1. DNA Packaging

4. Nucleosomes Assembly Nucleosomes are assembled in an ordered fashion!
Step 1: Two H3-H4 dimers form a tetramer
Step 2: H32:H42 tetramer binds to double stranded DNA
Step 3: Two H2A- H2B dimers bind are recruited to the DNA molecule forming a complete nucleosome
Step 1
Step 2
Step 3

5. Eukaryotic Chromosomal Organization
Nucleosomes and DNA Compaction
DNA wrapping around the histones (10nm) results in 6 fold compaction
Needs to be about 10,000 fold—depending on genome size
Nucleosomes link together to build a 30-nm fiber with help of Histone H1

8. True nature of the chromosome scaffold remains a mystery.
Further compaction
Histones alone are not the answer. The histone compaction (with Histone H1) results in about a 40 fold reduction. What does the rest?
True nature of the chromosome scaffold remains a mystery.

10. Histone Modifications
How are histone tails modified?
P=Phosphorylation
Me: Methylation
Ac=Acetylation
Ub: Ubiquitinylation

11. Histone Modifications
What do these modifications do?
Weaken the association of the nucleosome with DNA
Inhibit the higher order nucleosome assembly
Recruit specific proteins to the chromatin
Some proteins can recognize the modified histone tails

12. Histone Modifications
Acetyl groups carries a negative charge.
Acetylation of lysine cancels the positive charge
Histone tails are more weakly associated with the nucleosomes– thus cannot form higher order assemblies (ie. 30 nm fiber)
Tends to decreases packing
Methyl groups are neutral.
Histone tails maintain association with the nucleosomes.
Tends to increase packing