Although every cell in the human body has essentially the same DNA, as indicated in the epigenetics post, they are not the same and they do not behave the same. This difference in behaviour and in morphology, is based on how the genetic material is used, and when/which genes are active. One way cells have accomplished this is by utilizing chromatin in different ways. Chromatin is divided into two regions: euchromatin or heterochromatin; and the distribution of genes within them determines how a cell behaves - keep in mind that it differs between each type of cell.
Euchromatin
This part of the chromatin predominantly holds the active genomes. It is characterized by regions of the genome where nucleosomes are widely or loosely packaged, and certain histones are modified with distinct chemical tags [1]. These tags not only ensure euchromatin remains loosely packed, but also sends signals to the cell machinery that these regions of the genome should be active. Most cellular activity originates in the euchromatin of a cell.
Heterochromatin
These regions, similar to euchromatin, are also chemically tagged, but differ by holding inactive genomes and arranging their nucleosomes in a compact manner, making the genes less accessible to be translated. If these inactive genomes activate, the genome would be unstable, and those unnecessary genes could activate and alter the function of the cell. In fact, many studies show that activating inactive genomes is commonly linked to cancer cells, therefore, heterochromatin’s silencing is crucial [2-6].
These two regions are very important to create different types of cells and their functionality. All the cells have the same genome, but for example, the euchromatin region in cardiac cells contains the genes that are important for its function, while specific genes for liver function are inactive in the heterochromatin region. On rare occasions, cancerous or benign tumours (teratoma) composed of diverse types of tissues like bone, teeth or hair can growth in unique areas like the pelvis or the liver [7-9]. This is due to the euchromatin and heterochromatin roles being undefined, leading to the activation of genes that should be inactive. Even though teratomas are not fully understood, their development highlights how remarkable and dynamic our cells are.
In conclusion, cells hold all types of genes, and it is extraordinary how the combination of euchromatin and heterochromatin can activate and deactivate different genes to make specific types of cells. Not only do they control gene expression, but they also play a role in packaging the eukaryotic genome (more information here).
Figure 1. Highlight of the euchromatin and heterochromatin regions in the chromatin.
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Written by: Renard
Edited by: María and Natasha
BioDecoded is a volunteer group committed to sharing accurate scientific information. If you have any questions about this topic or would like to learn more, please comment below or send us your questions.
References:
Venkatesh S. and Workman JL (2015). “Histone exchange, chromatin structure and the regulation of transcription”, Nature Reviews Molecular Cell Biology; 16 (3):178-189. Available at: https://pubmed.ncbi.nlm.nih.gov/25650798/
Allshire, R. and Madhani, H. (2017) "Ten principles of heterochromatin formation and function", Nature Reviews Molecular Cell Biology, 19(4), pp. 229-244. Available at: https://www.nature.com/articles/nrm.2017.119
Pageau GJ, et al. (2007). “The diappearing Barr body in breat and ovarian cancers”, Nature Reviews Cancer;7(8):628-633. Available at: https://pubmed.ncbi.nlm.nih.gov/17611545/
Hansen KD, et al. (2011). “Increased methylation variation in epigenetic domains across cancer types”, Nature Genetics; 43(8):768-775. Available at: https://pubmed.ncbi.nlm.nih.gov/21706001/
Zhu Q., et al. (2011). “BRCA1 tumour suppression occurs via heterochromatin-mediated silencing”, Nature; 477(7363):179-184. Available at: https://pubmed.ncbi.nlm.nih.gov/21901007/
Ehrlich M., (2009). “DNA hypomethylation in cancer cells”, Epigenomics; 1(2):239-259. Available at: https://pubmed.ncbi.nlm.nih.gov/20495664/
What Is a Teratoma? (2023). WebMD. Available at: https://www.webmd.com/a-to-z-guides/what-is-teratoma (Accessed: 28 May 2023).
Da Silva TK., et al. (2015). “Teratoma: a set of teeth in the pelvis”, Radiologia Brasileira; 48(4):263-264. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4567366/
Kovalenko, Y. et al. (2021) "Rare primary mature teratoma of the liver: A case report", World Journal of Hepatology, 13(12), pp. 2192-2200. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8727195/
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