D1.2.1 Transcription as the synthesis of RNA using a DNA template Notes

Transcription as the Synthesis of RNA Using a DNA Template

1. It is the first step in gene expression, where genetic information flows from DNA → RNA → Protein.
2. The key enzyme in transcription is RNA polymerase, which performs multiple tasks to create an RNA strand.

Step by Step Breakdown Of Transcription

Transcription occurs in three stages: initiation, elongation, and termination.

Stage 1: Initiation

1. Transcription begins when RNA polymerase binds to a promoter, a specific DNA region (shown in green in the diagram) that signals the start of a gene.
2. The promoter acts as a recognition site, directing RNA polymerase to the correct location on the DNA.

Stage 2: Elongation

1. RNA polymerase unwinds the DNA double helix, separating the two strands:
   1. Template strand (3' to 5'): Used to synthesize RNA.
   2. Coding strand (5' to 3'): Has the same sequence as the RNA (except RNA uses uracil instead of thymine).
2. RNA polymerase moves along the template strand, adding RNA nucleotides one by one in the 5' to 3' direction (shown as the pink mRNA strand growing in the diagram).
3. This process relies on complementary base pairing:
   1. Adenine (A) on the DNA template pairs with Uracil (U) in RNA.
   2. Thymine (T) on the DNA template pairs with Adenine (A) in RNA.
   3. Cytosine (C) pairs with Guanine (G).
   4. Guanine (G) pairs with Cytosine (C).
4. RNA polymerase catalyzes the formation of covalent sugar-phosphate bonds, creating a continuous RNA strand.

Stage 3: Termination

1. Transcription ends when RNA polymerase reaches a termination sequence (shown in red in the diagram).
2. The enzyme releases the newly synthesized mRNA molecule, and the DNA helix reforms.
3. The mRNA is now ready for further processing (in eukaryotes) or translation (in prokaryotes).