4.2.3 Transcription

Steps of Transcription:

1. Unwinding and Unzipping DNA:

• The enzyme DNA helicase breaks the hydrogen bonds between complementary base pairs, causing the DNA to uncoil and expose the template strand (antisense strand).

2. Formation of mRNA:

• Free RNA nucleotides align with the exposed bases on the antisense strand by complementary base pairing:
• Adenine (A) pairs with uracil (U).
• Thymine (T) pairs with adenine (A).
• Cytosine (C) pairs with guanine (G).
• Guanine (G) pairs with cytosine (C).
• The enzyme RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides, creating a strand of mRNA.

3. Termination:

• Transcription stops when RNA polymerase reaches a stop codon on the DNA template. The newly formed pre-mRNA strand detaches from the DNA.

4. Reforming the DNA Double Helix:

• As RNA polymerase moves along the DNA, the hydrogen bonds between the DNA strands reform behind it, and the DNA recoils into a double helix.

5. RNA Splicing (Eukaryotic Cells Only):

• The pre-mRNA contains both introns (non-coding regions) and exons (coding regions).
• Enzymes remove the introns, leaving a strand of mature mRNA that contains only exons.
• The mRNA then exits the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm, ready for the next stage, translation.

Key Terms:

• Transcription: The process of making mRNA from a DNA template.
• Template Strand (Antisense Strand): The DNA strand used as a template for mRNA synthesis.
• Complementary Base Pairing: Bases pair specifically (A-U, T-A, C-G, G-C).
• RNA Polymerase: The enzyme that catalyses the formation of mRNA.
• Splicing: The removal of introns from pre-mRNA in eukaryotic cells.