Modern cells are like microscopic cities: They have power plants (mitochondria), trash dumps (lysosomes), local government (the nucleus, with DNA serving as the legal charter), and many other activities going on inside their boundaries. They also have a border patrol in the form of a double-layered membrane that uses a series of protein-powered pumps, pores and channels to let nutrients in and keep other chemicals and substances out.
But, cells were very different when life began 3.5 billion to four billion years ago. Rather than small metropolises, they were more like a purse that carried instructions—consisting of just a membrane with genetic information inside. They lacked the structures and proteins that now make them tick. The question is: How then were they able to take in the nutrients necessary to survive and reproduce?
Harvard Medical School researchers report in Nature that they have built a model of what they believe the very first living cell may have looked like, which contains a strip of genetic material surrounded by a fatty membrane. The membranes of modern cells consist of a double layer of fatty acids known as phospholipids. But in designing a membrane for their cell, scientists worked with much simpler fatty acids that they believe existed on a primeval Earth, when the first cell likely formed. The key, says study co-author Jack Szostak, a Harvard geneticist, was to develop one porous enough to let in needed nutrients (such as nucleotides, the units that make up genetic material, or DNA) but strong enough to protect the genetic material inside and keep it from slipping out after replicating.
In an attempt to duplicate an early cell, scientists put fatty acids (that were likely membrane candidates) and a strip of DNA into a test tube of water. While in there, the fatty acids formed into a ring, or membrane, around the genetic segment. The researchers then added nucleotides—units of genetic material—to the test tube to determine whether they would penetrate the membrane and copy the DNA inside it. Their findings: the nucleotides did enter the cell, latch onto and replicate the DNA over 24 hours.
What scientists now must figure out, Szostak says, is how the original and copycat DNA strands separated and this early cell divided or reproduced.
"We're trying to solve a whole series of problems, step by step," he says, "and build up to replicating an evolving system."
David Deamer, a biomolecular engineer at the University of California, Santa Cruz, says he believes the team is on its way to making a prototype of a primitive cell that has "essentially all the basic properties of life."