Cloning and gene isolation

What does "clone" mean?

If you look up the word clone in an older dictionary, you'll find the following definitions.

clone (noun): the stock of individuals derived asexually from one individual

clone (verb): to propagate asexually from an individual

These definitions describe common, old-time horticultural practices used by growers who want plants that are exactly like their parents. Because seeds don't produce plants that are exactly like their parents, growers had to find a way to grow new plants from parts of the parent plant. For example, all Golden Delicious apple trees came from part of one original plant or its cloned descendants. Another example of cloning is the sheep Dolly, the first cloned mammal, grown from the mammary cells of another sheep.

In the current era of biotechnology and genetic engineering, cloning takes on another meaning. Now, we use cloning to describe the isolation and movement of genes.

clone a gene (verb): to incorporate a piece of DNA in a vector which can replicate in another organism

How hard is it to clone a particular gene?

Until all the DNA of an organism is mapped out, this is a tough job.

The first task is to find the gene you want. There are about 100,000 genes in the human genome. So at its simplest, we have a 1 in 100,000 chance of picking the gene with the right piece of DNA. Because of the way the genes are organized, the chances of getting the right gene are actually 1 in 1,000,000. If the genes aren't grouped and organized, it's like looking for a needle in a haystack. Not very good odds.

However, a variety of new technologies, such as the polymerase chain reaction (PCR), and the development of large scale DNA sequencing programs, have now made gene cloning pretty easy. Also, researchers who have mapped out portions of an organism's genes keep that information as a sort of "gene library." They often share the information with other researchers who study the same organism. This helps speed the process.

A more difficult task nowadays is determining the function of all these genes that have been cloned, so called "functional genomics". The genome is analogous to a library where the individual books are equivalent to genes in the genome. In the nucleus, all of these books are mixed together, like having all the books piled up in a jumble on a table. It is virtually impossible to find the book you want among the mess on the table. However, when all the books are suitably filed on the shelves, it is possible not only to find the specific book that interests you, but also to take the book from the shelf and read its contents.

In 1995 the first complete DNA sequence of a free living organism was published. By now, many more bacterial genomes have been sequenced. The first eukaryote genome (brewers yeast) has also been completed. It is likely that the entire sequence of the human genome will be completed by 2005.