Question 1

What are the levels of DNA packaging?

Accepted Answer

DNA is packaged in a hierarchy of increasingly condensed levels: (1) the naked DNA double helix, (2) nucleosomes — DNA wrapped around histone octamers, giving the 'beads on a string' appearance, (3) the 30 nm chromatin fibre, where nucleosomes coil into a solenoid, (4) looped domains attached to a protein scaffold, and (5) the fully condensed metaphase chromosome.

Question 2

What is the overall packing ratio of DNA in a chromosome?

Accepted Answer

From the naked double helix to the fully condensed metaphase chromosome the DNA is compacted roughly 10,000-fold. This is why about 2 metres of DNA in each human cell can fit inside a nucleus only about 10 micrometres across.

Question 3

What is the 30 nm chromatin fibre?

Accepted Answer

The 30 nm fibre is the second major level of packaging. The 'beads on a string' (10 nm) array of nucleosomes coils up into a thicker, 30-nanometre-wide solenoid-like fibre, with about six nucleosomes per turn. The linker histone H1 helps stabilise this higher-order coiling.

Question 4

What is the difference between euchromatin and heterochromatin?

Accepted Answer

Euchromatin is loosely packed, lighter-staining chromatin that is transcriptionally active. Heterochromatin is densely packed, darker-staining and transcriptionally inactive. Both are made of the same packaging machinery — the difference is how tightly the chromatin is condensed.

Question 5

Why does DNA need to be packaged so tightly?

Accepted Answer

Each human cell contains about 2 metres of DNA that must fit inside a nucleus only ~10 µm wide. Packaging compacts the DNA roughly 10,000-fold so it physically fits, protects it from damage, and lets the cell organise, segregate and regulate which genes are switched on.

Question 6

Is DNA packaging an important NEET topic?

Accepted Answer

Yes. DNA packaging is part of the Class 12 chapter 'Molecular Basis of Inheritance' and is frequently tested in NEET. Common questions cover the order of the packaging levels, the role of histones and H1, the nucleosome composition, the 30 nm fibre, and the euchromatin vs heterochromatin distinction.

DNA Packaging — From Double Helix to Chromosome

Practice

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