Q.31 Histones are rich in
- Lysine and Arginine
- Lysine and Asparagine
- Glycine and Lysine
- Leucine and Arginine
Histones are rich in lysine and arginine, which are basic amino acids that give histones their positive charge for binding negatively charged DNA.
Question Breakdown
This multiple-choice question tests knowledge of histone protein composition, a key concept in molecular biology and chromatin structure.
Option Analysis
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Lysine and Arginine: Correct. Histones, especially core histones (H2A, H2B, H3, H4), contain about 20-30% basic amino acids, primarily lysine (Lys, K) and arginine (Arg, R). These positively charged residues (pKa >10) interact electrostatically with DNA’s phosphate backbone, enabling nucleosome formation. Lysine is abundant in tails for modifications like acetylation; arginine provides stronger binding.
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Lysine and Asparagine: Incorrect. Asparagine (Asn, N) is neutral and polar, not basic; it lacks the positive charge needed for DNA binding and is underrepresented in histones (~5-10%).
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Glycine and Lysine: Incorrect. Glycine (Gly, G) is the smallest, neutral amino acid with high flexibility but no charge; histones have moderate glycine (~8%) but rely on basics, not glycine, for charge.
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Leucine and Arginine: Incorrect. Leucine (Leu, L) is hydrophobic and non-polar, common in globular proteins but low in histones (~5-7%); it doesn’t contribute to basicity.
Introduction to Histone Composition
Histones are rich in lysine and arginine, making up ~25% of residues in these DNA-packaging proteins. This basic amino acid abundance creates a positive charge at physiological pH, essential for wrapping 147 bp of DNA around nucleosomes in chromatin. Eukaryotic cells rely on this for genome compaction and gene regulation.
Role of Basic Amino Acids
Lysine (ε-amino group, pKa ~10.5) and arginine (guanidino group, pKa ~12.5) protonate easily, forming ionic bonds with DNA’s negative phosphates. Histone tails, lysine/arginine-rich, undergo post-translational modifications (e.g., lysine acetylation neutralizes charge, loosening chromatin). Core histones show near-identical sequences across species, conserving this composition.
Histone Types and Variations
Histone Type Key Features Lys/Arg Content H2A, H2B Dimerize; flexible tails High Lys/Arg (~20%) H3, H4 Tetramer core; conserved Very high Lys/Arg (~25-30%) H1 (Linker) Chromatin folding Rich in Arg > Lys H3/H4 are most conserved; variants like H3.3 differ slightly but retain basics.
Biological Significance
This lysine and arginine richness enables tight DNA packaging (10,000-fold compaction) while allowing dynamic access for transcription. Disruptions link to diseases like cancer via epigenetic changes.
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