From the speed of replication, it is evident that a mammalian genome could be replicated in 1 hour if all replicons functioned simultaneously. S phase actually lasts for 6 hours in a typical somatic cell, though, which implies that no more than 15% of the replicons are likely to be active at any given moment.

TLS Online TPP Program

#Id: 1759

#Unit 4. Cell Communication and Cell Signaling

Mechanisms to protect an individual from this sort of anti-self immune attack must exist, and they do, called as tolerance

TLS Online TPP Program

#Id: 1760

#Unit 4. Cell Communication and Cell Signaling

Self-tolerance involving both the elimination of immune cells that can react against self-antigens and active inhibition of immune responses.

TLS Online TPP Program

#Id: 1761

#Unit 4. Cell Communication and Cell Signaling

Self-reactive B cells can undergo receptor editing.

In this process, the antigen-specific V region is “edited” or switched for a different V-region gene segment via V(D)J recombination,

TLS Online TPP Program

#Id: 1762

#Unit 4. Cell Communication and Cell Signaling

Central tolerance deletes self-reactive T- or B-cell clones before the cells are allowed to mature. occurs in the primary lymphoid organs

TLS Online TPP Program

#Id: 1763

#Unit 4. Cell Communication and Cell Signaling

Peripheral tolerance, which renders some self-reactive lymphocytes in secondary lymphoid tissues inactive and generates others that actively inhibit immune responses against self

TLS Online TPP Program

#Id: 1764

#Unit 4. Cell Communication and Cell Signaling

Self antigens are not uncommon in the periphery, Two factors contribute to this:                                                                                                                             a. not all self a. antigens are expressed in the central lymphoid organs where negative selection occurs

b. there is a threshold requirement for affinity to self antigens before clonal deletion is triggered, allowing some weakly self-reactive clones to survive the weeding-out process.