3.2

Transcription is the process of copying genetic information from DNA to RNA, occurring in the nucleus. Not all genes are transcribed at all times; cells selectively activate genes based on their needs. In humans, RNA polymerase 2 is responsible for synthesizing mRNA. Transcription occurs in three stages: initiation, elongation, and termination.

  1. Initiation: Proteins gather at the promoter region. The TATA-binding protein (TBP) marks the start site for transcription. General transcription factors help stabilize RNA polymerase 2, allowing it to bind to DNA and form the transcription initiation complex.

  2. Elongation: RNA polymerase opens the DNA locally and synthesizes RNA in the 5’ to 3’ direction, elongating the RNA transcript. General transcription factors typically remain at the promoter.

  3. Termination: RNA polymerase reaches a termination signal, linked to where the poly A tail is added. The RNA transcript is released, DNA reforms its double helix, and RNA polymerase disengages.

The newly synthesized RNA, pre-mRNA, undergoes processing before translation. Modifications include:

  • 5' Cap: A modified guanine nucleotide added for protection and ribosome recognition.

  • 3' Poly A Tail: A stretch of adenine bases that enhances stability.

Introns are removed, and exons are joined, sometimes in various combinations through alternative splicing, allowing a single gene to produce multiple protein variants. After processing, the mature mRNA exits the nucleus for translation, transitioning from gene structure to functional protein synthesis. RNA polymerase 1 and 3 produce different RNA types, but our focus is on RNA polymerase 2 and its role in mRNA production.