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Questions  >>  CBSE XII  >>  Biology
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Q)

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Shown above is the electron micrograph (EMI) picture of 'beads-on-string'.

(a)Identify and explain the detailed structure of a bead with the help of a labelled diagram.

(b) Describe the packaging of ‘beads –on -string' in a eukaryotic cell.

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A)
a.
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b. * The bead-like structures are the nucleosomes.
* Organization is much more complex.
* There is a set of basic proteins called histones.
* Depending upon the abundance of amino acid residues with charged side chains, a protein acquires charge.
* Histones are rich in basic amino acid residues-Lysine and Argenine which carry positive charges in their side chains.
* Histones are organized to form a unit of 8 molecules called a histone octomer.
* Negatively charged DNA is wrapped around positively charged histone octomer to form a nucleosome.
* A typical nucleosome consists of 200BP of double helix.
* The repeating units of chromatin are made up of nucleosomes.
* Nucleosomes in chromatin are seen as beads on a string.
* Chromatin further coils and condenses to form the chromosomes.
* Packaging of chromatin at higher level needs additional set of proteins that are collectively called Non histone chromosome proteins (NHC proteins).
* Some regions of chromatin are loosely packed and stains light and are called euchromatin. This is active transcriptionally.
* Some regions of chromatin that are densely packed and stains dark are called heterochromatin. This is transcriptionally inactive.
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