lec 5

• Transcription


  Making an RNA copy of a gene's DNA sequence

  
• Messenger RNA (mRNA)


  Carries the gene's protein information encoded in DNA

  
• In humans and other complex organisms, mRNA moves from the cell nucleus to the cell cytoplasm (watery interior), where it is used for synthesizing the encoded protein

  
• Transcription is like translating a book from one language to another. While DNA is more stable of a molecule, RNA is the more universal biological language

  
• Transcription in Prokaryotes


  • Prokaryotes do not have membrane-enclosed nuclei
  • Transcription, translation, and mRNA degradation can all occur simultaneously
  • The intracellular level of a bacterial protein can quickly be amplified by multiple transcription and translation events occurring concurrently on the same DNA template
  • Transcription in prokaryotes occurs in the cytoplasm

  
• Transcription in Prokaryotes

  1. Initiation

  2. Elongation

  3. Termination

  
• Initiation phase


  • RNA polymerase recognizes a promoter site on the DNA strand
  • Reading a promoter sequence unwinds the DNA strand and initiates transcription
  • The core enzyme of RNA polymerase then proceeds to synthesize RNA in the 5'-3' direction

  
• Termination


  When the RNA polymerase detects a termination sequence, it stops the transcription process, releases the synthesized RNA transcript, and dissociates from the DNA

  
• Transcription in Eukaryotes


  • Eukaryotes need additional proteins, known as transcription factors, to initially attach to the promoter region
  • Eukaryotes have three polymerases: RNA polymerase I, RNA polymerase II, and RNA polymerase III
  • RNA polymerase binds to the promoter and catalyses the polymerization in the 5' to 3' direction on the template strand
  • Once it reaches the terminator sequence, the process terminates and the newly synthesized RNA strand is released

  
• Stages of Transcription


  1. Initiation

  2. Elongation

  3. Termination

  
• Initiation


  • RNA polymerase attaches to the DNA molecule and moves along the DNA strand until it recognises a promoter sequence
  • The DNA double helix then unwinds and all the bases on each of the DNA strands are exposed
  • This acts as a template for a new mRNA strand

  
• Elongation


  Ribonucleotides are added to the template strand that enables the growth of mRNA growth

  
• Termination


  • RNA polymerase encounters a terminator sequence and the transcription stops
  • RNA polymerase then releases the DNA template

  
• Pre-mRNA


  The transcribed RNA that is processed further to convert it into mature RNA

  
• RNA Processing


  • Capping
  • Polyadenylation
  • Splicing

  
• Capping


  • A methylated guanine cap is added to protect the mRNA
  • It occurs at 5′ end of mRNA transcript
  • It protects the mRNA from degradation
• Polyadenylation


  • The endonucleases cleave the mRNA at a specific sequence
  • The enzyme polyA polymerase facilitates the addition of several adenine nucleotides
  • The poly-A tail also protects the mRNA from degradation

  
• Splicing


  Parts of the pre-mRNA (called introns) are chopped out, and the remaining pieces (called exons) are stuck back together

  
• Differences between Transcription in Prokaryotes and Eukaryotes

  • Transcription is a continuous process and occurs in the cytoplasm (Prokaryotes)
  • Transcription is a separate process and occurs in the nucleus (Eukaryotes)
  • Only one type of RNA polymerase is used (Prokaryotes)
  • Three types of RNA polymerase enzymes are involved (Eukaryotes)
  • No post-transcriptional modifications occur and the mRNA is devoid of a 5' guanosine cap and a poly A tail (Prokaryotes)
  • The mRNA produced has a RNA cap at the 5' end and a poly-A tail at the 3' end (Eukaryotes)