Gray Crescent 

The region opposite the point of sperm entry, where the greatest displacement of cytoplasm occurs (Gastrulation will begin)

Vegetal rotation: Vegetal cells asymmetrically press up against the inner blastocoel roof on the dorsal side.

Bottle cell formation and invagination: Localized apical constriction at the dorsal blastopore lip creates anisotropic forces that foster invagination.

Involution and cell migration: The leading edge of invaginating cells crawls up onto the blastocoel roof.

Convergence and extension: The targeted medial-to-lateral intercalation of cells on the midline (convergence) drives anterior-posterior axis elongation (extension).

#Id: 2439

#Unit 3. Fundamental Processes

The coupling between transcription and translation has important consequences for rho action. Rho must first have access to RNA upstream of the transcription complex and then moves along the RNA to catch up with RNA polymerase. 

#Id: 2440

#Unit 3. Fundamental Processes

In some cases, a nonsense mutation in one gene of a transcription unit was found to prevent the expression of subsequent genes in the unit even though both genes had their own ribosome binding sites, an effect called polarity.

#Id: 2441

#Unit 3. Fundamental Processes

Rho-dependent termination sites within a transcription unit are usually masked by translating ribosomes, and therefore rho cannot act on downstream RNA polymerases. 

#Id: 2442

#Unit 3. Fundamental Processes

Nonsense mutations release ribosomes within the RNA of a multigene operon, though, enabling rho to terminate transcription prematurely and prevent expression of distal genes in the transcription unit even though their open reading frames contained wild-type sequences

#Id: 2443

#Unit 3. Fundamental Processes

The need for rho factor in vitro is variable. Some (rho-dependent) terminators require relatively high concentrations of rho, whereas others function just as well at lower levels. 

#Id: 2444

#Unit 3. Fundamental Processes

Different terminators require different levels of rho factor for termination and therefore respond differently to the residual levels of rho factor in the mutants (rho mutants are usually leaky).