"IF I HAD A COW THAT GAVE SUCH MILK. ..

I'd clothe her in the finest silk," says an English drinking song. "I'd feed her on the choicest hay, and milk her forty times a day!"

But whiskey is nothing compared with what the new cows give.

The latest item of the phenomenal boom in biotechnology is genetically engineered cattle, sheep and goats that secrete phar-maceutical drugs in their milk. The drugs are then separated from the milk; the yield from one such animal is expected to amount to several millions of dollars per year. "Pharming" is the name of this latest development, doggedly opposed by Jeremiah Rifkin and other de-industrializing reactionaries.

But though the boom in biotechnology has centered mainly about the production of new drugs and diagnostic tools, it has also affected industry and agriculture, including energy production. Bugs directly affecting energy include bio-engineered bacteria to clean up oil spills, and potential applications range from biological gasification of coal in situ to the prevention of microbially induced corrosion in the condenser tubing of power plants.

There are several techniques of genetic engineering, but the best known and most widely used is Recombinant DNA or gene splicing, the one that truly engineers the properties of plants and animals by manipulating their genes.

Each cell in a living organism (in our bodies, say) has 46 chromosomes, one half inherited from our fathers, the other from our mothers. They are tightly coiled in a chemical sequence called DNA, a molecule of which has the structure of a double spiral, with rungs between the two spirals formed by four types of chemi-cals whose names begin with A, T, G or C. Each rung consists of either a CG or a TA compound, and a sequence of CGs and TAs codes a "message" just like a message in Morse code is a sequence of dots and dashes. But while the Morse code has only two elementary signals, the genetic code has four, since either the CG compound can be fitted between the two spirals as either CG or GC, depending on which end is on spiral 1 or 2. What is being coded is the instructions on how to build new cells, with the genetic code replicated in each new cell.

A gene is a piece of code that occupies a particular location on a chromosome and determines a certain hereditary trait in the or-ganism composed of these cells, such as blue eyes or vulnerability to diabetes. Certain enzymes (see below) will split the DNA chain wherever a certain gene occurs. The remaining fragments can be spliced with similarly formed fragments from other organisms, resulting in a recombinant DNA molecule, which now contains the new gene from the second organism, inserted by the genetic en-gineers. The resulting "host" cell now replicates the new sequence every time it divides. Most often the carrier of the host cells is a bacterium, and the bacteria are then grown either to harvest the new cell for insertion unto other organisms, or to alter the bacteria to perform a specific function.

As yet, genetic engineers cannot insert any gene of their choice into any organism, any more than chemists can make any com-pound with pre-specified properties; but like chemists, who know how to produce certain substances by subjecting the right com-ponents to the right processes, genetic engineers have certain (as yet much fewer) ways of selecting enzymes, genes, splicing proce-dures and host organisms to produce a certain outcome, and like the chemists, they keep experimenting to find ever new combina-tions to produce a desired result.

The enzyme used to split the DNA chain, or in some cases even split it in two places so as to take out the unwanted gene, is a catalyst; not in the nebulous sense used by politicians and their speech writers ("a means to facilitate . . ."), but in the strictly chemical sense: a substance that enables a chemical reaction of other compounds to proceed (or to proceed at a faster rate) without itself being consumed in the reaction.