CpG island - Definition (Aug/18/2005 )
What is CpG island, and I heard that some viruses have CpG DNA are they the same thing or what exactly???
If I'm remembering this correctly, but I should do a pubmed search also, cause there are loads of papers out there abut it. A CpG island is a region of DNA rich in C and Gs, which is unusual because 5 methyl dCTP when demethylated goes to thymine (I think, not sure if remembering this right) so C and Gs are depleted from the genome. CpG islands are somehow protected, I think they are regions of high expression, so don't get methylated
two definitions of a CpG island:
Frommer 1987 Any stretch of DNA greater than 200bp with a GC content of greater than 50% and an observed to expected CpG ratio of greater than 0.5
The new definition by Takai 2002, Any stretch of DNA greater than 500bp with a GC content of greater than 55% and an observed to expected CpG ratio of greater than 0.6
basically axlotl is right, but here are the formal definitions.
Now I'm getting confused. Methylation is a way of gene regulation. I thought there are a lot of genes of which the promoter is always methylated in cell types where the expression should be shut down. Am I wrong?
Methylation may control tissue or cell type specific gene expression. This is currently just a hypothesis, has not been confirmed yet.
methylation has a lot of functions - its role in gene regualtion is most obvious in the X chromosome - the Barr Bodies and areas where methylation has "switched off" the gene.
Methylation has aslo been seen to be irregular in some diseases such as Prader Willi Syndrome - however it is a wide, confusing and largely unexplored facet of the human genome
it's certainly being explored right now, very very exciting!