Regenesis by George M. Church

phrase is reminiscent of Pasteur’s omne vivum ex vivo — all life from life—and likewise seems to embody a vitalist position.)
    In general, cells are small objects. Bacteria range from 0.5 to 750 microns, human cells from 5 microns to 1 meter in length. (A micron is a millionth of a meter; for comparison purposes, a human red blood cell is about 5 microns across.) The longest cells are nerve cells, or neurons, some of which stretch from spine to toe. The very tiniest bacteria, members of the genus Mycoplasma , are less than a micron long. The physical volume taken up by a Mycoplasma bacterium is evidently the smallest amount of space that will accommodate all of the metabolic machinery necessary for life, or life as we know it, so far, here on earth.
    There’s one big division in the overall cellular universe: some cells exist only as entities embedded within other cells of the same type, for example, muscle cells or brain cells; other cells exist as free-floating entities by themselves, for example, E. coli cells or yeast cells.
    What we might call an average or generic cell is composed of eight major classes of polymers: polynucleotides (like DNA and RNA), polypeptides (like collagen and vancomycin), polyketides (like fats and tetracycline), polysaccharides (like cellulose and starch), polyterpenes (like cholesterol and rubber), polyaminoacids (like lignin and polyalkaloids), polypyrroles (like heme and vitamin B12), and polyesters (like PHA, PHV). First in importance are the nucleic acids, DNA and RNA, which contain the genetic information, the software of life. This software runs the cell in as literal a sense as a computer’s operating system runs the computer. It directs the formation of proteins. It contains the cell’s own recipe,the complete instruction set necessary and sufficient for making another nearly identical cell. The cell is not controlled by its genome exclusively, but also by its environment, its history, and the choices that the cell makes in response to these. Emergent behaviors arise as a function of the cell’s being greater than the sum of its parts individually.
    Second, cells are made of proteins, which constitute some 20 percent of a given cell by weight. The term “protein” comes from a Greek word that means “primary” or “first thing,” and a typical bacterium may possess several thousand different types of them. The proteins perform most of the cell’s housekeeping, self-repair, and other workaday tasks. Some of them, the enzymes, or biological catalysts, are shaped with distinctive clefts or pockets that assist in certain chemical reactions. Structural proteins have ends that attach themselves to surfaces to provide rigidity and support to compartmented cells. Transmembrane proteins allow selected types of molecules to enter or leave the cell. Since we are made up of cells, proteins are another thing that cells and humans have in common.
    A cell’s membrane, which constitutes its outer surface, is composed of lipid molecules. Lipid molecules possess special properties that suit them to the cell’s watery interior and to its fluid external environment. A lipid molecule is a long linear structure whose two ends behave differently when immersed in water, which in the case of living cells is most of the time. One end of the molecule, the “head,” carries an electrical charge, and this makes that end hydrophilic (water-loving), meaning that it seeks out or orients itself toward water. The molecule’s other end, the “tail,” is uncharged, which makes it hydrophobic (water-fearing), meaning that it hides or sequesters itself from water. This bipolar feature makes lipid molecules into active, dynamic entities, for when placed in water they will spontaneously associate and collectively adopt the shape suited to the opposed chemical affinities of their ends: generally they will form a sphere, with the outer surface