NCERT grounding
This subtopic is section 5.1.2 of the NCERT Class XII chapter Molecular Basis of Inheritance, titled Packaging of DNA Helix. It follows directly from the double-helix structure of DNA: once the helix is described, the textbook poses a physical question — a length of DNA "far greater than the dimension of a typical nucleus" must somehow be folded into that nucleus. NCERT answers it by introducing the nucleoid in prokaryotes and, in eukaryotes, the histone octamer, the nucleosome, chromatin and the euchromatin/heterochromatin distinction.
The NIOS supplement (Lesson 23, Molecular Inheritance and Gene Expression) reinforces the same hierarchy: DNA coils around a histone-octamer "core particle" to make a nucleosome; the nucleosome string then coils into a solenoid, which super-coils to build the metaphase chromosome. Both sources agree on the central facts you must lock down — the eight-histone octamer, the 200 bp nucleosome, and the beads-on-string image seen under the electron microscope.
"The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix." — NCERT, Class XII Biology, section 5.1.2
Packaging the DNA helix
The packaging problem begins with a simple multiplication. In the double helix the distance between two consecutive base pairs is 0.34 nm, that is 0.34 × 10⁻⁹ m. A diploid human cell has 6.6 × 10⁹ base pairs. Multiplying the number of base pairs by the distance between them gives the total contour length of the DNA: 6.6 × 10⁹ bp × 0.34 × 10⁻⁹ m/bp ≈ 2.2 metres. That single thread of DNA must be accommodated inside a nucleus whose diameter is only about 10⁻⁶ m. The DNA is therefore roughly a million times longer than the compartment that has to hold it, and it cannot simply lie loose — it must be folded, coiled and condensed in an organised way.
The scale problem
The DNA helix of a typical mammalian cell is about 2.2 metres long, calculated as 6.6 × 10⁹ bp × 0.34 nm/bp. The nucleus that must contain it is only about 10⁻⁶ m across — packaging bridges this gap of roughly a million-fold.
Prokaryotes: the nucleoid
Prokaryotes solve a smaller version of the same problem. A bacterium such as E. coli has no defined nucleus, yet its DNA is not scattered randomly through the cytoplasm. The DNA, being negatively charged because of its phosphate backbone, is held together with some positively charged proteins in a region called the nucleoid. Within the nucleoid the DNA is organised into large loops held in place by these proteins. The proteins involved here are not histones — that is a eukaryotic feature — so prokaryotic packaging is comparatively simple, but it still relies on the same basic principle of charge attraction between acidic DNA and basic proteins.
Histones: the positively charged scaffold
In eukaryotes the organisation is far more elaborate. The key players are histones — a set of positively charged, basic proteins. A protein acquires its charge from the amino acid residues with charged side chains in its sequence. Histones are rich in the basic amino acid residues lysine and arginine; both carry positive charges in their side chains, which makes the overall histone protein basic and positively charged. This positive charge is functionally vital: it is what allows histones to grip the negatively charged DNA. Opposite charges attract, and the electrostatic attraction between basic histones and acidic DNA is the physical glue of eukaryotic packaging.
Histones are organised to form a unit of eight molecules called the histone octamer. The octamer is built from four histone types — H2A, H2B, H3 and H4 — with two copies of each. A fifth histone, H1, exists but is not part of the octamer; its separate role is discussed below. Keeping these two groups distinct — the four octamer histones versus the standalone H1 — is a frequent NEET checkpoint.
The five histones at a glance. Four types build the octamer core; H1 acts separately as a linker. All are basic proteins rich in lysine and arginine.
Core histones
H2A · H2B · H3 · H4
two copies of each
Eight molecules together form the histone octamer around which DNA winds.
Linker histone
H1
outside the octamer
Sits on the linker DNA between nucleosomes; seals the wrap and aids higher-order folding.
Why basic
Lysine · Arginine
positively charged side chains
These residues make histones basic and positive, so they bind the negatively charged DNA.
The nucleosome: the repeating unit
When the negatively charged DNA is wrapped around the positively charged histone octamer, the resulting structure is a nucleosome. This is the single most important unit in the whole subtopic and deserves precise treatment. A typical nucleosome contains 200 base pairs of DNA helix wound around its octamer core. The nucleosome is the repeating unit of chromatin — the thread-like, stained (coloured) bodies seen in the nucleus. The word "chromatin" itself reflects this: it refers to material that takes up stain.
Each octamer is wrapped by a stretch of DNA, and consecutive octamers are joined by short segments of free DNA called linker DNA. The repeating pattern of bead (octamer plus wound DNA) and connecting thread (linker DNA) produces, under the electron microscope, the famous "beads-on-string" appearance. The beads are the nucleosomes; the string is the DNA running between them. This is the least condensed visible form of chromatin and the starting point for all higher-order packing.
Figure 1. A single nucleosome is roughly 200 bp of DNA wound around an octamer of eight histones. A run of nucleosomes joined by linker DNA gives the beads-on-string image seen under the electron microscope — beads are nucleosomes, the string is DNA.
Histone H1 and higher-order folding
The beads-on-string thread is still far too extended to fit the nucleus. It is packed further into chromatin fibres, which are themselves coiled and condensed. At metaphase of cell division this condensation reaches its maximum, producing the compact chromosomes visible under a light microscope. The NIOS account names the intermediate stages: the nucleosome string coils into a solenoid, which super-coils repeatedly until the metaphase chromosome is formed — a progression that takes the fibre from 2 nm of bare DNA up to a chromosome roughly 1400 nm wide.
This higher-order packing needs additional proteins. The packaging of chromatin at higher levels requires an extra set of proteins collectively called Non-histone Chromosomal (NHC) proteins. Separately, histone H1 plays a sealing role: it sits on the linker DNA where the DNA enters and exits the octamer and locks the wrap in place. The association of H1 with a nucleosome is the signal that DNA is being condensed into a chromatin fibre rather than remaining as exposed beads-on-string — a point NEET 2017 tested directly.
From naked helix to metaphase chromosome
-
Level 1
DNA double helix
The bare 2.2 m thread, about 2 nm wide.
-
Level 2
Nucleosome
≈200 bp of DNA wound on a histone octamer.
-
Level 3
Beads-on-string
Repeating nucleosomes — the unit of chromatin.
-
Level 4
Chromatin fibre
String coiled and condensed; H1 and NHC proteins assist.
-
Level 5
Chromosome
Maximally condensed at metaphase.
Figure 2. The packaging hierarchy: a thin DNA helix winds on octamers to make nucleosomes, the nucleosome thread coils into a chromatin fibre, and that fibre is condensed further into the compact metaphase chromosome.
Euchromatin and heterochromatin
Chromatin is not uniformly packed across the nucleus. Some regions are loosely packed and stain light — these are called euchromatin. Other regions are more densely packed and stain dark — these are called heterochromatin. The difference is not merely cosmetic. Euchromatin is said to be the transcriptionally active chromatin: because it is loosely packed, the transcription machinery can reach the genes. Heterochromatin, being tightly packed, is transcriptionally inactive. The level of condensation therefore directly controls whether the genes in a region can be expressed.
Euchromatin
Loose
packing density
- Loosely packed chromatin region
- Stains light under the microscope
- Transcriptionally active
- Genes here can be reached and expressed
Heterochromatin
Dense
packing density
- Densely packed chromatin region
- Stains dark under the microscope
- Transcriptionally inactive
- Tight packing blocks the machinery
A useful mnemonic ties the four properties together: light stain, loose packing and "active" all go with euchromatin (think eu = "true/good", the working chromatin), while dark stain, dense packing and "inactive" all go with heterochromatin. NEET frequently scrambles exactly these pairings — for instance claiming heterochromatin is transcriptionally active — so memorise them as a fixed set rather than guessing on the day.
Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive.
NCERT Class XII Biology · Section 5.1.2
Worked examples
A DNA double helix in a typical mammalian cell has 6.6 × 10⁹ base pairs, and the distance between two consecutive base pairs is 0.34 nm. Calculate the total length of the DNA.
Length = (number of base pairs) × (distance per base pair) = 6.6 × 10⁹ bp × 0.34 × 10⁻⁹ m/bp. Multiplying the numerical parts, 6.6 × 0.34 = 2.244, and the powers of ten cancel (10⁹ × 10⁻⁹ = 1). The length is therefore approximately 2.2 metres. This is the same calculation NCERT uses to frame the packaging problem, and NEET 2020 set it as a direct numerical question.
How many histone molecules make up a histone octamer, and how many distinct histone types occur in it?
An octamer contains eight histone molecules — that is the meaning of "octamer". These eight are not eight different proteins: they are two copies each of four types — H2A, H2B, H3 and H4. Histone H1 is excluded because it is a linker histone outside the octamer. So the answer is eight molecules of four types. A common error is to count five types because H1 exists; H1 simply does not belong to the octamer.
Why can the negatively charged DNA bind so tightly to histone proteins?
DNA carries a net negative charge because of the phosphate groups in its sugar-phosphate backbone. Histones are basic, positively charged proteins, since they are rich in the amino acid residues lysine and arginine, both of which carry positive charges on their side chains. Opposite charges attract, so the positively charged histone octamer and the negatively charged DNA are held together by strong electrostatic attraction. This charge complementarity is the physical basis of nucleosome formation.
Common confusion & NEET traps
Packaging questions are almost always fact-recall, and the examiners exploit a small set of predictable slips. The points below are the ones that decide a mark in this subtopic.