7.3 Transcription

7.3.1  State that transcription is carried out in a 5' - 3' direction

Transcription is carried out in a 5' - 3' direction (of the new RNA strand)

7.3.2  Distinguish between the sense and antisense strands of DNA

DNA consists of two polynucleotide strands, only one of which is transcribed into RNA

  • The antisense strand is transcribed into RNA
    • Its sequence will be complementary to the RNA sequence and will be the "DNA version" of the tRNA anticodon sequence 
  • The sense strand is not transcribed into RNA
    • Its sequence will be the "DNA version" of the RNA sequence (identical except for T instead of U)

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

A gene is a sequence of DNA which is transcribed into RNA and contain three main parts:

  • Promoter:  Responsible for the initiation of transcription (in prokaryotes, a number of genes may be regulated by a single promoter - this is an operon)
  • Coding Sequence:  The sequence of DNA that is actually transcribed (may contain introns in eukaryotes)
  • Terminator:  Sequence that serves to terminate transcription (mechanism of termination differs between prokaryotes and eukaryotes)

Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence and involves a number of steps:

  • RNA polymerase binds to the promoter and causes the unwinding and separation of the DNA strands
  • Nucleoside triphosphates (NTPs) bind to their complementary bases on the antisense strand (uracil pairs with adenine, cytosine pairs with guanine)
  • RNA polymerase covalently binds the NTPs together in a reaction that involves the release of two phosphates to gain the required energy
  • RNA polymerase synthesises an RNA strand in a 5' - 3' direction until it reaches the terminator
  • At the terminator, RNA polymerase and the newly formed RNA strand both detach from the antisense template, and the DNA rewinds
  • Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts
  • Post-transcriptional modification is necessary in eukaryotes

Overview of Transcription

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

  • Euakaryotic genes may contain non-coding sequences called introns that need to be removed before mature mRNA is formed
  • The process by which introns are removed is called splicing
  • The removal of exons (alternative splicing) can generate different mRNA transcripts (and different polypeptides) from a single gene