Heterochromatin

Transcriptionally inactive chromatin is densely packed during interphase as observed by electron microscopic smdies and is referred to as heterochro-matin transcriptionally active chromatin stains less densely and is referred to as euchromatin. Generally, euchromatin is repficated earfier than heterochromatin in the mammafian cell cycle (see below). 

Goff SP (2001) Retroviridae the retrovirases and their rephcation. In Knipe DM, Howley PM (eds) Fields virology, 4th edn. Lippincott Williams and Wilkins, Philadelphia, pp 1871-1940 Grewal SI, Moazed D (2003) Heterochromatin and epigenetic control of gene expression. Science 301(5634) 798-802... 

On replication, insertion or deletion of bases may occur. Chain scission and chromosome breaks are also possible. Quinacrine is useful in human cytogenetics, since it intercalates significantly into the heterochromatin of the Y chromosome, making it fluoresce and rendering it identifiable cytologically. Detection of the Y chromosome is important in prenatal sex determination. Other dyes present in our environment are potentially mutagenic. For example, some hair dyes were shown to be mutagenic for E. coli. 

The state of the chromatin has influence as well on the velocity with which is produced the recognition between the receptor dimer and the HRE sequences. The inactive heterochromatin has methylated histones, so that a different... 

A 35- to 40-fold incorporation (relative to other fractions) of labelled mercury into a nonhistone fraction of rat kidney nuclei has been reported [44]. By using flameless atomic absorption, a 12 to 15-fold enrichment of mercury was found in the euchromatin fraction of mouse liver nuclei [45, 46]. Mercury was not detected in the inactive heterochromatin. 

In addition to the role for the nucleosome-nucleosome interactions, the histone tails are known as the region that undergoes post-transcriptional modifications, such as acetylation, phosphorylation, and methylation (Fig. Ic) (Peterson and Laniel, 2004). These modifications trigger the formation of euchromatin (acetylation), heterochromatin (methylation), or metaphase chromosome (phosphorylation). The details of these modifications will be described in chapters 8-11. 

Acetylation of the histone tails correlates with the activities of genes (Kimura et al., 2005). However, the detailed analyses of the acetylation on the individual lysine residue have revealed that the relationship between the acetylation and the chromatin-compaction is not simple. There are 1-6 lysine residues in each histone subunit, that could be acetylated the Lys of H2A, Lys, Lys, Lys, and Lys ° ofH2B, Lys , Lys , Lys, Lys, Lys, andLys of H3, and Lys, Lys, Lys, and Lys of H4. In mammal, more than ten HATs (7/istone Acetyl Transferases) have been identified, each of which acetylates a specific lysine residue. Acetylation frequently occurs in euchromatin regions, and some in heterochromatin regions. For example, the acetylation of Lys of H4 leads to a telomeric silencing (Kelly et al., 2000). In Drosophila, Lys of H4 is acetylated specifically in the... 

Close observations of immunofluorescence signals showed that there are three different staining patterns, which correspond to three different nucleolar compartments FC (Fibrillar center), DFC (dense fibrillar component), and GC (granular component). Nucleolus is surrounded by heterochromatin. When the cells are in very active state of its proliferation, the nucleolar compartments and heterochromatin are integrated into a highly intricate structure called nucleolonema . A recent study has suggested that the chromatin associated with the nucleolus is less mobile than... 

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Hiragami K, Festenstein R (2005) Heterochromatin protein 1 a pervasive controlling influence. Cell Mol Life Sci 62 2711-2726... 

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Meneghini MD, Wu M, Madhani HD (2003) Conserved histone variant H2A.Z protects euchromatin from the ectopic spread of silent heterochromatin. Cell 112 725-736 Mermoud JE, Costanzi C, Pehrson JR, Brockdorff N (1999) Histone macroH2A1.2 relocates to the inactive X chromosome after initiation and propagation of X-inactivation. J Cell Biol 147 1399-1408... 

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