BIO102 Ch17a

Chapter 17: Transcription and RNA Processing

Introduction to Transcription, RNA Processing, and Translation

• Protein synthesis: Cells build proteins from instructions in their genes.

• Role of DNA: DNA is transcribed into messenger RNA (mRNA), which is then translated into proteins.

Key Concepts

• Types of RNA:

  • mRNA: Messenger RNA that serves as a template for protein synthesis.

  • rRNA: Ribosomal RNA, vital for ribosome structure and function.

  • tRNA: Transfer RNA, which carries amino acids to the ribosomes during translation.

  • Other small RNAs: Various roles in the cell, such as regulatory functions.

Transcription Overview

• Polymerization process: Includes three main stages:

  • Initiation: Begins transcription.

  • Elongation: RNA synthesis progresses.

  • Termination: Ends transcription.

Initiation of Transcription in Bacteria

• RNA polymerase core needs a sigma protein to start transcription.

• Promoter regions: Specific DNA sequences where RNA polymerase binds, directing it to start transcription.

• Holoenzyme formation: RNA polymerase plus sigma factor form a holoenzyme.

• Different sigma proteins recognize different promoters, allowing gene activation in response to environmental changes.

Elongation Phase

• Takes place after the sigma factor disassociates from the RNA polymerase.

• RNA polymerase moves along the DNA template, synthesizing RNA in the 5' to 3' direction, also has proofreading activity.

Termination of Transcription in Bacteria

• Involves a transcription-termination signal that causes RNA to form a hairpin loop, leading to RNA polymerase detaching from the DNA and completing transcription.

Eukaryotic Transcription

• More complex than bacterial transcription, involving three types of RNA polymerases:

  • RNA polymerase I: Transcribes rRNA (except 5S rRNA).

  • RNA polymerase II: Transcribes mRNA and some snRNA.

  • RNA polymerase III: Transcribes tRNA, 5S rRNA, and some snRNA.

• Eukaryotic promoters tend to be more diverse with additional regulatory elements.

Initiation in Eukaryotes

• Requires a core (basal) promoter.

• Basal transcription factors (like TATA-binding proteins) bind to the core promoter before RNA polymerase can initiate transcription.

Termination in Eukaryotes

• A polyadenylation signal is transcribed, resulting in the mRNA being cut and RNA polymerase detaching from DNA.

RNA Processing in Eukaryotes

• Involves modifications to the primary RNA transcript (pre-mRNA) including:

  • Splicing: Removal of introns and joining of exons to produce mature mRNA.

  • 5’ capping: Addition of a 5' cap for stability and recognition.

  • 3’ polyadenylation: Addition of a poly(A) tail to the mRNA.

Comparison of Transcription in Bacteria and Eukaryotes

Conclusion

• Understanding transcription and RNA processing is essential for grasping how genes dictate protein synthesis, with notable differences between prokaryotic and eukaryotic processes.