Chromosomes Organize into “Yarns”

Chromosomes, the molecular basis of genetic heredity, remain enigmatic 130 years after their discovery in 1882 by Dr. Walther Flemming. New research published online on April 11, 2012 in Nature by the team of Edith Heard, Ph.D., from the Curie Institute and Job Dekker, Ph.D., from the University of Massachusetts Medical School (UMMS), reveals a new layer in the complex organization of chromosomes. The scientists have shown that chromosomes fold in a series of contiguous "yarns" that harbor groups of genes and regulatory elements, bringing them in contact with each other and allowing them to work in a coordinated manner during development. Chromosomes are relatively large molecules that, when spread out, can measure up to the length of an entire human arm. Despite their size, however, they are actually confined within the small space of the cell nucleus which is just a few micrometers in size. Furthermore, within each cell nucleus are multiple chromosomes. In humans, for example, there are 23 pairs of chromosomes. In order to fit all this material into this small area, chromosomes are folded, compacted, and mingled in the three-dimensional space of the nucleus. So do chromosomes fill the nucleus just like spaghetti fills a plate? "Not quite," said Elphege Nora, PhD, a post-doctoral fellow on the team of Dr. Heard, head of the Genetics and Developmental Biology Lab at the Curie Institute. "Chromosome folding follows a pattern, and this actually turns out to be important for ensuring their proper function." "We have known for decades that the DNA of individual genes is wrapped around nucleosomes to form the classical 'beads-on-a-string' structure," said Dr. Dekker, co-director of the Program in Systems Biology at UMMS.
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