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Regulating the nuclear architecture of the cell

some inheritance, and other processes. Karpen and Peng have identified the molecular pathways that regulate two of heterochromatin's important functions.

One of these is control of repeated DNA sequences in and outside the heterochromatin. The other is the organization and structure of the nucleolus, the ribosome factory, which is situated at the specific site in the heterochromatin where ribosomal DNA -- consisting of large genes, repeated 300 to 400 times -- codes for the production of the RNA from which ribosomes are built.

"This work on pathways that control the organization of the nucleolus is the first to be published that deals with an organelle," says Karpen. "Even though the gross organization of chromosomes and other nuclear elements is well known in cell biology, learning about the regulation of nuclear architecture is in its early stages."

The most striking feature of any nucleus is its chromosomes. These are made of chromatin, which combines DNA with a set of proteins known as histones; four similar histones join together to form a cylindrical spool around which the DNA wraps. Each of these bundles is called a nucleosome, and many nucleosomes are bound together by the continuing strand of DNA, which forms a string of beads that further coils to form one of two kinds of chromatin, either euchromatin or heterochromatin.

Most genes reside in euchromatin, which is of relatively low density and where the DNA is more accessible to the machinery of gene transcription. By contrast, heterochromatin is dense and contains relatively few genes; most of the DNA in heterochromatin, including numerous short repeated sequences, does not code for proteins.

Heterochromatin is typically found at the ends of a chromosome, where it abuts the telomeres -- chromatin structures best known for limiting, by their diminishing length, how many times a cell can replicate. Heterochromatin also flanks the centromere in the cent
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Source:DOE/Lawrence Berkeley National Laboratory


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