In a darkened lab in the north of England, a research associate is intensely focused on the microscope in front of her. She carefully maneuvers a long glass tube that she uses to manipulate early human embryos.

"It's like microsurgery," says Laura Irving of Newcastle University.

Irving is part of a team of scientists trying to replace defective DNA with healthy DNA. They hope this procedure could one day help women who are carrying genetic disorders have healthy children.

"We are talking about conditions for which there is currently no cure," says Dr. Doug Turnbull, a professor of neurology at Newcastle University who is leading the research. These mitochondrial diseases are caused by hereditary defects in human cells.

"Mitochondria are like little power stations present in all our cells," Turnbull says. These power stations provide the energy that cells need. If the mitochondrial DNA is defective, the cells don't work right. The cells in effect run out of energy.

"I see the anguish of the families in every clinic that I do," says Turnbull. The severity of the disease can vary, with some families seeing their babies die in the first few hours of life. For others it can be a slow, progressive illness often leading to an early death.

Mitochondria have their own DNA, separate from the DNA that helps control the color of our eyes and hair, the shape of our noses, and how tall or smart we are. The mitochondrial DNA is passed down from mothers to their children.

Replacing defective mitochondrial DNA with healthy DNA might prevent these diseases from occurring. And that's exactly what Turnbull and his team want to do: DNA transplants.

Newcastle University scientists perform DNA transplants on very early embryos. The scientists hold the embryo very still with the tip of a pipette, left. They pull out the nuclear DNA of a mother and father hoping to have a healthy baby. It's then inserted into the embryo of a donor with healthy mitochondrial DNA. That embryo has had the rest of its DNA removed.

But the idea of scientists manipulating a human being's DNA in this way is very controversial. It would be the first time genetic changes have been made in human DNA that would be passed on, down the generations, through the germline. Any baby born this way would have genes from three different people.

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Combining The DNA Of Three People Raises Ethical Questions