Stories about: epigenetics

Mapping out DNA’s extra bases

5-methylcytosine (L) and 5-hydroxymethylcytosine (R): the two DNA bases you didn't learn about in high school biology. (Image: Wikimedia Commons)

Adenine and thymine, cytosine and guanine. We all learned the names of the four DNA bases in high school biology class. But, just like the list of planets, the list of bases may not be set in stone.

Over the years, epigenetics researchers have identified two alternate forms of cytosine whose biology differs enough from that of their parent base that may count as fifth and sixth DNA bases. These additional cytosines, called 5-methylcytosine (5mc) and 5-hydroxymethylcytosine (5hmc), each have a group of atoms called a methyl group added onto their central ring, a feature normal cytosine lacks.

Apart from making biology textbook editors very unhappy, these two bases may play unique roles in biology. Adding methyl groups (or methylation “marks”) to cytosines and other components of the genome is a well-known epigenetic mechanism that gives the cell exquisite control of gene activity, which in turn greatly influences how the cell will behave. For instance, patterns of methylation marks on the genes of embryonic stem (ES) cells are linked the cells’ ability to develop into more mature cells. And the genomes of cancer cells often have methylation marks in the wrong places.

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