10-1

Overview of Translation and RNA Editing

  • Week 10 materials focus on translation, specifically RNA editing

RNA Editing

  • Definition: RNA editing refers to the alteration of RNA molecules post-transcription, which modifies the information content of genes.

  • Mechanism:

    • It involves the modification of mRNA guided by small RNA molecules known as guide RNAs (gRNAs) that function similarly to GPS, directing enzymes where to edit or delete sequences.

    • Examples of RNA modifications include insertions, deletions, and changes to specific nucleotide bases.

Guide RNA and CRISPR-Cas9

  • Guide RNA:

    • Plays a role in RNA editing by providing templates for enzymes to perform precise edits.

    • Essential in correcting incomplete or faulty mRNA, ensuring the mRNA is fully functional for translation.

  • CRISPR-Cas9 System:

    • Found in certain bacteria and archaea, used for defense against viral infections.

    • Functions by integrating pieces of viral DNA into its own genome as spacers, which guide subsequent attacks by targeting the corresponding DNA in invading viruses.

    • This system allows for precise editing of genomes by guiding the Cas9 enzyme to specific DNA sequences, resulting in cuts that facilitate gene modification.

RNA Editing Examples

  • Deamination:

    • Process where an amine group is removed from a nucleotide, converting cytosine (C) to uracil (U).

    • Example:

    • Cytosine in apolipoprotein B RNA is deaminated to uracil, changing codon CAA (for glutamine) to UAA (a stop codon), resulting in a truncated protein with altered function relating to lipid binding.

  • Implications of Editing:

    • Changes in structure lead to variations in protein length and function, particularly noted in different tissues (e.g., liver vs intestine).

Mechanisms of RNA Editing

  • Guide RNAs serve to identify where edits must occur within the unedited mRNA by pairing with its complementary sequence.

  • The editing process involves removing incorrect uridine bases and incorporating the correct ones, producing mature mRNA ready for translation.

Transfer RNA (tRNA) Modifications

  • Structure: tRNA molecules are characterized by a folded structure resembling a cloverleaf.

    • Contains a 5’ end and a 3’ end, the latter where amino acids are covalently attached.

  • Anticodon:

    • Positioned at the bottom of the tRNA, complementary to mRNA codons, crucial for accurate translation.

  • Rare Bases:

    • Modified ribonucleotides in tRNA that enhance structural stability, codon recognition, and protect against degradation.

  • Modification Process:

    • tRNA undergo modifications such as intron removal (self-splicing) and the addition of the CCA sequence at the 3’ end, necessary for amino acid bonding.

Ribosomal RNA (rRNA)

  • Differences between Prokaryotic and Eukaryotic rRNA:

    • Prokaryotes: 70S ribosomes composed of 30S and 50S subunits, while eukaryotes possess 80S ribosomes formed from 40S and 60S subunits.

    • Svedberg units (S): Measure sedimentation rates based on size, shape, and density, not additive (e.g., 30S + 50S = 70S).

  • Functionality:

    • 16S (prokaryotes) essential for mRNA binding; 23S (prokaryotes) contributes to peptide bond formation; respective counterparts in eukaryotes serve similar roles.

  • Nucleolus Function: In eukaryotic cells, the nucleolus synthesizes rRNA and assembles ribosomal subunits, which then migrate to the cytoplasm to facilitate translation.

Types of Non-Coding RNAs

  • Small Nuclear RNA (snRNA): Important for splicing during mRNA processing.

  • Micro RNA (miRNA) and Small Interfering RNA (siRNA): Regulate gene expression through various mechanisms.

  • Long Non-Coding RNA (lncRNA): Function to control gene expression at transcriptional and translational levels, often larger in size than other RNA types.