7.3 Transcription

Question 1

State the direction in which transcription occurs

Answer 1

Transcription occur in a 5’-3’ direction

Question 2

Outline the features of the antisense strand

Answer 2

• Transcribed into RNA
  
  • Carries the complementary version of the genetic code
  • Its sequence will be complementary to the RNA sequence and will be the “DNA version” of the tRNA anticodon sequence

Question 3

Outline the features of the sense strand

Answer 3

• Not transcribed into RNA
  
  • Contains the genetic code
  • Its sequence will be the “DNA version” of the RNA sequence (identical except for T instead of U)

Question 4

Explain the process of transcription in prokaryotes

Answer 4

1. Transcription begins at a promoter region: a short series of bases which are not transcribed, but allow RNA polymerase to attach and determine which is the antisense strand.
   
   2. RNA polymerase adds nucleoside triphosphates complementary to the antisense strand to create a chain in the 5’-3’ direction, forming phosphodiester bonds between the nucleosides. As triphosphates are added, 2 phosphates are released creating RNA nucleotides, which form the mRNA strand.
   3. Transcription is stopped by a terminator sequence, a sequence of bases on the antisense DNA strand that when transcribed cause RNA polymerase to detach from DNA, ending the transcription process.

Question 5

Outline why and how mature mRNA is formed

Answer 5

Sections of the mRNA strand called introns (interrupting sequences) carry a base sequence not required for translation. These introns are spliced out, leaving only exons (expressed sequences). Exons are joined to form a mature mRNA strand, carrying the mRNA version of the gene’s base sequence.