood -- but it does reveal traces of gunshot residue (GSR)! Because GSR can be transferred if people make contact, the CSI team collects all the clothing in the laundry basket, and finds a much larger size 14 blouse with far more GSR adhering to it. This blouse belongs to the biological mother's own mother. Asking about her, the CSI team learns that her husband had recently died -- his photograph reveals a man with a prominent mole between his eyes! As a final touch, they find in her purse a red pen.

The long journey of crime scene investigation is over, this murder solved with the help of DNA.

With the program over and the crime solved, step back and consider the journey you have just taken. The amount of science you have had presented to you -- and that you have had to understand -- as you followed the CSI investigators through the case is more than many college courses would present, and it didn't hurt a bit.

CSI television shows have presented similar stories each week, in each of several cities, for years. Seeing this, I cannot help but be impressed at the power of television as a teaching tool. Imagine if we used it wisely to educate viewers about even one of the many important issues that face our uncertain future.

NOTE: Most of the illustrations came from SXC.HU.

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