Q.Distinguish between heterochromatin and euchromatin. Which of the two is transcriptionally active?
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DNA Packaging and the Nucleosome
The very long DNA of a eukaryotic cell is condensed and organised by packaging it with proteins into chromatin. The basic repeating unit of chromatin is the nucleosome.
A nucleosome consists of DNA (about 200 base pairs) wrapped roughly two turns around a histone octamer — a core of eight histone proteins: two molecules each of H2A, H2B, H3 and H4. The linker histone H1 sits outside the core, sealing the DNA where it enters and exits and helping fold nucleosomes into higher-order solenoid fibres. …
Chromatin is not uniformly packed throughout the nucleus - it exists in two states that differ in condensation, and this packing density determines whether the DNA in that region is accessible for transcription. …
Chromatin exists in two states based on packing density - heterochromatin (condensed, inactive) and euchromatin (loose, active) - and only euchromatin is transcribed.
Heterochromatin: Tightly/densely packed chromatin. It stains darkly with basic dyes (e.g., in Feulgen staining) because the DNA is condensed and less accessible. Because RNA polymerase and transcription factors cannot easily access such tightly packed DNA, heterochromatin is transcriptionally inactive.
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- CBSE 2026Set A1 markMCQQ.A typical nucleosome is composed of(a) 200 base pairs of DNA(b) Histone octamer(c) Both (A) and (B)(d) Acidic protein
›Reveal solutionSolution
A typical nucleosome = histone octamer + about 200 bp of DNA wound around it.
In eukaryotic chromatin, DNA is packaged with positively charged histone proteins. Eight histones (two each of H2A, H2B, H3, H4) form a histone octamer, and negatively charged DNA of about 200 base pairs is wrapped around this core to form a nucleosome, the repeati …
- CBSE 2026Set ANNUAL1 markMCQQ.Some region of chromatin are loosely packed in a typical nucleus, they are called(a) Euchromatin(b) Histone(c) Nucleosome(d) Heterochromatin
›Reveal solutionSolution
Chromatin exists in two packing states: loosely packed euchromatin (active) and densely packed heterochromatin (inactive).
In an interphase nucleus, chromatin stains unevenly with basic dyes. Regions that are loosely/diffusely packed stain lightly and are called euchromatin - this DNA is transcriptionally active. Regions that are densely packed stain darkly and are called heterochromatin - this DNA is transcriptionally inactive. Histone is a chr …
- CBSE 2026Set ANNUAL1 markMCQQ.A typical nucleosome consists of(a) 50 base pair of DNA helix(b) 100 base pair of DNA helix(c) 150 base pair of DNA helix(d) 200 base pair of DNA helix
›Reveal solutionSolution
A nucleosome is the basic unit of chromatin packaging: a histone octamer core wrapped by DNA, with a typical nucleosome spanning about 200 bp of DNA helix.
Chromatin packaging begins with the nucleosome — the fundamental repeating unit of chromatin. A histone octamer (two molecules each of H2A, H2B, H3, and H4) forms the core, around which about 146 bp of DNA is tightly wrapped nearly twice. Including the additional linker DNA that connects one nucleosome to the next (associated with histone H1), NCERT describes a typical nucleosome as containing about **200 bp of …
- CBSE 2025Set 57/4/11 markMCQQ.In its extended 'beads-on-string' form of chromatin, the 'beads' in the string represent : (A) Linker DNA (B) Histone proteins (C) Nucleosomes (D) NHC proteins
›Reveal solutionSolution
The "beads" in the beads-on-string model of chromatin are nucleosomes — each bead is a unit of DNA wrapped around a histone core, connected by linker DNA (the string).
The beads-on-string model is the classic visual for the first level of DNA packaging in eukaryotes. When you see a diagram of chromatin at low resolution, it looks like a necklace: a thin thread with periodic bumps. The thread is the linker DNA, and each bump is a nucleosome.
A nucleosome is the fundamental repeating subunit of chromatin. It consists of about 146–147 base pairs of DNA wrapped around a core of eight histone proteins (two each of H2A, H2B, H3, and H4). So the "bead" is not just the histones alone, nor the linker DNA, but the entire complex of DNA + histone octamer.
Let's walk through the options:
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Linker DNA — This is the "string" between beads, not the bead itself. It's the stretch of DNA that connects one nucleosome to the next. So (A) is incorrect.
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Histone proteins — These are part of the bead, but they are the core inside it. The bead as a whole includes both the histones and the DNA wrapped around them. Calling the bead just "histone proteins" would be incomplete — the DNA is an essential structural component. So (B) is not the best answer.
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Nucleosomes — This is exactly what each bead represents: the nucleosome, which is the complete unit of DNA plus histone core. This is the standard terminology in every textbook and exam. So (C) is correct. …
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- CBSE 2025Set 57/5/11 markMCQQ.The histone core in a nucleosome of chromatin thread is a/an : (A) pentamer (B) hexamer (C) heptomer (D) octamer
›Reveal solutionSolution
The histone core of a nucleosome is built from eight histone protein subunits — two copies each of H2A, H2B, H3, and H4 — making it an octamer.
The Architecture of Chromatin Packaging
DNA in eukaryotic cells faces a geometric challenge: roughly two meters of linear DNA must fit into a nucleus only micrometers across. The solution is chromatin, and the fundamental repeating unit of chromatin is the nucleosome. Understanding what forms the core of this structure reveals how cells achieve this remarkable compaction.
The nucleosome core particle consists of DNA wrapped around a protein complex. This protein complex is the histone core, and its composition follows a precise stoichiometry that has been conserved across eukaryotes for hundreds of millions of years.
Building the Histone Octamer
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The histone family members
Five major histone proteins exist: H1, H2A, H2B, H3, and H4. Of these, four participate in forming the nucleosome core. Each is a small, positively charged protein (rich in lysine and arginine residues) that binds tightly to the negatively charged DNA backbone.
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Pairing into dimers
The core histones first associate as heterodimers: H2A pairs with H2B, and H3 pairs with H4. These dimers form through specific protein-protein interactions at their histone-fold domains.
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Assembly into the octamer
The nucleosome core is constructed from:
- Two copies of the H3–H4 dimer (giving 2 × H3 and 2 × H4)
- Two copies of the H2A–H2B dimer (giving 2 × H2A and 2 × H2B)
This yields a total of eight histone proteins: (H2A)2, (H2B)2, (H3)2, (H4)2.
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DNA wrapping
Approximately 147 base pairs of DNA wrap 1.65 turns around this histone octamer in a left-handed superhelix. The result is the nucleosome core particle, the "beads-on-a-string" structure visible in electron micrographs of partially decondensed chromatin. …
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- CBSE 2025Set A1 markQ.Fill in the blank with the correct answer: DNA is ______ charged molecules.
›Reveal solutionSolution
DNA carries a net negative charge due to the phosphate groups along its backbone.
Each nucleotide in DNA consists of a nitrogenous base, deoxyribose sugar and a phosphate group. The phosphate groups link adjacent sugars via phosphodiester bonds, forming the sugar-phosphate backbone that runs along each strand. At physiological pH, these phosphate groups are ionised and carry a negative charge, making the entire DNA molecule negatively charged overall. Thi …
- CBSE 2024Set ANNUAL1 markQ.Which type of charge is found in DNA segments?
›Reveal solutionSolution
DNA is negatively charged because of the phosphate groups in its sugar-phosphate backbone.
Each nucleotide of DNA is made of a nitrogenous base, a deoxyribose sugar, and a phosphate group. In the polynucleotide chain, phosphate groups link adjacent sugars through phosphodiester bonds, and each phosphate carries an ionisable –OH group that loses a proton at cellular pH, leaving a negatively charged PO4- group. This is why DNA is an acidic mo …
- CBSE 2023Set ANNUAL1 markMCQQ.The length of DNA in Escherichia coli has(a) 5386 bp(b) 48502 bp(c) 4.6×10⁶ bp(d) 3.3×10⁹ bp
›Reveal solutionSolution
E. coli's genome is roughly 4.6 million base pairs (~1.36 mm when stretched out).
The genome of the bacterium Escherichia coli consists of a single circular DNA molecule of approximately 4.6 × 10⁶ base pairs — quite large compared to the size of the bacterial cell itself (if stretched out, it would be about 1,000 times longer than the cell). By comparison, phage lambda has about 48,502 bp, …
- CBSE 2021Set ANNUAL1 markQ.Under microscope, chromatin is seen as ‘beads-on-string’ like structure. Here, ‘beads’ represent the structures called ______.
›Reveal solutionSolution
The 'beads' seen in the beads-on-a-string chromatin structure are nucleosomes.
Chromatin is the DNA-protein complex found in the nucleus of eukaryotic cells. Under the electron microscope, chromatin fibre appears as a structure with repeating bead-like units strung along a thread - this is called the 'beads-on-string' structure.
- Each 'bead' is a nucleosome - the basic unit of packaging of chromatin. …
- CBSE 2020Set ANNUAL1 markQ.What is Euchromatin?
›Reveal solutionSolution
Euchromatin = loosely coiled, gene-rich, actively transcribed chromatin.
Within the interphase nucleus, chromatin exists in two states based on packing density. Euchromatin is the region where DNA is loosely packed, stains lightly with basic dyes, and is transcriptionally active because RNA polymerase and other proteins can access the DNA. It is contrasted with heterochromatin, which is densely packed, stains darkly, and is transcriptionally inactive. The proportion and distribution of e …
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