Comprehensive Study Notes on mRNA Processing and Translation

Transcription

  • Definition: Synthesis of RNA from a DNA template strand.

  • DNA Strands:

    • Non-template (coding) strand: Runs 5' to 3'.

    • Template strand: Runs 3' to 5'.

  • Example Transcription Relationship:

    • Given DNA Template Strand: 3' A T G C G T A C G A T T A 5'

    • Resulting mRNA Transcript: 5' U A C G C A U G C U A A U 3' (U replaces T).

Structure of a Gene

  • RNA Polymerase: Enzyme that produces RNA.

    • Binds at the Promoter Region of the gene.

    • Initiates transcription by copying the information in the template strand through complimentary base pairing.

    • Discerns between three regions:

    • Promoter: Region where RNA polymerase binds.

    • Transcribed Region: Contains coding sequence and non-coding sequences.

    • Terminator: Signals the end of transcription; often marked by a poly A signal.

Initiation of Transcription

  • Role of Transcription Factors:

    • General transcription factors assist RNA polymerase in binding to the TATA box in eukaryotic promoters.

    • The basal transcription complex needs further activation by additional proteins to be fully functional.

Transcription Process

  • RNA polymerase Functionality:

    • Moves along the gene, opening the DNA helix and synthesizing RNA in the 5' to 3' direction based on the template strand.

    • Different types of RNA polymerases exist for various RNA types (mRNA, tRNA, rRNA).

RNA Processing in Eukaryotes

  • Location: Occurs within the nucleus.

  • Key Components:

    • RNA Splicing: Introns (non-coding sequences) are removed from pre-mRNA.

    • 5’ Cap Addition: Protects mRNA from degradation and aids in ribosomal recognition.

    • 3’ Poly A Tail Addition: Critical for mRNA stability and eventual translation readiness.

RNA Splicing Process

  • Illustration of Splicing:

    • Components:

    • Intron: Non-coding regions that are removed.

    • Exon: Coding regions that remain in the mature mRNA.

    • The spliceosome mechanism involves snRNPs (small nuclear ribonucleoproteins).

    • Spliceosome assembly process includes:

    1. Binding to the start and end of the intron, and to a branch site within the intron.

    2. Cut intron, forming a lariat structure and joining exons together.

Alternative Splicing

  • Definition: Pre-mRNA may be spliced in various ways.

    • Introns are always removed, but exons can also be omitted to create different mature mRNA variants.

    • Consequence: Multiple protein isoforms can be produced from a single gene, increasing biological diversity.

Overview of Translation

  • Translational Mechanics:

    • Mature mRNA exits the nucleus, where ribosomes bind and read the mRNA three bases (codons) at a time to assemble chains of amino acids.

    • Codons: Specific three-base sequences on mRNA that are decoded into amino acids.

Genetic Code Specifications

  • Most codons correspond directly to amino acids.

  • Special codons:

    • AUG: Start of translation; specifies Methionine (Met).

    • STOP Codons: UAA, UAG, UGA; signal termination of protein synthesis.

  • Specific but Redundant: The genetic code can specify the same amino acid through different codon arrangements.

Role of Ribosomes in Translation

  • Composed of two subunits: large and small, containing both proteins and ribosomal RNA (rRNA).

  • Unique feature of rRNA: Possesses catalytic ability (ribozymes) to speed up translation.

Transfer RNA (tRNA) Role

  • Functions:

    • Transfers specific amino acids to the ribosome.

    • Each tRNA has a 3’ attachment site for a corresponding amino acid and an anticodon that base-pairs with mRNA codons, ensuring the correct amino acid is added.

Steps of Translation

  • Initiation:

    1. The small ribosomal subunit, along with an initiator tRNA (carrying Methionine), recognizes and binds to the mRNA 5’ cap.

    2. Scanning continues until the tRNA binds to the start codon (AUG).

    3. The large ribosomal subunit attaches, positioning the starter tRNA in the P site of the ribosome.

  • Elongation:

    • Charged tRNAs enter the A site, binding if their anticodon matches the A site codon.

    • The polypeptide chain is formed as tRNA in the P site transfers its amino acid to the A site, creating a peptide bond.

    • The ribosome translocates, moving the uncharged tRNA to the E site and preparing for the next cycle.

  • Termination:

    1. A STOP codon occupies the A site.

    2. A release factor recognizes the STOP codon and binds to the A site.

    3. Ribosome disassociates from the mRNA, releasing the synthesized protein.

mRNA Degradation

  • Process:

    • Enzymatic degradation of the 3’ end of mRNA occurs, especially when the poly A tail is critically short.

    • Upon degradation, cells may need to synthesize more of that protein type if production adapts to the current cellular needs.