RNA Synthesis and Classes Quiz

RNA Synthesis, Processing, and Modification

• RNA synthesis is a complex process using DNA-dependent RNA polymerase enzymes and additional proteins.
• General steps involve initiation, elongation, and termination.
• RNA must be processed to become mature and active.
• mRNA synthesis and metabolism are crucial for adaptation to environmental changes and maintaining cell structure and function.
• Errors in synthesis, processing, splicing, stability, or mRNA function can cause diseases.

RNA Classes

• Two major types of RNA exist in eukaryotic cells:
  • Messenger RNA (mRNA): Protein-coding, usually unstable or moderately stable.
  • Non-protein coding RNA (ncRNA): Includes rRNA, lncRNA, tRNA, and miRNA, involved in regulation or protein synthesis.
• mRNA directly encodes proteins.
• rRNA is part of the ribosome for translation.
• tRNA transfers amino acids to ribosomes.
• lncRNA and miRNA are involved in gene regulation, either by influencing mRNA stability or modulating transcription.

RNA Synthesis from a DNA Template

• RNA synthesis is similar to DNA synthesis, using initiation, elongation, and termination with 5'→3' polarity, multicomponent complexes, and Watson-Crick base pairing.
• Differences include:
  • Ribonucleotides are used instead of deoxyribonucleotides.
  • Uracil (U) replaces thymine (T).
  • RNA polymerases can initiate synthesis without a primer.
  • RNA synthesis transcribes specific genome portions.
  • RNA polymerases have less proofreading, leading to higher mutation rates.

Transcription

• The template strand of DNA is transcribed into RNA, read in 3'→5' direction.
• The non-template strand corresponds to the mRNA sequence (except T→U).
• RNA polymerase binds to the promoter, a specific DNA region, to begin transcription.
• Transcription starts at the transcription start site (TSS).
• A transcription unit includes initiation, elongation, and termination signals, often a single gene in eukaryotes, but multiple in prokaryotes.
• The primary transcript is the initial RNA product.

Bacterial RNA Polymerase

• Bacterial RNA polymerase (RNAP) has a core enzyme (ββ′α2ω) for RNA synthesis and a sigma (σ) factor for promoter binding.
• The sigma factor is crucial for promoter recognition; it reduces non-promoter affinity and increases promoter affinity.
• Multiple sigma factors exist; each recognizes different promoters, leading to the expression of different genes and regulating gene expression in response to environmental changes (e.g., sporulation, nutrient deprivation, heat shock).

Eukaryotic RNA Polymerases

• Mammalian cells possess three nuclear DNA-dependent RNA polymerases and a mitochondrial RNA polymerase.
• Eukaryotic RNA polymerase II transcripts are modified with a 7-methylguanosine cap, essential for processing, translation, and mRNA protection.

Summary

• RNA synthesis relies on specific enzymes and protein factors.
• RNA processing is necessary for functional RNA types.
• Transcription and RNA structure are crucial for many cellular processes.