Transcription

• What are the basic requirements for transcription

  • Ribonuleoside triphosphates to create the new strand
  • Template to copy
  • Enzyme to do the polymerisation (RNA polymerase)
  • Energy source
• RNAP is recruited to the promoter region which contains specific sequences to RNAP
• RNAP stops at transcription terminators 
• RNAP rejoins the promoter regin to create multiple copies of RNA from the same template. These are called transcripts.
• Different genes produce different amounts of RNA
• RNAP can open up the helicase so a small section of DNA can be used for transcription. This is called a transcription bubble.
• The template strand is the strand that moves from 3' to 5'
• RNAP always moves from 3' to 5'
• Describe transcriptional termination

  RNA is transcribed so it pairs back on itself producing a hairpin structure. This hairpin structure gets in the way of RNAP so it pauses as the structure gets in the way of the progression of the enzyme. DNA also has a T rich region so when transcribed there is a U region, which is weaker as there are only 2 hydrogen bonds and therefore RNA dissociates.
• The area between the start and stop codons are referred to as open reading frames which are the parts that are translated
• Prokaryotes often have RNAs that code for multiple proteins. This structure is known as an operon.
• Eukaryotic mRNAs are modified by adding 7-methylguanosine to the 5' end known as the 5' cap
• Eukaryotic mRNAs are modified by adding -250 adenosine nucleotides known as the polyA tail
• The 5' cap and polyA tail provide stability, promote translation and are important for exporting RNAs from the nucleus into the cytoplasm.
• The 5' cap and polyA tail are added to the RNA without a DNA template. Proteins also bind to these regions to protect the RNA from degradation.
• What is splicing?

  When introns are removed and the remaining exons are joined together
• What is the advantage of splicing?

  Produces greater protein diversity out of single genes. This is because exons can be joined together in different combinations through alternative splicing.
• Where can gene expression be controlled?

  At transcription, RNA stability, translation and at the point of protein stability
• What is positive regulation?

  RNAP has little affinity to the promoter region so activating transcription factors are able to recognise a specific sequence and interact with RNAP (happens in most eukaryotic genes)
• What is negative regulation?

  Gene is expressed unless a repressor protein is present. RNAP has a strong affinity to the promoter region so the repressor protein blocks access to RNAP.