Promises and Plasmids

CELLS are very small. About 500 average-sized cells could fit on the period at the end of this sentence. Yet each one of those cells generally contains all the DNA needed to construct a living creature, such as you.

Obviously, if cells are small DNA molecules must be very tiny indeed. They are shaped like long, twisted threads, so long that all the DNA in your body laid end to end would stretch to the sun and back many times! But the threads are very thin, only about one ten-millionth of an inch (1/400,000th mm) across.

To complicate matters, these long, thin threads of DNA must somehow be packed inside the cells, and the only way to fit them in is for them to be twisted up into very tight bundles. This makes it difficult for scientists to locate the exact areas of the particular DNA molecules they may be interested in, the genes. Scientists cannot just put a cell under a microscope, find the gene they want and then extract it with tweezers and put in another gene.

Plasmids to the Rescue

It turns out, however, that bacteria often contain some DNA molecules that are easier to work with. These strands of DNA are more or less independent from the rest of the DNA in the bacteria, forming loops all to themselves that can easily be passed from one bacterium to another. They are called plasmids. At present plasmids are the keys to gene-splicing.

Splicing genes into plants and animals is not so easy because these cells do not have plasmids, and their genetic regulatory systems are much more complicated. But scientists are hopeful that such splicing will soon be possible. If they succeed, then they will be able to put genes in plants from bacteria that fix nitrogen in the soil so that it will not be necessary to add nitrogen fertilizer to the soil. They are also hoping that someday they will be able to cure genetically caused diseases, like sickle-cell anemia, by replacing defective genes in humans.

How Do You Splice a Gene?

SUPPOSE you wanted to splice a gene. How would you go about it?

First, you would need the gene, a section of DNA containing the “code,” or “master drawing,” for a specific protein. “Gene machines” are now available to synthesize simple genes from inert chemicals. More complicated genes might have to be located and isolated from the DNA of living cells.

Next, you would need a plasmid and a special chemical called a restriction enzyme, which chemical would break the plasmid open at the right spot, leaving “sticky ends” for the attachment.

You might also need to make sure that your new gene was properly attached to a special gene that acts like an “on switch” for the gene you want to splice. Otherwise your new gene might never work. After all, neither the plasmid nor the bacteria you are putting it into have any real use for the new gene. The gene is not doing them any good, so why should the bacteria waste time and energy producing whatever the gene codes for?

The idea of the “on switch” is to trick the bacteria into thinking they are producing something they need, when really they are producing something you need. The switches are called “regulatory genes.”

Now, put the combined regulatory gene and the new gene together and mix them up with lots of sticky plasmids. Some of the plasmids will hook up with the new genes and form themselves back into loops. Next, put the “spliced” plasmids in a dish with lots of bacteria, and some of the bacteria will absorb some of the plasmids. Bacteria swap plasmids commonly. Plasmids, for example, are usually where they get new genes that make them immune to antibiotics.

If all has gone well, at least some of the bacteria will have absorbed plasmids with your new genes on them, and at least some of the plasmids will be operating inside the bacteria, using the bacteria’s ribosomes and other “workers” to produce whatever you want produced. The bacteria have become a tiny “factory” at your service. But this factory has the special advantage of reproducing itself. The bacteria divide and produce more bacteria, all containing your special gene, all making the protein you want.

[Picture on page 8]

gene + plasmid = modified plasmid → absorbed by bacterium