Primitive Streak

First arises from a local thickening of the epiblast at the posterior edge of the area pellucida (Koller’s sickle)

By stage 2, the primitive streak cells form a third layer that lies between the hypoblast and epiblast cells.

 By stage 3, the primitive streak has become a definitive region of the epiblast, with cells migrating through it to become the mesoderm and endoderm.

TLS Online TPP Program

#Id: 6665

#Unit 3. Fundamental Processes

There are three type of Splicing Machinery

TLS Online TPP Program

#Id: 6666

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6667

#Unit 3. Fundamental Processes

The group I and group II introns have the remarkable ability to excise themselves from an RNA. This is called autosplicing, or self splicing. 

Group I introns are more common than group II introns.

TLS Online TPP Program

#Id: 6668

#Unit 3. Fundamental Processes

Two transesterification reactions takes place for 2 exon -1 intron configuration. First transesterification is catalysed variably either by external G or branch point while second transesterification is always caused by exon 1 -3’OH.

TLS Online TPP Program

#Id: 6669

#Unit 3. Fundamental Processes

There are three type of Splicing signals 

 Major U2-type or GU-AG introns

Minor U12-type or AU-AC introns

Minor U12-type with GU-AG introns (hybrid type)

TLS Online TPP Program

#Id: 6670

#Unit 3. Fundamental Processes

UACUAAC is branch point sequence for U2 type introns CCUUAAC is branch point sequence for U12 type introns