We have so far defined a cell, compared the two different categories of cells: eukaryotic and prokaryotic, outlined the functions of organelles, and discussed the nucleus of a cell. As we continue diving into a cell, let's take a closer look at how our genetic information is packaged and safely stored.
If one were to take the DNA from one of your cells, and stretch it out from end to end, it would be approximately 2 metres long [1]. However, the average diameter of a human nucleus is only about 10 microns - which means it is 200 000 times smaller than the total length of DNA! Yet, all the genetic material is still somehow able to fit neatly and compactly within a nucleus, and be readily accessible whenever the cell needs it. Humans, and other eukaryotic organisms in general, have overcome this size and packaging paradox by creating chromatin.
What is chromatin?
Chromatin is a combination of DNA and proteins, which together create the chromosomes of a cell. The basic repeating unit of chromatin is the nucleosome, the first structure to genome compaction and organization [2].
The nucleosome
Nucleosomes are composed of two copies of four proteins: Histone 3 (H3), Histone 4 (H4), Histone 2A (H2A) and Histone 2B (H2B). One can imagine a nucleosome being similar to the beads of a pearl necklace. These eight histones are positively charged, which facilitates the winding of the negatively charged DNA around them - in this case the DNA will act as a necklace thread. The space in between each nucleosome is linked together by “linker DNA,” and the entire structure is commonly known as the “beads-on-a-string” [3-4]. However, this arrangement is still not sufficient to ensure the 2 metres worth of DNA can fit within a nucleus, thus a greater level of compaction is still required.
Figure 1. Nucleosome structure.
Further compaction?
To gain an even higher order of packaging, eukaryotic cells employed yet another protein known as Histone 1 (H1). It binds the linker DNA and organizes the “beads-on-a-string” into a more compact and condensed structure [4].
Figure 2. Compacter structure with linker histone (H1).
However, the latest structure has not been truly studied, and there are ongoing debates on whether this compaction exists [5]. Experiments done in labs have shown that arranging histones and DNA within test tubes containing the right conditions, can lead to the development of this compacted structure. Whereas chromatin in its natural environment is shown to be more dynamic and would not necessarily need this structure. As of now, the current consensus is that this structure does exist, but it is far more complex and disordered than what is seen in test tubes [5].
Although the last stages of compaction are not clearly defined at the moment, the compaction from the nucleosomes allow DNA to fit within a tiny nucleus. To learn how DNA is unwrapped and read, stay tuned for next week’s post!
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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:
Piovesan, A. et al. (2019) "On the length, weight and GC content of the human genome", BMC Research Notes, 12(1). Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6391780/
Kornberg, RD. (1977) “Structure of chromatin”, Annual Review of Biochemistry, 46:1, 931-954. Available at: https://www.annualreviews.org/doi/abs/10.1146/annurev.bi.46.070177.004435
Luger, K. et al. (1997) "Crystal structure of the nucleosome core particle at 2.8 Å resolution", Nature, 389(6648), pp. 251-260. Available at: https://www.nature.com/articles/38444
Li, G. and Zhu, P. (2015) "Structure and organization of chromatin fiber in the nucleus", FEBS Letters, 589(20PartA), pp. 2893-2904. Available at: https://www.sciencedirect.com/science/article/pii/S0014579315002811#f0020
Mansisidor, A. and Risca, V. (2022) "Chromatin accessibility: methods, mechanisms, and biological insights", Nucleus, 13(1), pp. 238-278. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9683059/
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