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Only Half of the Chromosome Consists of DNA

Since their first discovery in 1882, mitotic chromosomes have been the subject of intensive study.

Remarkably, despite significant advances in light and electron microscopy over the years, the detailed organization of chromosomes remains a mystery. A research team of scientists at the University of Edinburgh developed a precise method of microscopy that allows them to examine the structure of chromosomes in detail. This method, called

3D-CLEM , combines light and scanning electron microscopy with computer modeling software to produce high resolution 3D images of chromosomes.

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Figure 1. Booth et al developed a microscopy method called 3D-CLEM . They challenged the classical definition of chromosomes and found with this method that the majority of mitotic chromosomes are not composed of chromatins (1).

The researchers 3D-CLEM method used the length, width, surface area, volume of all normal human chromosomes, DNA packing density and a small synthetic artificial chromosome . 19459009] allows determination of DNA content.

Edinburgh University – Department of Biological Sciences, Dr. Daniel Booth (co-lead author) adds that the imaging technique they developed for studying chromosomes was truly groundbreaking. “Describing the structure of all 46 human chromosomes for the first time forced us to rethink the idea that chromosomes consist almost entirely of chromatin; this is an assumption that has not been discussed for nearly 100 years. ”

Figure 2. An upper mitotic RPE (retinal pigment epithelium) cell. The images below show the complete set of chromosomes (i) and the split chromosomes (ii); arrows represent single chromosome patterns in panels zoomed in; these include metacentric (light blue) and submetacentric (red, orange) chromosome samples. Scale bar – 10 μm. (1).

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Analysis of the images DNA and the material containing the promoter protein known as chromatin constitute 53% -70% of the total chromosome content.

The remaining 30% -47% consists of the outer surface of the chromosome.

With the exact function of this sheath unknown, Dr. Booth and colleagues suggest that it can isolate chromosomes from each other during cell division. They say that this so-called chromosome surface can help prevent errors (a sign of some types of cancer and diseases associated with birth defects) that occur during division of cells.

Also from Edinburgh University – Department of Biological Sciences, Prof. Bill Earnshaw (co-lead author) adds: “We now have to rethink how chromosomes are formed and how they separate during cell division, because the genetic material is covered by this thick layer of material.”

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Further Reading
1) Booth D, G., Beckett AJ, Molina O., Samejima I., Masumoto H., Kouprina N., Larionov V., Prior I., A., Earnshaw WC 2016. 3D-CLEM Reveals that a Major Portion of Mitotic Chromosomes Is Not Chromatin. Mol Cell. 2016 Nov 17; 64 (4): 790-802. doi: 10.1016 / j.molcel.2016.10.009. Epub 2016 Nov 10.

Visual Sources
Cover Image: https://images.sciencedaily.com/2016/11/161121170310_1_900x600. jpg
Figure 1: http://cdn.sci-news.com/images/enlarge3/image_4393_1e- Chromosome-DNA.jpg
Figure 2: http: //cdn.sci-news.com/images/enlarge3/image_4393_2e- Chromosome-DNA.jpg

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