A countercurrent system is a system in which the inflow runs parallel to, counter to, and in close proximity to the outflow for some distance. This occurs for both the loops of Henle and the vasa recta in the renal medulla

TLS Online TPP Program

#Id: 6886

#Unit 3. Fundamental Processes

Recruitment of Polycomb repressor complexes (PRC1 and PRC2), which trigger a series of chromosome-wide histone modifications (H2AK119 ubiquitination, H3K27 methylation, H4K20 methylation, and H4 deacetylation). Late in the process, an inactive X-specific histone variant, macroH2A

TLS Online TPP Program

#Id: 6887

#Unit 3. Fundamental Processes

Inactivation of the X chromosome in females is governed by the n–1 rule

TLS Online TPP Program

#Id: 6888

#Unit 3. Fundamental Processes

Regardless of how many X chromosomes are present, all but one will be inactivated. 

In normal females there are of course two X chromosomes, but in rare cases where nondisjunction has generated a 3X or greater genotype, only one X chromosome remains active

TLS Online TPP Program

#Id: 6889

#Unit 3. Fundamental Processes

Xic is a cis-acting locus that contains the information necessary to count X chromosomes and inactivate all copies but one. Inactivation spreads from Xic along the entire X chromosome.

TLS Online TPP Program

#Id: 6890

#Unit 3. Fundamental Processes

Xic is a complex genetic locus that expresses several long noncoding RNAs (ncRNAs). The most important of these is a gene called Xist (X  (X inactive specific transcript), which is stably expressed only on the inactive X chromosome.

TLS Online TPP Program

#Id: 6891

#Unit 3. Fundamental Processes

The Xist transcript is regulated in a negative manner by Tsix, its antisense partner.