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Melting away misconceptions: The strucure of the mitotic chromosome
Posted by peanutbutter in cell biologu on December 9, 2008
I am sure many of you remember sitting in a science class as a child, or an early undergraduate course, being taught about cell replication. How DNA is passed from one cell to the next via either mitosis or meiosis in order to effect DNA replication and gene expression, so that the genetic information content of the DNA can be passed from one generation to the next.
DNA can be organised inside packages within cells. These packages are called chromatin, which are found inside the nuclei of eukaryotic cells and the nucleoid of prokaryotic cells. Chromatin  is a complex combination of DNA, RNA and protein that forms a chromosome.
To date, the commonly accepted hypothesis is that chromatin can take the following three organisational forms
- DNA wrapping around nucleosomes – The “beads on a string” structure.
- A 30 nm condensed chromatin fiber consisting of nucleosome arrays in their most compact form.
- Higher level DNA packaging into the metaphase chromosome
A paper published by Eltsova M, MacLellan KM, et al, 2008 disproves the theory of the existence of a 30 nm condensed chromatin fiber, by providing high resolution in situ images (structures of less than 0.7nm can be resolved) that have not been technologically possible until now. They use a new technique called Cryo-electron microscopy of vitreous sections (CEMOVIS)  which allows them to observe the cells structures in a close-to-native state. The technique of CEMOVIS involves the vitrification of a sample by high pressure freezing which is sliced to 20-100nm sections (thin sectioning) and then is imaged in a cryo-electron microscope, without any further chemical treatment or staining, which ensures immobilization of all of the macromolecules in the specimen in a close-to-native state . Eltsova M, MacLellan KM, et al, hypothesis, that it is the harsh nonphysiological treatments that chromatin samples have been exposed to in other investigations, such as, hypotonic buffers, chemical fixation, alcohol dehydration and embedding in plastic, that might have generated artificial de novo folding of chromatin into 30nm fibres. Instead, their results show no evidence of 30nm fibres. Rather they view a highly disordered and interdigitated state – which they call a chromatin melt.
This paper truly, challenges the fundamental beliefs of what we understand of DNA replication. Not by more theories or hypothesis, however, instead by the actual visualisation of an unordered structure, using an advance technique which they claim allows the close-to-native state observation of biological phenomenon. Can you believe your eyes? As this technique of CEMOVIS matures, what other closely held hypothesis will be challenged with visual evidence?
1. Maeshima K, Eltsov M. Packaging the genome: the structure of mitotic chromosomes. J Biochem. 2008 Feb;143(2):145-53. Epub 2007 Nov 2. [PMID: 17981824]
2. Eltsova M, MacLellana KM, Maeshimad K, Frangakisb AS and Dubocheta J. Analysis of cryo-electron microscopy images does not support the existence of 30-nm chromatin fibers in mitotic chromosomes in situ. PNAS December 8, 2008 DOI: 10.1073/pnas.0810057105
3. Al-Amoudi A, Chang J, Leforestier A, McDowall A, Salamin LM, Norlén LPO, Richter K, Blanc NS, Studer D and Dubochet J. Cryo-electron microscopy of vitreous sections. EMBO J 23:3583–3588. [PMID: 15318169]