NEW DNA REPAIRING ENZYME






Information of all living creature is stored on DNA double helix molecule. Just five round pinheads full of DNA could hold all the information of the earth’s entire human population. Just one of these pinheads would have 2 million times the information content of a 2 TB hard drive. And each of our 100 trillion cells has 3 billion DNA ‘letters’ called nucleobases worth of information.
But chemically, DNA is actually a very reactive molecule and RNA is even more so, so it’s highly implausible that it could have arisen in a hypothetical primordial soup. Indeed, about a million DNA ‘letters’ are damaged in a cell on a good day. One common form of DNA damage is called alkylation—this means a small hydrocarbon group is attached to one of the ‘letters’, and there are many places for the attachment. This changes the shape enough so it can no longer fit into the double helix. This can prevent DNA replication or reading the gene.
 


So living creatures must have elaborate DNA repair machinery. University of Chicago biologist James Shapiro points out that:  all cells from bacteria to man possess a truly astonishing array of repair systems which serve to remove accidental and stochastic sources of mutation. Multiple levels of proofreading mechanisms recognize and remove errors that inevitably occur during DNA replication. … cells protect themselves against precisely the kinds of accidental genetic change that, according to conventional theory, are the sources of evolutionary variability. By virtue of their proofreading and repair systems, living cells are not passive victims of the random forces of chemistry and physics. They devote large resources to suppressing random genetic variation and have the capacity to set the level of background localized mutability by adjusting the activity of their repair systems.
For example, there is ‘base excision repair’: special enzymes called DNA glycosylases run down the DNA molecule, detect the damaged ‘letter’, grab it, put it in a specially shaped pocket, then chop it out. Then other enzymes repair the resulting gap.




Scientists at North American universities have discovered another ingenious repair enzyme in bacteria, called AlkD. This has a very different structure. It works by flipping a positively charged damaged base—highly unstable—and the one it’s paired with, from the inside to the outside of the helix. Then they are both detached, and the gap filled. Understanding these enzymes could lead to more effective chemotherapy.