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NEET asks about DNA packaging every few years, and the questions are predictable: why histones are positive, how many base pairs sit in a nucleosome, and what euchromatin means. The numbers come straight from NCERT. This post works through them, links them to the experiments behind the "beads-on-string" picture, and lists the traps.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists "DNA packaging" under Molecular Basis of Inheritance in Unit 7, Genetics and Evolution. The matching text is section 5.1.2, "Packaging of DNA Helix", in the NCERT Class 12 Biology chapter Molecular Basis of Inheritance (2026-27 reprint). Read that section once with this post open beside it.
How long is the DNA in one cell?
NCERT gives the method. The distance between consecutive base pairs is 0.34 nm, and a typical mammalian cell has 6.6 × 10⁹ base pairs.
Length = 6.6 × 10⁹ × 0.34 × 10⁻⁹ m = 2.244 m, about 2.2 m.
NCERT compares this with a typical nucleus of about 10⁻⁶ m. So the DNA is over a million times longer than the space it must fit into: 2.2 ÷ 10⁻⁶ = 2.2 × 10⁶.
NCERT also asks a reverse question. If E. coli DNA is 1.36 mm long, how many base pairs does it have?
Base pairs = 1.36 × 10⁻³ m ÷ 0.34 × 10⁻⁹ m = 4.0 × 10⁶ bp.
Keep the units in metres throughout. Most wrong answers come from mixing mm and nm.
How do prokaryotes and eukaryotes differ?
In prokaryotes such as E. coli there is no defined nucleus. NCERT explains that the negatively charged DNA is held by positively charged proteins in a region called the nucleoid, organised in large loops.
Eukaryotes use a set of positively charged, basic proteins called histones. Histones are rich in lysine and arginine, two amino acids with positively charged side chains. Eight histone molecules form a histone octamer, and the negatively charged DNA wraps around it. That unit is a nucleosome.
What does a nucleosome look like?
NCERT says a typical nucleosome contains 200 bp of DNA. The atomic detail came later. In 1997, Luger and colleagues published the crystal structure of the nucleosome core particle at 2.8 Å resolution in Nature. Their abstract describes 146 base pairs of DNA organised into a superhelix around the histone octamer, with histone tails passing between the turns of DNA to contact neighbouring particles. So in a typical 200 bp repeat, most of the DNA is wound on the bead and the rest is the connecting stretch between beads.
Where does "beads-on-string" come from?
Under the electron microscope, chromatin looks like beads on a string, and NCERT shows this image. One of the first reports was by Ada and Donald Olins (1974) in Science. They saw linear arrays of spherical particles about 70 Å across in chromatin from rat thymus, rat liver and chicken red blood cells, joined by connecting strands about 15 Å wide.
Worked example: how many nucleosomes in one cell?
NCERT asks you to estimate this.
Nucleosomes ≈ 6.6 × 10⁹ bp ÷ 200 bp per nucleosome = 3.3 × 10⁷.
That is about 33 million beads. It is an estimate, since the spacing varies along the genome, but it is the answer NCERT's numbers give.
How is chromatin packed further?
NCERT describes the next levels briefly. The beads-on-string fibre coils into chromatin fibres, which condense further at metaphase to form chromosomes. Higher-level packaging needs non-histone chromosomal (NHC) proteins.
Within a nucleus, chromatin is not packed evenly:
Euchromatin is loosely packed, stains light and is transcriptionally active.
Heterochromatin is densely packed, stains dark and is inactive.
A link worth making: genes in tightly packed regions are harder for the cell to read. That is why packaging also sits close to replication and transcription in the chapter, covered in our post on DNA replication and transcription.
Which mistakes cost marks?
Histones are acidic. No: they are basic and positively charged.
Octamer means eight types. It means eight molecules.
Nucleosome = chromatin. The nucleosome is the repeating unit; chromatin is the whole thread.
Mixing up the stains. Euchromatin stains light and is active; heterochromatin stains dark and is inactive.
Unit errors in length questions. Convert mm and nm to metres before dividing.
A practice routine
Read NCERT section 5.1.2 twice, then close the book and draw a labelled nucleosome from memory.
Redo the three calculations above without looking.
Make a three-column list comparing prokaryotic and eukaryotic DNA packaging.
Solve ten past NEET questions from this section and note which line of NCERT each one tests.
If you want to revisit DNA structure before packaging, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, one lesson at a time.
Today, calculate the length of DNA in a cell with 3.3 × 10⁹ bp and compare it with your answer above.
