Both the pi helix and the 310 helix occur rarely and usually only at the ends of a helices or as single-tum helices. They are not energetically favorable, since the backbone atoms are too tightly packed in the 310 helix and so loosely packed in the n helix that there is a hole through the middle. 

Only in the a helix are the backbone atoms properly packed to provide a stable structure.

TLS Online TPP Program

#Id: 1997

#Unit 3. Fundamental Processes

The polyadenylation reaction passes through two stages. 

First, a rather short oligo(A) sequence (10 residues) is added to the 3’ end. This reaction is absolutely dependent on the AAUAAA sequence and poly(A) polymerase performs it under the direction of the specificity factor. 

In the second phase, the nuclear poly(A) binding protein (PABP II) binds the oligo(A) tail to allow extension of the poly(A) tail to the full 200 residue length.

TLS Online TPP Program

#Id: 1998

#Unit 3. Fundamental Processes

The rapid phase requires the binding of multiple copies of a poly(A)-binding protein containing the RRM motif 

TLS Online TPP Program

#Id: 1999

#Unit 3. Fundamental Processes

In wild-type genes, RNA polymerase II terminates transcription at any one of multiple possible sites within about

2 kb of the polyadenylation signal. When the polyadenylation signal is mutated, RNA polymerase II does not terminate transcription, but continues transcription until the next poly(A) site.

TLS Online TPP Program

#Id: 2000

#Unit 3. Fundamental Processes

Histone mRNAs are not polyadenylated; their 3’ ends is recognized by hairpin structure.

TLS Online TPP Program

#Id: 2001

#Unit 3. Fundamental Processes

Histone biogenesis is primarily controlled by the regulation of histone mRNA abundance during the cell cycle. At this G1/S transition, the abundance of histone mRNAs is increased more than 30-fold due to elevated transcription; this process is regulated by the Cyclin E/Cdk2 complex.

TLS Online TPP Program

#Id: 2002

#Unit 3. Fundamental Processes

The rise in histone mRNAs is followed by a rapid decay of histone mRNAs at the end of S phase.