Gene Expression and Regulation

• Transcription

  Rewriting information from one form to another, in this case from DNA to mRNA
• Transcription in eukaryotes

  1. DNA to pre-mRNA

  2. Pre-mRNA processing (add 5' cap, poly-A tail, splice out introns)

  3. Mature mRNA
• Transcription in prokaryotes

  DNA to mRNA
• RNA polymerase

  • Attaches to promoter to know where to start
  • Separates DNA strands
  • Encodes RNA in 5' to 3' direction
• Template strand

  The strand of DNA that RNA polymerase interacts with and uses as a template
• Coding strand

  The strand of DNA that contains the same sequence as the resulting mRNA
• Terminator


  Sequence that signals RNA polymerase to stop transcription
• 5' cap

  • Modified guanine added to 5' end of mRNA to help in translation
• Poly-A tail

  • Sequence of adenines added to 3' end of mRNA to help stabilize it
• Introns


  Sequences in pre-mRNA that do not code for the protein and are spliced out
• Exons


  Sequences in pre-mRNA that do code for the protein and remain after splicing
• Post-transcriptional regulation


  Regulation that occurs once DNA has been transcribed into mRNA
• Post-transcriptional regulation


  1. DNA transcribed into RNA

  2. RNA gets a "haircut" (introns spliced out)

  3. RNA gets a 5' cap and 3' poly-A tail

  4. Processed mRNA exits nucleus
• Post-transcriptional regulation

  • Occurs in eukaryotes only
  • Helps stabilize mRNA to protect from degradation before translation
• Exons


  Sequences in RNA that code for the protein product
• Introns


  Non-coding sequences in RNA that get spliced out
• Splicing

  1. Spliceosome binds to either side of intron

  2. Loops intron into a circle

  3. Cleaves off intron

  4. Ligates exposed exons together
• 5' cap


  Modification at 5' end of mRNA that protects from degradation, promotes ribosomal binding, and regulates nuclear export
• Poly-A tail


  Multiple adenosine monophosphates added to 3' end of mRNA to protect from degradation, promote translation, and regulate nuclear export
• Polyadenylation


  1. Catalyzed by polyadenylate polymerase using ATP as substrate

  2. Poly-A tail built to ~250 nucleotides long
• RNA editing


  Process that results in sequence variation in RNA molecule, catalyzed by enzymes
• RNA editing enzymes

  • Adenosine deaminase acting on RNA (ADAR)
  • Cytosine deaminase acting on RNA (CDAR)
• RNA editing is being extensively studied in relation to infectious diseases as it alters viral enzymes and their function
• Translation

  1. Start with DNA

  2. Transcribe to mRNA

  3. mRNA travels to ribosome

  4. Ribosome reads mRNA and synthesizes polypeptide
• Codon

  A sequence of three nucleotides that codes for a specific amino acid
• Start codon


  AUG, where the ribosome initially attaches to begin translation
• There are 64 possible codons (4 x 4 x 4)
• There are 22 standard amino acids, 21 typically found in eukaryotic cells
• Stop codons


  UAA, UAG, UGA - codons that signal the ribosome to stop translation
• Transfer RNA (tRNA)


  Carries specific amino acids and has an anticodon that pairs with the complementary codon on mRNA
• Ribosome


  • Has 3 sites: A-site (where aminoacyl-tRNA binds), P-site (where polypeptide chain forms), E-site (where used tRNA exits)
• Ribosome translation process

  1. A-site: aminoacyl-tRNA binds

  2. Peptide bond forms between amino acids

  3. Ribosome shifts, used tRNA moves to P-site then E-site

  4. Repeat until stop codon reached
• Antibiotics can disrupt bacterial ribosome function without affecting eukaryotic ribosomes