mRNA Processing Notes

mRNA Processing

Introduction

  • This lecture covers eukaryotic mRNA processing during transcription.

  • Key processes:

    • 5' capping

    • Polyadenylation (poly(A) tail addition)

    • Splicing (intron removal)

  • These steps ensure mRNA transport and proper translation.

  • Alternative splicing and non-coding RNAs are also discussed.

Pre-mRNA

  • Pre-mRNA is the initial RNA after transcription.

  • Pre-mRNA contains:

    • Exons: Coding regions for protein synthesis.

    • Introns: Non-coding regions that must be removed.

  • Exons are expressed regions.

Essential mRNA Processing Steps

  • mRNA must undergo these modifications before transport:

    • 5' capping

    • Polyadenylation

    • Splicing

Timing of Processing Events

  • 5' capping starts as the 5' end emerges from RNA polymerase.

  • Splicing begins when introns are visible.

  • Polyadenylation occurs near the end of transcription.

  • These processes happen quickly.

5' Capping

  • Adds 7-methylguanosine to the 5' end.

  • Forms a 5'-5' triphosphate linkage.

Polyadenylation

  • Adds adenine nucleotides to the 3' end.

  • Poly(A) tail length affects mRNA lifespan.

Detailed Look at 5' Capping

  • RNA is synthesized 5' to 3', so the 5' end appears first.

  • An enzyme adds 7-methylguanosine, creating a 5'-5' triphosphate linkage.

Functions of the 5' Cap and Poly(A) Tail

  • Protection: Protect mRNA from degradation.

  • Stability: Increase mRNA stability.

  • Translation: 5' cap is essential for ribosome binding.

Polyadenylation Signals and Mechanism

  • A specific sequence signals the end of the coding region.

  • The complex recognizes this sequence.

  • Cleavage occurs 10-30 nucleotides downstream.

  • A U-rich sequence identifies the cleavage site.

  • After cleavage, PAP adds a poly(A) tail to the 3' end.

Poly(A) Polymerase (PAP)

  • PAP doesn't need a template.

  • It adds adenine nucleotides to the 3' end.

Role of Poly(A) Binding Proteins (PABPs)

  • PABPs bind to the poly(A) tail and protect it.

  • They maintain mRNA integrity.

  • PABPs bind to the large subunit of the ribosome.

Consequences of Errors in mRNA Processing

  • Incorrect 5' capping or poly(A) tail addition can prevent proper translation.

  • Both modifications are crucial.

Preview of Splicing and Alternative Splicing

  • Splicing increases protein variety from a single gene.

Key Concepts and Acronyms

  • CPSF: Cleavage and Polyadenylation Specificity Factor

  • PAP: Poly(A) Polymerase