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