Transcription

• Central Dogma of molecular biology

  DNA -> RNA -> Protein
• Information flows from DNA to RNA to Protein
• RNA


  Ribonucleic acid
• RNA is used so that information can leave the nucleus
• We can now make a specific protein at a specific location
• In eukaryotes, DNA cannot leave the nucleus
• Ribosomes that synthesize proteins are found in the cytoplasm
• mRNA


  Messenger RNA
• mRNA is able to leave the nucleus
• Phases of Transcription


  1. Initiation

  2. Elongation

  3. Termination

  4. Processing (Eukaryotes only)
• RNA polymerase


  • Main enzyme involved in transcription
  • Binds to DNA at a promoter sequence
  • Able to locally unzip DNA with its own built-in helicase activity
  • Constructs an RNA transcript using the DNA as a template
• Promoter Sequence


  • DNA sequence upstream of the gene being transcribed
  • Various transcription factors bind to the promoter and affect RNA polymerase's ability to bind
  • Usually rich in Thymine and Adenine -> "TATA" box
• Elongation


  • One strand of the unzipped DNA acts as a template for RNA synthesis
  • As the RNA polymerase moves along the template strand it creates a transcript that has complementary bases
• A key difference between RNA and DNA is that RNA contains Uracil (U) instead of Thymine (T)
• The RNA transcript is nearly identical to the template strand (non-template strand with U's switched for T's), which is why it is called the coding strand
• mRNA transcription


  1. DNA unwinds only in the region that transcription is occurring

  2. After transcription the DNA recoils
• Multiple RNA polymerases can work on a single gene at once
• Terminator sequence on the coding strand tells RNA polymerase when to stop transcribing the mRNA
• When the terminator sequence is transcribed

  A hairpin loop structure forms because the code is complementary to itself
• When the hairpin loop structure forms

  It causes polymerase to stall and become unstable
• Typically the terminator sequence is rich in G's and C's
• Pre-mRNA


  The RNA transcript in eukaryotic cells that must still be modified before it becomes mRNA and leaves the nucleus
• Why process the pre-mRNA?

  • Protects from degradation in the cytoplasm
  • Removes unnecessary information
• Pre-mRNA processing

  1. Add 5' cap

  2. Add 3' poly-A tail
• 5' cap

  A modified guanine that protects the transcript from degradation and acts as an attachment site for the ribosome during translation
• Poly-A tail


  A sequence of 100-200 A's added to the 3' end that also protects the transcript from degradation
• Genes


  • Contain coding regions (exons) and non-coding regions (introns)
  • Introns must be removed to produce the final mRNA transcript
• Splicing


  1. Small nuclear RNAs (snRNAs) and proteins assemble into the spliceosome

  2. The spliceosome binds to sequences at the beginning and end of an intron forming a loop

  3. The loop is removed and the remaining exons are linked
• Genes can be alternatively spliced


  • To have multiple variations of the same gene