Topic 7.3 Transcription

Question 1

7.3.1 State that transcription is carried out in a _ → _ direction.

Answer 1

Transcription is carried out in a 5’ → 3’ direction.

Question 2

7.3.2 Distinguish between the sense and antisense strands of DNA.

Answer 2

Antisense strand: transcribed into RNA

• its sequence will be complementary to the mRNA sequence and will be the “DNA version” of the tRNA anticodon sequence

Sense strand: not transcribed into RNA

• mRNA matches this strand (except with uracil instead of thymine)

Question 3

7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.

Answer 3

1. RNA polymerase binds to the promoter region (short sequence of bases that is not transcribed; promoter may be on different strands of DNA, depending on which gene is being coded for)
   
   • promoter region determines which DNA strand is the antisense strand and initiates transcription
2. RNA polymerase separates area of DNA double helix to expose base sequence of nucleotides → transcription bubble occurs (contains antisense DNA strand, RNA polymerase & growing RNA transcript)
3. antisense strand is copied
   
   • free nucleoside triphosphates bond to their complementary bases on the template strand
   • resulting mRNA molecule will have same sequence as sense strand, except with uracil instead of thymine
   • mRNA is built in a 5’ → 3’ direction
4. as nucleoside triphosphates bind, they become nucleotides and release energy by losing 2 phosphate groups
5. RNA polymerase, with energy provided by release of 2 phosphates from NTP, forms covalent bonds between the nucleotides → this is called elongation
   
   • RNA polymerase keeps moving along until it reaches the terminator
6. Terminator (sequence of nucleotides) signals RNA polymerase to stop transcription; RNA polymerase is released & mRNA detaches from the DNA
7. DNA rewinds

Question 4

7.3.4 State that eukaryotic RNA needs the removal of ______ to form mature mRNA.

Answer 4

Eukaryotic RNA needs the removal of introns to form mature mRNA.

Introns are non-coding DNA and must be removed to make a functional mRNA strand.

Note: prokaryotic mRNA doesn’t require processing b/c no introns are present