NEW DISCOVERY OF MicroDNA CIRCLES: Your genes and mine differ at thousands of individual points (SNPs, or single nucleotide polymorphisms) and in the number of copies of individual genes (CNV, or copy number variations). But did you know that there are differences in DNA between the individual cells of your body?
• Searching for evidence of genetic mosaics within the brain, researchers found tens of thousands of tiny circles of DNA representing a completely different form from the usual chromosomes. These microDNA circles are typically 200-400 base pairs long, and represent only 0.2% of the total chromosomal DNA, but given that we have 3,156,105,057 base pairs of DNA that’s still a lot. They are thought to arise from genes due to errors in replication or repair. Both single and double stranded circles were found. The electron microscope image shows a microDNA circle with a larger molecular model in color on the outside.
• Researchers reasoned that microDNA would leave behind microdeletions, or small gaps in the chromosome. Indeed, when they set about looking for these gaps, they found them at a rate of 1 in 2000, giving rise to considerable genetic variation between cells. Not only could these deletions potentially affect gene function, but they may serve as a genetic cache of information that may play a role in non-Mendelian inheritance.
• Fun fact: Do you know the origin of the word genome? It is thought to come from the Greek (γίνομαι) for “I become, I am born, to come into being”.
• Source: Extrachromosomal microDNAs and chromosomal microdeletions in normal tissues. Shibata Y, Kumar P, Layer R, Willcox S, Gagan JR, Griffith JD, Dutta A. Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA, USA. Science. 2012 Apr 6;336(6077):82-6. http://www.ncbi.nlm.nih.gov/pubmed/22403181