Activated T cells express increased levels of Fas and FasL. In both B and T cells, engagement of Fas by FasL induces a rapid apoptotic death known as activation-induced cell death (AICD).

TLS Online TPP Program

#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.

TLS Online TPP Program

#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

TLS Online TPP Program

#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.

TLS Online TPP Program

#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.

TLS Online TPP Program

#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,”

TLS Online TPP Program

#Id: 398

#Unit 2. Cellular Organization

Modification of histones is associated with replication and transcription, and specific modifications also facilitate DNA repair.