When a gene "switches on," it eventually makes a protein, but it does not do so directly. First the gene codes an intermediary molecule called mRNA. To transfer a gene's information from DNA to mRNA, base pairing is used. However, there is one change: an adenine base (A) in the DNA matches with a new base called uracil (U) in the mRNA. This difference helps to distinguish mRNA from DNA.
![Genes Switch On](https://webarchive.library.unt.edu/eot2008/20090131033815im_/http://newscenter.cancer.gov/images/Documents/5d8fef4b-34da-44ba-8087-191d81c95c88/cancer7.jpg)
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