11.3 RNA Transcription

RNA transcription is the process of creating RNA from a DNA template. It's a crucial step in gene expression, allowing genetic information to be transferred from DNA to RNA. This process involves initiation, elongation, and termination, with RNA polymerase as the key enzyme.

Transcription differs between prokaryotes and eukaryotes, mainly in location and complexity. Prokaryotes transcribe in the cytoplasm with one RNA polymerase, while eukaryotes use three types in the nucleus. Understanding these differences is essential for grasping cellular processes.

RNA Transcription

Process of RNA synthesis

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• RNA synthesis or transcription creates RNA from a DNA template
• Occurs in the 5' to 3' direction catalyzed by the enzyme RNA polymerase
• Initiation
  • RNA polymerase binds to a promoter sequence upstream of the gene to be transcribed
  • Unwinds the DNA double helix exposing the template strand for transcription
• Elongation
  • RNA polymerase reads the template strand 3' to 5' adding complementary RNA nucleotides to the growing RNA strand 5' to 3'
    • Adenine (A) pairs with Uracil (U)
    • Thymine (T) replaced by Uracil (U) in RNA
    • Guanine (G) pairs with Cytosine (C)
  • RNA sugar-phosphate backbone formed as nucleotides are added (ribose sugar in RNA vs deoxyribose in DNA)
• Termination
  • RNA polymerase reaches a termination signal on the DNA
  • Newly synthesized RNA strand released and RNA polymerase dissociates from the DNA template

Transcription in prokaryotes vs eukaryotes

• Similarities
  • Both use DNA as a template and require RNA polymerase to catalyze RNA synthesis
  • Follow the basic steps of initiation, elongation, and termination
• Differences
  • Prokaryotic transcription
    • Occurs in the cytoplasm using a single type of RNA polymerase for all RNA synthesis
    • Transcription and translation can occur simultaneously (no nuclear membrane)
  • Eukaryotic transcription
    • Occurs in the nucleus using three types of RNA polymerase (I, II, and III) for different RNA molecules
      • RNA polymerase I: ribosomal RNA (rRNA)
      • RNA polymerase II: messenger RNA (mRNA)
      • RNA polymerase III: transfer RNA (tRNA) and other small RNAs
    • Transcription and translation are spatially and temporally separated
    • Newly transcribed RNA undergoes post-transcriptional modifications in the nucleus before translation (splicing, capping, polyadenylation)

Components of transcription process

• Promoters
  • Specific DNA sequences located upstream of the gene serving as binding sites for RNA polymerase
  • Contain consensus sequences recognized by RNA polymerase and transcription factors (TATA box in eukaryotes, -10 and -35 sequences in prokaryotes)
  • Determine the starting point and direction of transcription
• RNA polymerase
  • Enzyme that catalyzes RNA synthesis from a DNA template
  • Unwinds the DNA double helix exposing the template strand
  • Reads the template strand 3' to 5' and synthesizes RNA 5' to 3'
  • Maintains stability of the transcription bubble during elongation
  • Recognizes and binds to promoter sequences to initiate transcription
• Termination signals
  • Specific DNA sequences signaling the end of a gene and termination of transcription
  • Prokaryotic termination signals
    1. Rho-dependent termination: Rho factor protein binds to RNA causing RNA polymerase to dissociate from DNA
    2. Rho-independent termination: Inverted repeat sequences followed by A-T base pairs form a stem-loop structure in RNA causing RNA polymerase to stall and dissociate
  • Eukaryotic termination signals
    • Less well-defined involving recognition of specific sequences and binding of termination factors to the RNA polymerase complex

DNA and RNA structure

• Nucleotides: Building blocks of DNA and RNA, consisting of a sugar, phosphate group, and nitrogenous base
• Template strand: The DNA strand used as a template for RNA synthesis during transcription
• Coding strand: The DNA strand complementary to the template strand, identical in sequence to the RNA transcript (with T replaced by U)
• Base pairing: Specific hydrogen bonding between complementary nucleotides (A-T/U and G-C) that ensures accurate transcription

Genetic code and protein synthesis

• Gene: A segment of DNA that codes for a specific protein or RNA molecule
• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or stop signal during translation
• Anticodon: A sequence of three nucleotides in tRNA that is complementary to a specific codon in mRNA