Transferring Genes Using Microinjection

DNA microinjection methods are used to transfer genes between animals and are a popular technique for creating transgenic organisms, particularly mammals.

An Explanation of DNA

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can be found in the mitochondria, called mitochondrial DNA or mtDNA.

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99% of those bases are the same in all people.

The sequence of these bases determines the information available for building and maintaining an organism. This system is similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

Nucleotides

DNA bases pair up with each other (i.e., A with T, and C with G) to form units called base pairs. Each base is attached to a sugar molecule and a phosphate molecule. When the three are put together (a base, a sugar, and a phosphate) it becomes a nucleotide.

Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA from the old cell.

The Process of DNA Microinjection

In DNA microinjection, also known as pronuclear microinjection, a very fine glass pipette is used to manually inject DNA from one organism into the eggs of another.

The best time for injection is early after fertilization when the ova have two pronuclei. When the two pronuclei fuse to form a single nucleus, the injected DNA may or may not be taken up.

In mice, the fertilized eggs are harvested from a female. The DNA is then microinjected into the eggs, and the eggs are re-implanted into a pseudopregnant female mouse (the ovum is transferred into the oviduct of a recipient female, or foster mother, that has been induced by mating with a vasectomized male).

Results of Microinjection

The University of California (San Diego) Moore's Cancer Center Research and Training Center reports over 80% survival rate for transgenic mouse implants.

The Transgenic Mouse Facility at the University of California San Diego (Irvine) reports an estimated success rate of 10% to 15% based on experiments with mice testing positive for transgenes.

If the DNA is incorporated into the genome, it is done so randomly. Because of this, there is always a chance the gene insert will not be expressed (the cell won't produce the molecules it needs) by the GMO, or may even interfere with the expression of another gene on the chromosome.