more on dynamics

As one of the previous posts describes, heterochromatin is not always cold and inert. It goes through dynamic transcription during cell cycle progression. The fact that all of the earlier studies used mixed population of cells is the reason of the controversial heterochromatin transcription.

To expand this a little bit, in fact most of modern molecular and cellular biology researches (except cell cycle studies) are done using cells staged at different cell cycle phase. If you think about it, the cell cycle approach could be applied to most of these researches. It is not unexpected if a lot of our current knowledge is refined by taking this approach, for example cancer formation and development. A new science direction is approaching.

an important example of heterochromatin, showing the major heterochromatin regions in mouse and its amazing global structure in a cell. The detail properties will be described later on.

http://www.jcb.org/content/vol166/issue4/images/large/200403109f1.jpeg

(copyright JCB)

marks

These are two important hallmarks of heterochromatin:


Cytological level - electron microscopy picture


molecular level: HP1-H3K9Me and the HMTase

dynamic heterochromatin

This is relatively new. But I think it is necessary to put it here before all the historical information. You have to keep this dynamic thing in mind whenever you think about heterochromatin. --I am sure this will be huge in the future.


http://www.jcb.org/cgi/content/full/179/3/411
Proliferation-dependent and cell cycle–regulated transcription of mouse pericentric heterochromatin

http://www.landesbioscience.com/journals/cc/article/6206
Cell cycle regulated transcription of heterochromatin in mammals vs. fission yeast: Functional conservation or coincidence?

wikipedia on heterochromatin

http://en.wikipedia.org/wiki/Heterochromatin

This is a description of heterochromatin on wikipedia. I am amazed that it is basically correct and even covers most of the concepts. It is a good introduction of this interesting kind of chromatin.

However, I have to correct some of the obvious mistakes.

1. first paragraph. "Its major characteristic is that transcription is limited." This is definitely going to change with the current research showing transcription from heterochromatin in many species. At least it is not the "major" characteristic anymore. The concept of heterochromatin is being reshaped dramatically recently.

2. still first paragraph. "As such, it is a means to control gene expression, through regulation of the transcription initiation." this is correct but certainly not the most important function of heterochromatin. Also it is not clear if the regulation is only through transcription initiation.

3. structure: about genes in heterochromatin regions, there are some, especially in fruit fly. About its replication timing, in most species heterochromatin replicates late in S phase. But in fission yeast, it replicates very early. --Joel Huberman nicely demonstrated this years ago.

4. On the potential involvement of RNAi in heterochromatin formation in higher eukaryotes like mammalian systems, the evidence is very controversy currently. Almost all supporting evidence came from one single lab. My opinion is that it is not as general. There might be some similarity, but models of fission yeast cannot be simply adopted to mammalian systems.

I will expand this hot area later on.

5. One important thing missing is the molecular hallmark of heterochromatin, i.e. the histone H3 methylation at lysine 9 (H3K9)- HP1 (heterochromatin protein 1) interaction and the histone H3K9 methytransferases.

Ok. Enough for wikipedia.

more coming...