How is Genetic Testing Embryos Performed?
The process of screening embryos for genetic abnormality has become much more useful because of a new technology called Comparative Genomic Hybridization (CGH). It is now possible to determine the complete chromosomal composition of an embryo (that is all 46) from testing a single cell.
Each chromosome is composed of two strands of DNA that are chemically attracted to each other in a way that holds them together for their entire length. These two strands of DNA are referred to as complementary strands and it is the chemical attraction between complementary strands that is the basis for testing chromosomes with CGH. CGH is done by first making many copies of the DNA obtained from the nucleus of the cell which has been removed from the embryo to be tested. The two strands of DNA are then separated and only one strand from each chromosome is used for testing. The DNA is then broken up into very small pieces and then stained with a dye of a certain color.
DNA obtained from a cell which is known to have normal chromosomes is then broken up into the exact same small pieces and stained with a dye of a different color. A glass slide is then coated with the pieces of DNA that are from the complementary strand of the stained pieces of DNA to be tested. These pieces of DNA are from a cell with normal chromosomes and are arranged in a pattern on the slide referred to as a microarray. Because of their ability to attract the complementary strand, pieces of DNA on the microarray will attract the stained pieces of DNA when it is added.
The stained pieces of DNA from the test cell and the control cell are then added to the microarray. Since each cell has been stained a different color, it is possible to see how much DNA from each cell has been attracted to the pieces of DNA on the microarray by determining how much of each color is present. The pieces of DNA on the microarray represent all of the chromosomes in a normal cell and the location of all of the pieces that make up each chromosome are known. Therefore it is possible to determine how many copies of each chromosome are present by looking at the amount of each color that is present on all of the pieces of DNA that make up a particular chromosome on the microarray. For example, if the test cell has two copies of a particular chromosome then it is normal. So when the microarray is examined, there would be an equal amount of each color present for that chromosome since the control cell also has two copies of each chromosome. If however, the test cell has three copies of a particular chromosome, then it is abnormal and referred to as a trisomy. This is the case in the condition known as Down Syndrome where there are three copies of chromosome 21. The microarray from a trisomy would have more of the color from the test cell than the color from the control cell.