Eukaryotic transcripts posttranscriptional

RNA Posttranscriptional Processing Predict the likely effects of a mutation in the sequence (5 )AAUAAA in a eukaryotic mRNA transcript. 

We begin by examining the interactions between proteins and DNA that are the key to transcriptional regulation. We next discuss the specific proteins that influence the expression of specific genes, first in prokaryotic and then in eukaryotic cells. Information about posttranscriptional and translational regulation is included in the discussion, where relevant, to provide a more complete overview of the rich complexity of regulatory mechanisms. 

A primary transcript is a linear copy of a transcriptional unit—the segment of DNA between specific initiation and termination sequences. The primary transcripts of both prokaryotic and eukaryotic tRNAs and rRNAs are post-transcriptionally modified by cleavage of the original transcripts by ribonucleases. tRNAs are then further modified to help give each species its unique identity. In contrast, prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic nrRNA is extensively modified posttranscriptionally. 

Prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic mRNA is extensively modified posttranscriptionally. For example, a 7-methyl-guanosine "cap" is attached to the 5-terminal end of the mRNA through a triphosphate linkage by guanylyl-transferase. A long poly-Atail—not transcribed from the DNA—is attached to the 3 -end of most mRNAs. Many eukaryotic mRNAs also contain introns that must be removed to make the mRNA functional. Their removal requires small nuclear RNAs. 

The RNA polymerase is a metalloenzyme and contains Zn2+. The first nucleotide is often an ATP or GTP and the triphosphate is maintained unless the 50 end of the transcript is posttranscriptionally modified, as is the case in eukaryotes (Chapter 25). The RNA polymerization reaction is chemically very similar to DNA synthesis in that the nascent RNA strand is synthesized in the 50 to 30 direction. In this reaction, the 30 hydroxyl group of the existing RNA chain undergoes a nucleophile attack on the a-phosphate of the incoming nucleotide which then releases PPi. 

The separation of transcription and translation means there are many more opportunities for control in eukaryotes. Eor example, expression can be controlled at a posttranscriptional level. The cell can alter the rate at which RNA transcripts are... 

Messenger RNA processing/modification. Most primary mRNA transcripts in prokaryotes function in translation without further modification. Eukaryotic mRNA transcripts, however, undergo extensive posttranscriptional modifications. 

Polyadenylation is a critical posttranscriptional processing step in the maturation of eukaryotic mRNA [1], The location where the pre-mRNA is cleaved (also known as the poly(A) site) marks the end of a mRNA transcript. Many eukaryotic genes possess two or more poly(A) sites [2-4], and thus are involved in alternative polyadenylation (APA). APA is a powerful pathway that entails the selection of alternate poly(A) sites in a pre-mRNA and leads to the production of multiple mature mRNA isoforms from the same gene, resulting in potential gene expression regulation [1]. 

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