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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance3 Marks
Question
A typical mammalian cell has 2.2m long DNA molecule, whereas the nucleus in which it is packed measures about 10m. Explain how such a long DNA molecule is packed within a tiny nucleus in the cell.
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Answer

  1. In the mammalian cells (or eukaryotes) there is a set of positively-charged basic proteins, called histones.
  2. Histones are organised to form a unit of eight molecules, called histone octamer.
  3. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure, called nucleosome.
  4. A typical nucleosome contains 200 bp of DNA helix.
  5. The nucleosomes constitute the repeating units of chromatin, which appear as beads-on-string structure under an electron microscope.
  6. These are further packaged to form the chromatin fibres, which condense to form chromosomes.
  7. The packaging of chromatin at higher levels requires additional set of proteins called non-histone chromosomal (NHC) proteins.

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