In mammalian cells, Cdt1 is targeted for degradation by the action of a protein complex that is recruited to the origin of replication by PCNA, the eukaryotic counterpart of the bacterial b clamp.

TLS Online TPP Program

#Id: 4748

#Unit 5. Developmental Biology

Once PAR-1 is in the cortical cytoplasm, it phosphorylates PAR-3 causing PAR-3 and its binding partners PAR-6 and PKC-3 to leave the cortex.

TLS Online TPP Program

#Id: 4749

#Unit 5. Developmental Biology

At the same time, the sperm microtubules induce the contraction of the actin-myosin cytoskeleton toward the anterior, thereby clearing PAR-3, PAR-6, and PKC-3 from the posterior of the 1-cell embryo.

TLS Online TPP Program

#Id: 4750

#Unit 5. Developmental Biology

During first cleavage, the metaphase plate is closer to the posterior, & fertilized egg is divided into two cells:

one having the anterior PARs (PAR-6 and PAR-3)

one having the posterior PARs (PAR-2 and PAR-1)

TLS Online TPP Program

#Id: 4751

#Unit 5. Developmental Biology

C. elegans demonstrates both the conditional and autonomous modes of cell specification. 

TLS Online TPP Program

#Id: 4752

#Unit 5. Developmental Biology

The P1 cell develops autonomously without the presence of AB, generating all the cells it would normally make, and the result is the posterior half of an embryo.

TLS Online TPP Program

#Id: 4753

#Unit 5. Developmental Biology

AB cell in isolation makes only a small fraction of the cell types it would normally make.