Each day in a healthy human body, at least a trillion cells divide.

White blood cells proliferate into fresh legions of T cells, B cells, macrophages and other gladiators of the immune system. The cells that line the stomach divide daily to keep a seamless seal around the belly's caustic juices.As the uppermost layer of the skin sloughs off, newborn dermal cells poke their way up from below. Hair grows; nails grow. Cell division is synonymous with life.

The mystery of how a cell knows when to divide and when to cease division is one of the fundamental puzzles of biology. And lately, through an extraordinary convergence of research from a broad spectrum of disciplines, scientists have made enormous progress in unraveling the pivotal molecular events that control cell division.

Some of the new results were reported recently at a conference at Rockefeller University in New York presented by the General Motors Cancer Research Foundation.

``So many things are coming together from fields that have been developing on their own for years,' said Dr. Raymond Erikson, a molecular biologist at Harvard Medical School who attended the meeting. ``There's a new synergism that is really exhilarating.'

Biologists believe they are close to a deep understanding of the cell cycle, the intricate dance that begins when a cell awakens from its normal state of rest and glissades with balletic precision through the replication of its chromosomes and the apportioning of them into two progeny cells.

This knowledge has coalesced with a swiftness exceptional even for basic biology, which has grown accustomed to a dizzying pace ever since the advent of recombinant DNA technology.

``Our knowledge of the cell cycle compared to just two or three years ago is really the difference between day and night,' said Dr. David Beach of Cold Spring Harbor Laboratory in Cold Spring Harbor, N.Y.

As part of the new dawn, biologists have identified two types of proteins that are indispensable to beginning and completing cell division. Either species of protein on its own is useless - ``a little lump of clay,' said Dr. Joan Ruderman of Harvard Medical School, a pioneer in the field.

But when the proteins clasp together they take on the vitality of young lovers, galvanizing a cascade of changes in the cell that culminates in division. The paired proteins seem to work by altering the shapes and duties of a string of other proteins in the cell, and scientists have identified many of those target proteins.

But the coupling is short-lived: researchers have discovered that after each cycle of division, one of the two proteins rapidly disintegrates, an event that seems to protect against untrammeled cell growth.

So exquisitely do the master molecules perform their job that nature has decided to make wide use of them. Among the many exciting findings about the cell cycle is this universality of the central players: the same proteins commanding cell division in primitive cells like yeast are also at the helm in human tissue.

This has made life far easier for scientists; any discoveries they make in simple and easily manipulated laboratory organisms can be extended almost directly up to humans.

Scientists are also beginning to knit together the findings about the cell cycle with recent studies of the hormones and peptides in the bloodstream known to stimulate cell growth.

By combining discoveries about the growth signals that bombard the cell from the outside with knowledge of the internal machinery that orchestrates growth, scientists hope to form a complete and finely detailed portrait of the dividing cell.

That information will in turn permit them to better understand cell division gone awry, the hallmark of cancer.

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