Q.37 Arrange the following in the correct sequence of DNA packaging –

• (A) DNA scaffold
• (B) Nucleosome formation
• (C) Chromatosome formation
• (D) Histone dimerization

Choose the correct answer from the options given below:

1. (D), (A), (C), (B)
2. (D), (C), (B), (A)
3. (B), (A), (D), (C)
4. (D), (B), (C), (A).

   Packaging Steps Explained

   DNA packaging compacts 2 meters of DNA into the nucleus through hierarchical levels, beginning at the molecular scale with histones.

   • Histone dimerization (D): Core histones H2A-H2B and H3-H4 pair into dimers, forming the base for octamer assembly.

   • Nucleosome formation (B): Two H3-H4 dimers and two H2A-H2B dimers create an octamer; ~147 bp DNA wraps around it 1.65 times.

   • Chromatosome formation (C): Linker histone H1 binds, stabilizing ~167 bp DNA (core + linker) into a chromatosome unit.

   • DNA scaffold (A): Higher-order folding attaches chromatosomes to protein scaffold (topoisomerase II, SMC) for chromosome condensation.

   Option Analysis

   • (D), (A), (C), (B): Wrong; scaffold (A) can’t precede basic nucleosome structure.

   • (D), (C), (B), (A): Wrong; chromatosome requires prior nucleosome core.

   • (B), (A), (D), (C): Wrong; nucleosome can’t form without histone dimers first.

   • (B), (C), (D), (A): Wrong; dimerization initiates histone assembly before nucleosomes.(B), (C), (D), (A)

   Introduction to Sequence of DNA Packaging

   Sequence of DNA packaging begins with histone dimerization and progresses to compact chromosomes, critical for GATE Life Sciences molecular biology prep. This hierarchy fits eukaryotic DNA into the nucleus efficiently.

   Step-by-Step DNA Packaging Sequence

   • Histone dimerization (D): H2A/H2B and H3/H4 dimers assemble into octamers.

   • Nucleosome formation (B): DNA superhelix wraps octamer (“beads-on-string”).

   • Chromatosome formation (C): H1 linker histone seals chromatosome.

   • DNA scaffold (A): Loops attach to scaffold proteins for mitotic chromosomes.

   
   Progressive DNA packaging from naked DNA to metaphase chromosome.

   Biological Significance

   Packaging regulates access for replication/transcription; disruptions link to cancer.

   GATE Exam Strategy

   Recall D-B-C-A: dimers build cores, H1 compacts, scaffold condenses. Common trap: swapping early steps.