Co and N atoms of the peptide grouping are both in planar sp2 hybridization and the Co and O atoms are linked by a p bond, leaving the nitrogen with a lone pair of electrons in a 2p orbital.

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.

TLS Online TPP Program

#Id: 6893

#Unit 3. Fundamental Processes

There are two de novo methyltransferases (Dnmt3A and Dnmt3B) in the mouse; they have different target sites, and both are essential for development.

TLS Online TPP Program

#Id: 6894

#Unit 3. Fundamental Processes

A maintenance methyltransferase acts constitutively only on hemimethylated sites to convert them to fully methylated sites. Its existence means that any methylated site is perpetuated after replication.  There is one maintenance methyltransferase (Dnmt1) in mice, and it is essential.