RNA processing in eukaryotes is a complex dance of molecular modifications. After transcription, pre-mRNA undergoes several key steps: 5' capping , 3' polyadenylation , and splicing . These changes prepare the RNA for its journey from the nucleus to the cytoplasm.
The process involves removing non-coding introns and joining coding exons . This splicing allows for alternative forms of proteins from a single gene. Other RNA types, like tRNA and rRNA, have their own unique processing steps, all crucial for proper cellular function.
RNA Processing in Eukaryotes
Steps in eukaryotic RNA processing
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Transcription
RNA polymerase synthesizes a pre-mRNA molecule using a DNA template as a guide
Pre-mRNA contains both exons which are coding sequences and introns which are non-coding sequences
5' capping
7-methylguanosine cap is added to the 5' end of the pre-mRNA molecule
Protects mRNA from degradation by exonucleases and facilitates translation initiation by recruiting ribosomes
3' polyadenylation
Poly(A) tail consisting of multiple adenine nucleotides is added to the 3' end of the pre-mRNA
Enhances mRNA stability by preventing degradation and facilitates export from the nucleus to the cytoplasm
Splicing
Introns are removed from the pre-mRNA molecule by the spliceosome complex
Exons are joined together by the spliceosome to form the mature mRNA molecule ready for translation
The spliceosome recognizes specific splice sites at the intron-exon boundaries
Nuclear export
Mature mRNA is actively transported from the nucleus to the cytoplasm through nuclear pore complexes
In the cytoplasm, mature mRNA can be translated by ribosomes to produce functional proteins
Exons and introns in mRNA splicing
Exons
Coding sequences that contain the genetic information necessary for protein synthesis (amino acid sequence)
Exons are joined together during splicing to form the mature mRNA ready for translation
Alternative splicing allows for the production of different protein isoforms from a single gene (CD44 , tropomyosin )
Introns
Non-coding sequences that interrupt the coding regions of a gene but are removed during mRNA processing
Introns are removed during the splicing process by the spliceosome complex
Some introns contain regulatory sequences like enhancers that influence gene expression levels
Self-splicing introns found in some organisms can catalyze their own removal from the pre-mRNA (group I, group II introns )
Introns contain a branch point , which is crucial for the formation of the lariat structure during splicing
Processing of mRNA vs tRNA vs rRNA
mRNA processing
Pre-mRNA undergoes 5' capping, 3' polyadenylation, and splicing to remove introns
Mature mRNA is exported to the cytoplasm where it serves as a template for protein synthesis
tRNA processing
Pre-tRNA is transcribed by RNA polymerase III and contains extra sequences on both ends
5' leader sequence is removed by RNase P and 3' trailer sequence is removed by RNase Z
Introns, if present, are removed by splicing enzymes specific to tRNAs
CCA sequence is added to the 3' end by a nucleotidyltransferase enzyme as the amino acid attachment site
Nucleotide modifications occur at various positions (pseudouridine , dihydrouridine ) to stabilize tRNA structure
rRNA processing
Pre-rRNA is transcribed by RNA polymerase I (28S, 18S, 5.8S rRNAs) and RNA polymerase III (5S rRNA )
Pre-rRNA is cleaved by endonucleases and exonucleases to generate mature rRNA molecules
Nucleotide modifications like pseudouridylation and 2'-O-methylation occur at specific sites to facilitate rRNA folding
Mature rRNAs assemble with ribosomal proteins to form the small (40S) and large (60S) ribosomal subunits
Additional RNA processing mechanisms
RNA editing : A process that alters the nucleotide sequence of an RNA molecule after transcription
Ribonucleoproteins : Complexes of RNA and proteins that play various roles in RNA processing and function
snRNPs (small nuclear ribonucleoproteins): Key components of the spliceosome that recognize and bind to specific RNA sequences during splicing