The cleavage activity is intrinsic to RNA polymerase itself, it is stimulated greatly by accessory factors that are ubiquitous in the three biological kingdoms. There are two such factors in E. coli, GreA and GreB, and eukaryotic RNA polymerase II uses TFIIS for the same purpose. 

TLS Online TPP Program

#Id: 1885

#Unit 4. Cell Communication and Cell Signaling

Development of T cells from hematopoietic stem cells on bone marrow stromal cells expressing the Notch ligand.

TLS Online TPP Program

#Id: 1886

#Unit 4. Cell Communication and Cell Signaling

T-Cell Development

a. Commitment of hematopoietic precursors to the T cell lineage

b. The initiation of antigen receptor gene rearrangements

c.  The selection and expansion of cells that have successfully rearranged one of their T-cell receptor genes (-selection).

TLS Online TPP Program

#Id: 1887

#Unit 4. Cell Communication and Cell Signaling

Time course of appearance of  thymocytes and  thymocytes during mouse fetal development.

TLS Online TPP Program

#Id: 1888

#Unit 4. Cell Communication and Cell Signaling

Allelic exclusion

Most T cells fully rearrange and express a TCR chain from only one of their two TCR alleles

TLS Online TPP Program

#Id: 1889

#Unit 4. Cell Communication and Cell Signaling

Thymic selection

The most distinctive property of the mature T cells is that they recognize only foreign antigen combined with self-MHC molecules.

TLS Online TPP Program

#Id: 1890

#Unit 4. Cell Communication and Cell Signaling

Two distinct selection processes are required:

 • Positive selection, which selects for those thymocytes bearing receptors capable of binding self-MHC molecules, resulting in MHC restriction

• Negative selection, which selects against thymocytes bearing high-affinity receptors for self-MHC/peptide complexes, resulting in self-tolerance.