Q.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

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 repeating unit of chromatin. So both statements (A) 200 base pairs of DNA and (B) histone octamer describe the nucleosome.

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 chromosomal protein, and a nucleosome is the repeating structural unit of chromatin (DNA wrapped around a histone octamer) - neither describes a 'loosely packed region'.

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 DNA helix.

This repeated 'beads-on-a-string' structure is what gives chromatin its characteristic appearance under the electron microscope and is the first level of DNA compaction inside the nucleus.

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 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.

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. This negative charge is exploited in techniques like agarose gel electrophoresis, where DNA fragments move towards the positive electrode (anode) under an electric field.

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 molecule (nucleic acid) and migrates towards the anode (positive electrode) during gel electrophoresis.

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, and phage ϕX174 has 5,386 bp (much smaller viral genomes); the human haploid genome is around 3.3 × 10⁹ bp, vastly larger than the bacterial genome.

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.

• A nucleosome consists of a histone octamer (two molecules each of the four core histones H2A, H2B, H3 and H4) around which about 200 bp of DNA is wrapped.

• Nucleosomes are connected to each other by a stretch of 'linker DNA', giving the beads-on-string appearance.

• This packaging allows the very long DNA molecule to be condensed and organised within the small nucleus.

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 euchromatin versus heterochromatin in a nucleus reflects how much of the genome is being actively expressed in that cell.