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: 4577

#Unit 6. System Physiology – Plant

Light wavelengths detected by plant light receptors.

TLS Online TPP Program

#Id: 4578

#Unit 6. System Physiology – Plant

Phytochrome Can Interconvert between Pr and Pfr Forms

TLS Online TPP Program

#Id: 4579

#Unit 6. System Physiology – Plant

Effects of red light (650–680 nm) on morphogenesis could be reversed by a subsequent irradiation with light of longer wavelengths (710–740 nm), called far-red light. This phenomenon was first demonstrated in germinating seeds.

TLS Online TPP Program

#Id: 4580

#Unit 6. System Physiology – Plant

In dark-grown or etiolated plants, phytochrome is present in a red light–absorbing form, referred to as Pr because it is synthesized in this form.

TLS Online TPP Program

#Id: 4581

#Unit 6. System Physiology – Plant

Pr, which to the human eye is blue, is converted by red light to a far-red light–absorbing form called Pfr, which is bluegreen.

TLS Online TPP Program

#Id: 4582

#Unit 6. System Physiology – Plant

Pfr, in turn, can be converted back to Pr by far-red light. Known as photoreversibility, this conversion/reconversion property is the most distinctive property of phytochrome.