DNA and the Developing Embryo

Inside almost every cell in every living organism is a long, twisted, ladder-like molecule known as DNA. The information contained in the DNA molecule provides a "blueprint," or a set of codes, for building other molecules used by the cell.

As the organism grows, different parts of the DNA molecule, called genes, are decoded and read by the cells. Each gene contains instructions for building a particular molecule that's needed by the growing body.

As the organism develops, different genes in different cells may be read. The ultimate fate of any one cell--whether it becomes a skin cell, a nerve cell, a kidney cell, or a bone cell--depends upon which genes are read. If any of the genes are missing, if they are misread, read out of sequence, or altered in any way, the cell or organism may dramatically change.

The timing of this process depends on the embryo's stage of development and the location of the cell. Developing eye cells, for instance, use different combinations of genes than do brain cells or skin cells. The embryos here are all in the same stage of development. Their physical similarities and differences correspond to variations in their genes.