RNA processing transforms raw transcripts into functional molecules. It's like turning rough diamonds into sparkling gems. Capping, polyadenylation, and splicing protect mRNA and fine-tune gene expression.
tRNA and rRNA also undergo extensive modifications. These changes are crucial for their roles in protein synthesis. RNA processing ensures the accuracy and efficiency of gene expression in cells.
RNA Capping and Polyadenylation
5' Capping and 3' Polyadenylation Processes
Top images from around the web for 5' Capping and 3' Polyadenylation Processes
How Genes Are Regulated · Concepts of Biology View original
Is this image relevant?
Transcription · Concepts of Biology View original
Is this image relevant?
Frontiers | Regulation of mRNA Stability During Bacterial Stress Responses View original
Is this image relevant?
How Genes Are Regulated · Concepts of Biology View original
Is this image relevant?
Transcription · Concepts of Biology View original
Is this image relevant?
1 of 3
Top images from around the web for 5' Capping and 3' Polyadenylation Processes
How Genes Are Regulated · Concepts of Biology View original
Is this image relevant?
Transcription · Concepts of Biology View original
Is this image relevant?
Frontiers | Regulation of mRNA Stability During Bacterial Stress Responses View original
Is this image relevant?
How Genes Are Regulated · Concepts of Biology View original
Is this image relevant?
Transcription · Concepts of Biology View original
Is this image relevant?
1 of 3
- 5' capping occurs during transcription when the RNA molecule reaches ~20-30 nucleotides in length
- Involves addition of a modified guanine nucleotide to the 5' end of the pre-mRNA
- Protects mRNA from degradation by exonucleases
- Facilitates recognition by ribosomes during translation initiation
- 3' polyadenylation takes place after transcription termination
- Adds a poly-A tail consisting of ~100-250 adenine nucleotides to the 3' end
- Enhances mRNA stability and export from the nucleus
- Influences translation efficiency and mRNA localization in the cytoplasm
- Both processes contribute to mRNA maturation and regulation of gene expression
RNA Editing Mechanisms
- RNA editing alters the nucleotide sequence of RNA molecules after transcription
- Occurs in both coding and non-coding regions of RNA
- Types of RNA editing include:
- Substitution editing (adenosine to inosine conversion)
- Insertion/deletion editing (addition or removal of nucleotides)
- Editing can change amino acid sequences, create or eliminate stop codons, or alter regulatory elements
- Plays crucial roles in expanding protein diversity and fine-tuning gene expression
- Regulated by specific enzymes (ADAR family for A-to-I editing)
RNA Splicing
Splicing Mechanism and Spliceosome Assembly
- Splicing removes introns and joins exons to form mature mRNA
- Spliceosome catalyzes the splicing reaction
- Large ribonucleoprotein complex composed of snRNPs and other proteins
- Assembles on pre-mRNA in a stepwise manner
- Splicing occurs through two transesterification reactions:
- First reaction: 5' splice site cleavage and lariat formation
- Second reaction: 3' splice site cleavage and exon ligation
- Introns contain conserved sequences for splice site recognition (GU at 5' end, AG at 3' end)
- Exons carry the coding information for protein synthesis
Alternative Splicing and Regulatory Mechanisms
- Alternative splicing generates multiple mRNA isoforms from a single gene
- Types of alternative splicing events:
- Exon skipping
- Mutually exclusive exons
- Alternative 5' or 3' splice sites
- Intron retention
- Regulated by cis-acting elements (splicing enhancers and silencers) and trans-acting factors (SR proteins, hnRNPs)
- Increases protein diversity and allows for tissue-specific or developmental stage-specific gene expression
- snRNPs (small nuclear ribonucleoproteins) play crucial roles in spliceosome assembly and catalysis
- Consist of snRNA (U1, U2, U4, U5, U6) and associated proteins
- Recognize specific sequences in pre-mRNA and facilitate splicing reactions
tRNA and rRNA Processing
tRNA Maturation and Modification
- tRNA processing involves multiple steps to generate functional molecules:
- Removal of 5' leader sequence by RNase P
- Trimming of 3' trailer sequence by endonucleases and exonucleases
- Addition of CCA sequence to 3' end by nucleotidyl transferase
- Extensive post-transcriptional modifications (over 100 known types)
- Modifications enhance tRNA stability, folding, and codon recognition
- Include methylation, pseudouridylation, and base isomerization
- Aminoacylation of mature tRNA by aminoacyl-tRNA synthetases for protein synthesis
rRNA Processing and Ribosome Assembly
- rRNA genes are transcribed as a single precursor RNA (pre-rRNA)
- Processing involves multiple cleavage and modification steps:
- Endonucleolytic cleavages separate 18S, 5.8S, and 28S rRNAs
- Exonucleolytic trimming refines rRNA ends
- Numerous chemical modifications (methylation, pseudouridylation)
- Coordinated assembly of rRNAs with ribosomal proteins
- Occurs in the nucleolus and requires many assembly factors and small nucleolar RNAs (snoRNAs)
Ribozymes and Their Catalytic Functions
- Ribozymes are RNA molecules with catalytic activity
- Examples of naturally occurring ribozymes:
- Group I and II introns (self-splicing introns)
- RNase P (processes tRNA 5' ends)
- Hammerhead ribozyme (found in some plant viroids)
- Catalyze various reactions including:
- Phosphodiester bond cleavage and ligation
- Peptide bond formation (in the ribosome)
- Provide evidence for the "RNA World" hypothesis in early evolution
- Have applications in biotechnology and therapeutic development (gene silencing, biosensors)