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Sunday, January 06, 2008

Euchromatin is the region of DNA that is relatively Uncondensed

013. Euchromatin is the region of DNA that is relatively:

1. Uncondensed.

2. Condensed.

3. Overcondensed.

4. Partially condensed.


1. Uncondensed.


Gray 38th Edition Page 50

Harper 26th Edition page 316








DNA is organized in chromosomes by a heterogeneous set of proteins to form a DNA-protein complex which is called chromatin. The protein constituents of chromatin are the histones and the non-histone proteins. The latter group of proteins is extremely heterogeneous and includes DNA and RNA polymerases, gene regulatory proteins and high-mobility group proteins (HMG proteins). However, the histones are the most abundant group of proteins in chromatin and they are primarily responsible for the 'packaging' of chromosomal DNA (see below). There are five histone proteins: H1, H2A, H2B, H3 and H4, and the last four of these combine to form a compact granule, the nucleosome core. A nucleosome core is a histone octomer comprising two each of the H2A, H2B, H3 and H4 proteins. The DNA molecule winds twice around each nucleosome core in such a manner that 146 nucleotide pairs are organized around it (2.36). This packaging organizes the DNA into a chromatin fibre 11 nm in diameter and gives this form of chromatin the appearance of 'beads-on-a-string' in electron micrographs, with each 'bead' separated by a length of DNA about 50 nucleotide pairs long.


When chromatin is arranged with the string of beads fully extended it is termed euchromatin and in this condition it is actively transcribed to form RNA.


Chromatin can also be highly folded by the aggregation of nucleosomes into spiral clusters (solenoids) about 30 nm thick rather than the 11 nm of the euchromatic form, or may form even thicker super-clusters. This is achieved by the binding together of H1 histones of adjacent nucleosomes.


Transcriptionally active chromatin is densely packed during interphase as observed by electron microscopic studies and

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