Transcription

• What are the free nucleotides involved in transcription?

  RNA nucleotides
• What is the product of transcription?

  An mRNA strand that is identical to the sense strand (original strand that codes for the actual protein)
• What is mRNA?

  A single-stranded helix - responsible for carrying genetic information from the DNA in the nucleus, to the ribosomes outside of the nucleus in order to perform protein-synthesis
• Why is it that mRNA binds to ribosomes outside and not DNA?

  DNA is too large to leave the nucleus - mRNA is smaller, therefore it leaves through the nuclear pores and binds to ribosomes.

  DNA contains the base thymine, which can get destroyed once it's outside of the nucleus in the cytoplasm - mRNA has the base uracil which is safer.
• Why's it dangerous for mRNA to stay in the nucleus?

  mRNA contains the base Uracil which may cause mutation.
• Transcription STEP 1

  DNA molecule consists of a sense strand and an antisense strand which is anti-parallel.

  
  

  RNA polymerase breaks down the hydrogen bonds between the complementary bases in the DNA double helix.

  This leaves with two strands of DNA with exposed bases - a sense strand and an anti-sense strand or template strand.
• Transcription STEP 2

  Free RNA nucleotides in the nucleus pair up with the exposed bases through complementary base pairing forming hydrogen bonds between them- Guanine with cytosine; RNA has no Thymine, it's replaced with Uracil, so Uracil pairs up with adenine.
• Transcription STEP 3

  RNA polymerase then joins the free RNA nucleotides together via condensation reaction, forming phosphodiester bonds between them, creating a sugar phosphate backbone.

  This forms a strand of mRNA.
• Transcription STEP 4

  mRNA leaves the molecule and the anti-sense strand joins up with the original sense strand, twists and coils up around each other and forms a double helix.
• Transcription - what are we left with?

  We're left with an mRNA strand that is identical to the original sense strand, that contains the genetic information needed to code for protein synthesis.