section_path: Properties/Non-canonical bases
section_name: Non-canonical bases
paragraph_text: Non-canonical bases
  
  Modified bases occur in DNA. The first of these recognized was 5-methylcytosine, which was found in the genome of Mycobacterium tuberculosis in 1925. The reason for the presence of these noncanonical bases in bacterial viruses (bacteriophages) is to avoid the restriction enzymes present in bacteria. This enzyme system acts at least in part as a molecular immune system protecting bacteria from infection by viruses. Modifications of the bases cytosine and adenine, the more common and modified DNA bases, play vital roles in the epigenetic control of gene expression in plants and animals.
  
  A number of noncanonical bases are known to occur in DNA. Most of these are modifications of the canonical bases plus uracil.