Packaging of DNA: Histones, Nucleosome and Chromatin
Biology · Molecular Basis of Inheritance · NEET
DNA is very long (about 2.2 metres in one human cell), so it must be packed to fit inside a tiny nucleus. The negatively charged DNA wraps around a positively charged histone octamer (a unit of 8 histone proteins) to make a nucleosome, and one nucleosome holds about 200 base pairs of DNA. Memory hook: "Negative DNA hugs Positive histones" (opposite charges attract), and "octamer = 8 = octopus arms."
Negatively charged DNA (red line) wraps around positively charged histone octamers of 8 molecules (blue beads) to form nucleosomes of about 200 bp. These beads-on-string nucleosomes make chromatin, which coils into fibres and finally condenses into chromosomes.
Your doubts, answered
Is DNA positively or negatively charged, and what about histones?
DNA is NEGATIVELY charged because of its phosphate groups. Histones are POSITIVELY charged basic proteins. The negative DNA wraps around the positive histone octamer. NEET reverses these charges as a trap almost every year, so remember: DNA negative, histone positive.
How many histone molecules make a histone octamer?
Eight (8) histone molecules join to form one histone octamer. 'Octa' means eight. This is one of the most repeated NEET facts (2016, 2026), so lock it: octamer = 8 molecules.
How many base pairs of DNA are in one nucleosome?
A typical nucleosome contains about 200 bp (base pairs) of DNA helix wrapped around the histone octamer. Remember the number 200 for direct NEET one-liners.
What is the difference between a nucleosome and a nucleoid?
A nucleosome is the DNA-plus-histone-octamer packaging unit found in EUKARYOTES. A nucleoid is the region in PROKARYOTES (like E. coli) where DNA is held in large loops by some positively charged proteins, without histones and without a nuclear membrane. NEET mixes these two words to confuse you.
Why does histones being basic help package DNA?
Histones are rich in the basic amino acids lysine and arginine, whose side chains carry positive charges. This positive charge attracts the negatively charged DNA (opposite charges attract), so DNA wraps tightly around the histone octamer. This is WHY NEET links 'basic protein' to 'positive charge' to 'lysine and arginine'.
What comes after the nucleosome in DNA packaging?
Nucleosomes are the repeating units of chromatin and look like 'beads-on-string' under the electron microscope. This beads-on-string chromatin is coiled into chromatin fibres, and at metaphase these condense into chromosomes. Higher-level packaging also needs Non-histone Chromosomal (NHC) proteins.
⚠️ The NEET trap ✗ In prokaryotes the positively charged DNA is held with negatively charged proteins in the nucleoid. ✓ DNA is always NEGATIVELY charged (phosphate groups); it is held with POSITIVELY charged proteins. In eukaryotes negative DNA wraps around the positive histone octamer to form a nucleosome. 🧠 NEET 2023 flipped both charges. Rule: DNA is NEGATIVE, packaging proteins/histones are POSITIVE. If a statement says DNA is positive, it is wrong.
Real NEET questions
2023
Given below are two statements: Statement I: In prokaryotes, the positively charged DNA is held with some negatively charged proteins in a region called nucleoid. Statement II: In eukaryotes, the negatively charged DNA is wrapped around the positively charged histone octamer to form nucleosome. In the light of the above statements, choose the correct answer:
A · Both Statement I and Statement II are true
B · Both Statement I and Statement II are false
C · Statement I is correct but Statement II is false
D · Statement I is incorrect but Statement II is true ✓
Solution: DNA is negatively charged and is held with positively charged proteins, so Statement I reverses both charges and is FALSE. Statement II correctly describes the nucleosome, so it is TRUE. Answer: Statement I is incorrect but Statement II is true.
2016
Which one of the following statements about Histones is wrong?
A · Histones are rich in amino acids - Lysine and Arginine
B · Histones carry positive charge in the side chain
C · Histones are organized to form a unit of 8 molecules
D · The pH of histones is slightly acidic ✓
Solution: Histones are BASIC (not acidic) proteins because they are rich in lysine and arginine and carry positive charges. They form a unit of 8 molecules (octamer). So the wrong statement is that their pH is slightly acidic.
2017
The association of histone H1 with a nucleosome indicates:
A · Transcription is occurring
B · DNA replication is occurring
C · The DNA is condensed into a Chromatin Fibre ✓
D · The DNA double helix is exposed
Solution: Nucleosomes (DNA wrapped around the histone octamer) form the beads-on-string chromatin. Histone H1 binds the linker DNA and packs the beads-on-string into the higher-order, more condensed chromatin fibre. So H1 association indicates the DNA is condensed into a chromatin fibre.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A nucleosome is the basic packaging unit of DNA in eukaryotes. It is about 200 base pairs of negatively charged DNA wrapped around a positively charged histone octamer (8 histone molecules).
Why must DNA be packaged?
The DNA in one human cell is about 2.2 metres long, but the nucleus is only about 10 micrometres wide. So DNA must be tightly packed around histones to fit inside the small nucleus. This is a common NEET reasoning point.
What are histones rich in?
Histones are rich in the basic amino acids lysine and arginine. Their side chains carry positive charges, which is why histones are positively charged basic proteins that attract the negative DNA.
What are Non-histone Chromosomal (NHC) proteins?
NHC proteins are an additional set of proteins needed to package chromatin at higher levels, beyond the histone octamer. NEET has asked this directly (2026), so remember NHC = higher-level packaging.
What is the order of DNA packaging?
DNA + histone octamer makes a nucleosome (200 bp); many nucleosomes make chromatin (beads-on-string); chromatin coils into chromatin fibres; these condense at metaphase into chromosomes.
