Eukaryotic cells also have type I and type II topoisomerases. The type I enzymes are topoisomerases I and III;

The eukaryotic type II topoisomerases cannot underwind DNA (introduce negative supercoils), but they can relax both positive and negative supercoils.

TLS Online TPP Program

#Id: 7221

#Unit 5. Developmental Biology

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

TLS Online TPP Program

#Id: 7222

#Unit 5. Developmental Biology

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: 7223

#Unit 5. Developmental Biology

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: 7224

#Unit 5. Developmental Biology

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.

TLS Online TPP Program

#Id: 7225

#Unit 5. Developmental Biology

TLS Online TPP Program

#Id: 7226

#Unit 5. Developmental Biology

Relationship between TCR affinity and selection