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So what is chromatin?
Chromatin is an extraordinarily complex structure of DNA, RNA, and proteins. The major group of proteins that make up chromatin are histone proteins. During cell division chromatin condenses to form chromosomes. It is essentially the raw material of chromosomes and it exists in two states - heterochromatin and euchromatin.
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What is the job of chromatin?
Chromatin carries out several important functions:-
- It packages DNA into a smaller volume to ensure that it fits inside the cell
- It is involved in the regulation of processes such as gene expression, DNA replication, transcription, and recombination
All these processes need ready access to DNA, which is controlled by the chromatin structure.
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What is the structure of chromatin?
This is a question that has stumped scientists for more than 30 years, and two alternative models exist to explain chromatin's internal structure, and how the nucleosomes are arranged.
First of all a definition - nucleosomes are the fundamental units of chromatin and consist of tightly coiled DNA around a histone protein spool.
The two chromatin models:-
1) A solenoid formation - here the nucleosomes follow a helical path so that adjacent nucleosomes lie side by side
2) A zigzag formation - here, according to some researchers, the nucleosomes follow a zigzag path along the chromatin fibre so that adjacent nucleosomes are opposite each other.
In August 2009 scientists from UC San Diego Jacobs School of Engineering, Penn State University, University of Massachusetts, and New York University came up with a third model, and it consists of both the solenoid and the zigzag formation that are in balance with each other and present at different times - a heteromorphic structure.
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Who discovered chromatin?
German biologist Walther Flemming (1843-1905) discovered chromatin when he was investigating the process of cell division. "Further study of the division phenomena requires a brief discussion of the material which thus far I have called the stainable substance of the nucleus. Since the term nuclear substance could easily result in misinterpretation..., I shall coin the term chromatin for the time being."
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What is the relationship between chromatin and cancer?
Cancer is a disease of cells and abnormal gene regulation. Chromatin plays a key role in maintaining the genetic stability of a cell, and this stability is compromised during tumourgenesis. Therefore, chromatin is a focal point of large bodies of research that are trying to understand how it contributes to stability. In addition, it's suspected that some of the proteins inside the chromatin structure are mutated or damaged in some way which can affect tumour suppressor genes.
Then there is the fact that DNA rarely exists on its own in higher organisms; it is nearly always associated with proteins inside chromatin. How chromatin exerts its influence over DNA to turn genes on or off is largely unknown. Understanding how it exerts its control will be a major advance in cancer studies.
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Grigoryev et al. Evidence for heteromorphic chromatin fibers from analysis of nucleosome interactions. Proceedings of the National Academy of Sciences, 2009; DOI: 10.1073/pnas.0903280106
Walther Flemming quote - www.todayinsci.com