Understanding DNA Transcription Process

Aug 31, 2024

DNA Transcription Lecture Notes

Overview of Transcription

  • Transcription: The process of synthesizing RNA from a DNA template.
  • Important to identify which strand of DNA will be transcribed.

DNA Strands

  • Strand A: Non-coding strand (template strand).
  • Strand B: Coding strand (sense strand).

Transcription Process

  1. Transcribing Strand A

    • RNA synthesized has complementary bases to Strand A.
    • Example: If Strand A has cytosine (C), mRNA will have guanine (G).
    • Resulting mRNA sequence is GGG, coding for the amino acid glycine.

  2. Transcribing Strand B

    • RNA synthesized has complementary bases to Strand B.
    • Example: If Strand B has guanine (G), mRNA will have cytosine (C).
    • Resulting mRNA sequence is CCC, coding for the amino acid proline.
  • Key Point: Transcribing different strands results in different polypeptides.
  • Strand B is the strand chosen for transcription in this example.

Coding vs Non-Coding Strands

  • Coding Strand (Strand A)
    • Identical nucleotide sequence to the synthesized mRNA.
    • Also known as the sense strand.
  • Non-Coding Strand (Strand B)
    • Complementary to the mRNA, not identical.
    • Also known as the antisense strand, transcribed strand, or template strand.

Steps of Transcription

1. Initiation

  • RNA Polymerase: The enzyme responsible for synthesizing RNA.
    • Scans DNA to find the promoter sequence (start signal for transcription).
  • Transcription Bubble: Formation of a bubble in the DNA where transcription occurs.

2. Elongation

  • RNA polymerase synthesizes RNA in the 5' to 3' direction.
  • Complementary nucleotides are added based on the DNA template:
    • G in DNA pairs with C in RNA.
    • A in DNA pairs with U in RNA (thymine replaced with uracil).
  • In prokaryotic cells, the rate of transcription is 40 nucleotides per second.

3. Termination

  • RNA polymerase encounters a terminator sequence (stop signal).
  • RNA polymerase detaches from the DNA, releasing the newly synthesized mRNA.
  • DNA rewinds and closes back up.
  • The mRNA is then sent to the ribosome for translation into a polypeptide chain.