PR segment – Conduction through AV node

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.

TLS Online TPP Program

#Id: 6892

#Unit 3. Fundamental Processes

To modify DNA at a new position requires the action of a de novo methyltransferase, which recognizes DNA by virtue of a specific sequence. It acts only on unmethylated DNA to add a methyl group to one strand.