How Mammals Get the Message

Abstract

gene expression in bacteria during the past 20 years that some biologists now profess a biology is dead philosophy. Not so! counter biologists enamored with the conversion of genes into moleCules of messenger RNA into proteins, in mammalian cells. The mammalian cell is a thousand times larger than the bacterium and has thousands more genes, messenger RNA molecules and proteins than the bacterium does. What's more, gene expression in mammalian cells is frontier research territory. If anything inflames those studying mammalian-gene expression. it is the manufacture of messenger RNA. They're becoming more and more convinced that the manufacture in mammals differs dramatically from the manufacture in bacteria. In the bacterium, the conversion of a gene (DNA) into a molecule of messenger RNA and then into a protein is immediate and automatic. The reason is that the bacterium has no nucleus and its manufacturing equipment sits cozily together in the cytoplasm. But in the mammalian cell, the process is more than a humdrum assembly line. Evidence building for a decade suggests that DNA in the nucleus of the mammalian cell makes large RNA molecules. As one of the discoverers recalls, We had no idea what in the hell the cell was doing to make molecules of such monstrous size. These large nuclear molecules give rise to much, much smaller (some 10 times as small) molecules of messenger RNA. The molecules of messenger RNA are then catapulted from the nucleus of the cell into the cell's cytoplasm, where they hook up with ribosomes and get on with protein production. Molecular biologists are now tackling the manufacture of messenger RNA molecules from the large nuclear RNA molecules, what properties they share or don't share and why. A property that has them especially intrigued is poly (A)-a string of nucleotides that contains adenylic acid only as its base composition. Two years ago, for instance, Mary Edmonds of the University of Pittsburgh and some other molecular biologists found that large molecules of RNA and molecules of messenger RNA have the same chemical handle on their right ends (the so-called 3' ends). The handle is added to each kind of molecule after it is made. Because both kinds of molecules have a poly (A) handle on their right ends, and because the handle is added only after they're transcribed, it appears that messenger RNA molecules might be made from the nucleotides toward the right ends of the large RNA molecules. Now another, shorter poly (A) sequence has How Mammals Get the Message