In-depth Notes on RNA Processing and DNA Methylation

RNA Processing Overview

  • RNA processing is essential for creating mature, functional mRNA in eukaryotic cells.
  • Major steps include:
    • Addition of a 5' cap
    • Polyadenylation at the 3' end
    • Intron splicing

DNA Methylation

  • Base methylation of DNA is crucial for regulating gene expression.
  • Not to be confused with histone methylation; primarily affects cytosine and adenine nucleotides.
  • Over 50% of CpG sequences in mammalian genomes are methylated on the C residue, which generally inhibits gene expression.
  • Mechanism: Methylation disrupts the binding of transcription machinery, silencing gene expression.
Observing Methylation
  • Methylation distribution can be analyzed using restriction enzymes such as HpaII (cuts only unmethylated CCGG) versus MspI (cuts all CCGG).
Methylation and Gene Silencing
  • Methylation can completely silence genes by preventing transcription machinery access or recruiting repressive complexes.
  • Constitutively expressed genes tend to remain unmethylated, while tissue-specific genes show variable methylation patterns.
Epigenetic Inheritance
  • Maintenance methyltransferase ensures that existing methylation patterns are inherited by daughter DNA strands during cell division, impacting gene expression across generations.
CG Islands
  • CG islands are unmethylated CpG-rich regions often found upstream of gene promoters, indicative of transcription initiation sites.
  • Approximately 60% of protein-coding genes have promoters embedded within CG islands, facilitating their transcription potential.

Eukaryotic mRNA Processing

5' RNA Cap
  • The 5' cap (G-cap) is crucial for mRNA stability and promotes translation.
    • Synthesized early during transcription by guanylyltransferase.
    • Binds to cap-binding complex (CBC) which recruits mRNAs for ribosome translation.
Structure and Function of the 5' Cap
  • Added in reverse orientation using a 5'-5' triphosphate link, with several methylation modifications.
  • Essential for protecting the mRNA and aiding in ribosomal attachment for translation.
3' Polyadenylation
  • In eukaryotes, transcription terminates at specific sites, with the 3' end defined by cleavage and polyadenylation.
  • The AAUAAA motif recognized by CPSF (cleavage and polyadenylation specificity factor) directs this process.
Functions of the Poly(A) Tail
  • Contributes to mRNA stability, translational initiation, and intron splicing near the 3' end.
  • Is bound by poly(A) binding protein (PABP) which interacts with the CBC and enhances translation initiation.

Intron Splicing

  • Pre-mRNA contains introns that are spliced out during processing, producing a continuous coding sequence.
  • Typically, the number of introns increases in higher organisms compared to simpler ones like yeast, contributing to protein diversity.
Alternative Splicing
  • Allows a single gene to produce multiple mRNA isoforms.
    • Over 90% of human genes undergo alternative splicing, resulting in diverse protein products.
  • Regulated by splicing factors that bind to splicing enhancers or silencers, allowing for tissue-specific expression patterns.
Examples of Alternative Splicing
  • Tissue-specific splicing differences can be seen in calcitonin and CGRP proteins derived from the same gene but with completely different functions.
  • A notable example in Drosophila concerns the Dscam locus, which can generate over 38,000 protein isoforms vital for neural connections.
Identifying Introns and Splicing
  1. cDNA Sequencing:
    • Utilizes first-generation sequencing for single genes and RNA-Seq for transcriptome-wide analysis.
  2. Splicing Mechanism Recognition:
    • Recognizes intron-exon boundaries for precise splicing through specific nucleotide sequences in pre-mRNA.
Conclusion
  • Detailed understanding of RNA processing mechanisms highlights their complexity and importance in gene regulation and expression.