The eukaryotic chromatin is composed of

Correct — B, DNA and Protein. Chromatin is the material a eukaryotic chromosome is made of, and it is a complex of DNA wound around histone proteins. About 147 base pairs of DNA coil round each histone octamer to make a nucleosome, nucleosomes string together like beads on a thread, and that thread folds further into the compact chromosome seen at cell division. Take away the protein and there is no chromatin, only naked DNA.

• (a)DNA only — DNA alone would be an unmanageable length — roughly two metres of it per human cell — with nothing to package it into a nucleus a few micrometres across.
• (c)DNA and RNA — RNA is present in the nucleus and small amounts associate with chromatin, but it is not what chromatin is built from. The structural partnership is DNA with histones.
• (d)RNA and Protein — Leaving DNA out removes the genetic material itself, which is the one component chromatin cannot do without.

Histones are small, positively charged proteins whose charge lets them bind the negatively charged phosphate backbone of DNA. Two copies each of four histone types form an octamer, DNA wraps nearly twice around it, and the resulting nucleosome is the repeating unit of chromatin. Chemical tags added to the histone tails loosen or tighten the packing and thereby control which genes a cell can read.

Packaging is the reason chromatin exists. Loosely packed euchromatin is accessible and generally active; densely packed heterochromatin is largely silent. Before a cell divides, chromatin condenses further into the rod-shaped chromosomes visible under a light microscope, and DNA replication requires the packing to be opened again. Prokaryotes have no histones of this kind and no nucleus, so their DNA is organised quite differently — which is one line of evidence that the two cell types are deeply distinct.

• Chromatin is DNA complexed with histone proteins, found in the nucleus of eukaryotic cells.
• The nucleosome, about 147 base pairs of DNA around a histone octamer, is its repeating unit.
• Euchromatin is loosely packed and largely active; heterochromatin is condensed and largely inactive.
• Chromatin condenses into visible rod-shaped chromosomes before cell division and unwinds for replication.
• Chemical modification of histone tails regulates which genes are accessible, a central idea in epigenetics.

• Equating chromatin with DNA alone and forgetting the protein half.
• Substituting RNA for protein because both are found in the nucleus.
• Treating chromatin and chromosome as different substances; they are the same material at different degrees of packing.

A one-line composition recall. The wrong options are built by deleting or swapping one of the two real components.

• practice — not a real PYQ

  The basic structural unit of chromatin is

  • (a)the nucleotide
  • (b)the nucleosome
  • (c)the centromere
  • (d)the ribosome

  Answer(b) the nucleosome — DNA wrapped around a histone octamer, repeated along the length of the fibre.

• practice — not a real PYQ

  Histone proteins bind readily to DNA mainly because histones are

  • (a)negatively charged
  • (b)positively charged
  • (c)electrically neutral
  • (d)hydrophobic

  Answer(b) positively charged — they are rich in lysine and arginine, and the charge attracts the negative phosphate backbone of DNA.