This 30-nm fiber is further folded, results in 1000-fold compaction in euchromatin in interphase chromatin and in mitotic chromosomes to achieve an overall 10,000- fold compaction in euchromatin.

#Id: 392

#Unit 2. Cellular Organization

The two H2A.H2B dimers each associate with 30 bp of DNA on either side of the central 60 bp of DNA bound by H3 and H4.

#Id: 393

#Unit 2. Cellular Organization

Using the clock analogy again, the DNA associated with H2A.H2B is located from 5 o’clock to 9 o’clock on either face of the nucleosome disc.

#Id: 394

#Unit 2. Cellular Organization

Together, the two H2A.H2B dimers form the bottom part of the histone octamer located across the disc from the DNA ends

#Id: 395

#Unit 2. Cellular Organization

Relatively short length (30 bp) of DNA bound by H2A.H2B dimers is not sufficient to prepare the DNA for H3.H4 tetramer binding.

#Id: 396

#Unit 2. Cellular Organization

Each nucleosome added to a covalently closed circular template changes the linking number of the associated DNA by approximately –1.2.

#Id: 397

#Unit 2. Cellular Organization

Nucleosomal DNA wraps around the histone protein 1.65 times, so create a writhe of –1.65 and so DLk should be -1.65. But when the change in linking number associated with each nucleosome was measured, the number was lower than this, approximately –1.2 for each nucleosome added. This discrepancy is referred to as the “Nucleosome Linking Number paradox,”