P53 Apoptosis Cancer

3 Easy Steps to Transgenic Mice

A strand of DNA is like a large library, composed of individual books called genes, each of which hold specific instructions. The onset of cancer, as discussed earlier, is a result of errors in these instructions, which can be likened to pages falling out of a book, individual sentences being marked out or added, or even a complete removal of the said book. Each of these changes is referred to as a mutation and it typically takes a chain of such mutations to create cancer.

Scientists, in their research, produced a set of transgenic mice, or mice which are made to express inappropriately the myc oncogene with mutations that are linked to certain cancer types. The first step in this procedure is DNA Recombination.

Image, used with permission, from http://www.accessexcellence.org/AB/GG/transgenic.html

DNA Recombination

DNA Recombination, put simply, is inserting a desired gene, which codes for a specific trait, into a stretch of DNA. This is analogous to the above-mentioned circumstance of removing pages from a book or adding sentences or completely deleting the book from the library. To accomplish this, the desired gene is inserted into a small strand of DNA, called a vector. Typically, a bacterial plasmid is used as this vector. Plasmids are circular strands of DNA, which are exchanged between bacteria as a form of sexual reproduction. Scientists extract these plasmids and then insert the gene that they want into it. The end product is the vector, which contains the gene of interest. The next process is described by some to be Introduction.

Injection of desired gene

Image, used with permission, from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/TransgenicAnimals.html)

There are two accepted methods in accomplishing this next step. As you can see from the diagram, one involves adding the vector to stem cells, and the other requires it to be incorporated into a fertilized egg. In the case of the mice used for Dox Treatment, a virus was used as a carrier to release the plasmids into the fertilized eggs. Regardless of which method is used, the final step involves implanting the vector into the mother.

Implantation and subsequent creation of transgenic mice

This last step is often referred to as in vitro fertilization. The created embryo at the end of the above two steps is now injected into the uterus of a female mouse, now called a surrogate mother (see diagram). If all goes well, the mother will soon give rise to a family of baby mice, some of who will express the desired trait.

Transgenic Mice and Dox Treatment

The scientists use a regulatory device, called the tetracycline regulatory system, to either turn on or off expression of the myc oncogene, encoded in the aforementioned vector, in the process called Dox Treatment. The ability to regulate this gene expression is what has made the initial experiments successful. The tetracycline regulatory system acts as a switch through an exogenous agent, doxycycline,a tetracycline derivative. It is only with this tetracycline derivative that such control could exist. The end product is controlled expression of the myc oncogene. This type of control is known as conditional expression, meaning it is only turned on under certain circumstances. The next page goes into detail on how this experiment was conducted by the scientists.

Sources:

1. http://isi3.webofscience.com/CIW.cgi?PeJyWccEmxoAADi5eo8_3D2D9209_PeJyWccEmxoAADi5eo8-0

2. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/TransgenicAnimals.html

3. http://www.accessexcellence.org/AB/GG/transgenic.html

Back | Forward

Group Number 7

http://student.biology.arizona.edu/honors2002/group07/home07.html

Last Updated: Dec 9, 2002