RNA splicing and Spliceosome

Gene Expression: Transcription and Translation

Overview of Gene Expression

• Gene expression in both prokaryotes and eukaryotes involves two main steps: transcription and translation. Transcription is the process where DNA information is copied into RNA, while translation uses this RNA to synthesize polypeptides.

Differences Between Prokaryotic and Eukaryotic Gene Expression

• In prokaryotes, there is no membrane-bound nucleus; thus, transcription and translation occur simultaneously in the same cellular compartment. The RNA produced serves as the actual messenger RNA (mRNA) for protein synthesis.

• Eukaryotic cells exhibit compartmentalization, with transcription occurring in the nucleus and translation in the cytoplasm. The initial RNA synthesized (pre-mRNA) undergoes processing before becoming mature mRNA.

The Importance of RNA Processing

Pre-mRNA Characteristics

• Pre-mRNA contains both coding regions (exons) and non-coding regions (introns). Exons are expressed regions that code for proteins, while introns interrupt these coding sequences. This structure necessitates processing to produce a functional mRNA molecule.

Formation of Mature mRNA

• The primary transcript undergoes splicing to remove introns and join exons correctly, resulting in mature mRNA that carries an uninterrupted genetic message ready for translation. This processing occurs before mRNA exits the nucleus.

Mechanisms of RNA Processing

Modifications During Processing

• Key modifications during pre-mRNA processing include adding a 5' cap at one end and a poly-A tail at the other end, along with splicing out introns through a process known as RNA splicing. These modifications enhance stability and facilitate export from the nucleus.

Role of Spliceosome in Splicing

• The spliceosome is responsible for recognizing conserved sequences at exon-intron junctions to carry out splicing effectively. It consists of small nuclear RNAs (snRNAs) combined with proteins forming small nuclear ribonucleoproteins (snRNPs).

Steps Involved in Pre-mRNA Splicing

Assembly of Spliceosome

• Initial binding involves snRNP U1 attaching to the 5' splice site while U2 binds to the branch point's A residue, leading to spliceosome assembly with additional snRNP components joining subsequently. This forms a complex capable of looping out introns for removal.

Cleavage and Joining Process

• The process culminates with cleavage at both ends of the intron; it attaches to form a lariat structure while exons are joined together seamlessly after intron release, which is then degraded by enzymes for reuse in future splicing reactions.