5.1–5.5

DNA: what is it, and what does it do?

Children climb on a DNA sculpture.

In 1981, Julius Ruffin, a 27-year-old maintenance worker at Eastern Virginia Medical School, got onto an elevator and lost more than 20 years of his life. Several weeks earlier, a nursing student at the school had been raped by an attacker who broke into her apartment. When Ruffin got on the elevator, the student thought she recognized him as her attacker and called the police, who immediately arrested him. Ruffin’s girlfriend testified that he was with her at the time of the attack, but on the basis of the victim’s eyewitness testimony, a jury found Ruffin guilty and sentenced him to life in prison. Ruffin maintained his innocence—to no avail, until 2003. At that time, the state’s Division of Forensic Science performed a DNA analysis on a swab of evidence that remained from the investigation. The analysis revealed that Ruffin was not the attacker. Rather, the analysis showed that the DNA perfectly matched that of a man who, in 2003, was already in a Virginia prison, serving time for rape. Ruffin was freed, but only after having served more than two decades in prison (FIGURE 5-1).

Figure 5-1Vindicated by DNA evidence. Julius Ruffin was released from prison after two decades of wrongful incarceration.

Julius Ruffin’s case is tragic, but it is not unusual. He is one of a group of 316 people in the United States (as of early 2014) who have been freed from prison as a result of DNA analyses. These unjustly imprisoned people spent an average of 13.5 years behind bars. Eighty percent had been convicted of sexual assault; 28% had been convicted of murder. In three-quarters of the cases, inaccurate eyewitness testimony played an important role in the guilty verdict. (Recall, from Chapter 1, the experiments that revealed the unreliability of eyewitness identification.)

In this chapter, we take a close look at DNA, the molecule responsible for Julius Ruffin’s exoneration and the deferred justice served to the 315 other people. All living organisms—people, plants, animals, bacteria, and otherwise—carry DNA in almost every cell in their body (with just a few exceptions). Like a social security number, every person’s DNA is unique (with the exception of identical twins). In addition to being contained in our living cells, our DNA exists in what we leave behind. It is in our saliva, hair, blood, and even the dead skin cells that fall from our bodies. This is why DNA can serve as an individual identifier; it is a trail we all leave that is increasingly being used to ensure greater justice in our society.

The importance of DNA goes far beyond its function as an individual identifier, however. The information carried within this molecule, which is organized into individual units called genes, is among the most important of all biological knowledge. It contains instructions for the function of every enzyme and cell in our bodies and carries a record of the evolutionary history of lineages of cells and organisms. And, as witnessed by the following excerpts, it is often in the news.

In fact, it’s nearly impossible to open a newspaper or watch a news report these days without being informed that yet another complex human characteristic or trait has been linked to a newly discovered gene (FIGURE 5-2). But what does that actually mean? What is this molecule that apparently exerts influence over everything from our fidelity to our ruthlessness? We explore these issues and more in this chapter, beginning with a look at the structure of DNA itself and how it contains the information for producing organisms. In later sections of the chapter, we learn how modern manipulations of DNA are having far-reaching implications not just for human health but for agriculture as well.

Figure 5-2Genetics issues are in the news.