Question
Shown above is the electron micrograph (EM) picture of 'beads-on-string'.
- Identify and explain the detailed structure of a bead with the help of a labelled diagram.
- Describe the packaging of 'beads-on-string'in a eukaryotic cell.
Shown above is the electron micrograph (EM) picture of 'beads-on-string'.
✓
Answer
- The bead-like structures are the nucleosomes:
- There is a set of positively charged proteins, called histones, rich in basic amino acid residues, lysine and arginine.
- 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 nucleosome; a nucleosome contains 200 bp of DNA helix.
- The nucleosomes form the repeating units of the chromatin fibre in the nucleus; these nucleosomes of a chromatin are seen as the beads-on-string structure under electron microscope.
- The chromatin fibres are further coiled and condensed at metaphase stage to form the chromosomes.
- The packaging of chromatin at higher levels requires another set of proteins, called non histone chromosomal (NHC) proteins.
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