Transcription Overview and Steps

Transcription Overview

• Transcription occurs in cytoplasm for prokaryotes and in the nucleus for eukaryotes.
• DNA serves as the template to synthesize mRNA.

Steps of Transcription

• Initiation:
  • RNA polymerase holoenzyme binds to the promoter.
  • Sigma subunit facilitates DNA unwinding, allowing RNA synthesis to commence.
• Elongation:
  • RNA polymerase synthesizes RNA in the 5’ to 3’ direction while moving 3’ to 5’ along the DNA.
  • Formation of phosphodiester bonds between ribonucleotides is catalyzed by RNA polymerase.
  • The β subunit ensures correct ribo-nucleotide addition and catalyzes RNA synthesis.
  • A proofreading mechanism enables correction of any incorporated errors during transcription.
• Termination:
  • RNA polymerase encounters a termination signal, leading to the release of the synthesized RNA.
  • Cleavage occurs, and a polyadenylation signal (AAUAAA) prompts the addition of a poly(A) tail.

Post-Transcription Modifications in Eukaryotes

• Capping:
  • 5’ cap (7-methylguanosine) added to the nascent RNA transcript for protection and translation initiation.
• Splicing:
  • Introns (non-coding regions) are removed, while exons (coding sequences) are joined by the spliceosome to form mRNA.
• Polyadenylation:
  • Addition of a poly(A) tail to 3’ end increases RNA stability and facilitates export to cytoplasm.
• RNA Editing:
  • Nucleotide sequence may be modified post-transcription, involving insertions, deletions, or substitutions.
• RNA Transport:
  • Mature RNA transported from nucleus to cytoplasm via export signals and proteins.
• RNA Stability and Degradation:
  • Lifespan of RNA is regulated by intrinsic sequences affecting stability.
• Alternative Splicing:
  • Different proteins generated from a single gene through splicing variations increases proteomic diversity.

Prokaryotic vs. Eukaryotic RNA Polymerase

• Prokaryotic polymerase is a core enzyme with five subunits (α, β, β’, ω) plus a sigma factor to initiate transcription.
• Eukaryotic RNA polymerase has three types:
  • RNA Polymerase I: Synthesizes rRNA.
  • RNA Polymerase II: Synthesizes mRNA and some snRNA.
  • RNA Polymerase III: Synthesizes tRNA and small RNAs.

DNA Transcription Unit

• Composed of:
  • Promoter: Upstream sequence signaling transcription start.
  • Coding region: DNA sequence that is transcribed, including exons and introns.
  • Terminator: Downstream sequence signaling the end of transcription.

Translation Overview

• Translation decodes mRNA to produce proteins, occurring in the cytoplasm.

Key Components in Translation

• mRNA: Carries genetic instructions from DNA to ribosome.
• Ribosomes: Complexes of rRNA and proteins that synthesize proteins from amino acids.
• tRNA: Adapters that transport specific amino acids to ribosomes, matching anticodons with mRNA codons.
• Amino Acids: Building blocks of proteins, with 20 different types assembled in specific sequences.

Steps of Translation

• Initiation:
  • Small ribosomal subunit binds to start codon (AUG) on mRNA.
  • Initiator tRNA binds with methionine; large ribosomal subunit joins to form full ribosome.
• Elongation:
  • Ribosome reads codons on mRNA, bringing in corresponding tRNAs with the nucleotides.
  • Peptide bonds form between amino acids as the polypeptide chain grows.
  • Ribosome has three functional sites: A (aminoacyl), P (peptidyl), and E (exit).
• Termination:
  • When a stop codon (UAA, UAG, UGA) is reached, no tRNA pairs, triggering release factors.
  • Newly synthesized polypeptide chain is discharged; ribosome subunits dissociate for reuse.

Post-Translational Modifications

• New proteins may undergo folding, cleavage, and functional group additions to achieve full functionality.
• Translation is a highly regulated process, ensuring accuracy and efficiency according to mRNA instructions.

This quiz covers the essential processes of transcription in prokaryotes and eukaryotes, including initiation, elongation, and termination. It also highlights post-transcription modifications in eukaryotic cells. Test your understanding of RNA polymerase functions and the significance of mRNA synthesis.