7.4.1 Transcription: RNA Synthesis

7.4.1 Transcription: RNA Synthesis

Transcription is the process by which DNA direct the synthesis of mRNA molecules that carry the coded information needed for protein synthesis. It is carried out by RNA polymerase.

The enzyme couples together the ribonucleoside triphosphates ATP, CTA, GTA and UTP on DNA templates releasing PPi.

mRNA production via transcription is actually a “two-step” process in which an hnRNA molecule is initially produced and then is “edited” to yield the desired mRNAmolecule. The mRNA molecule so produced then functions as the carrier of the information needed to direct protein synthesis.

Within a strand of a DNA molecule are instructions for the synthesis of numerous hnRNA/mRNA molecules. During transcription, a DNA molecule unwinds, under enzyme influence, at the particular location where the appropriate base sequence is found for the hnRNA/mRNA of concern and the “exposed” base sequence is transcribed.

A short segment of a DNA strand so transcribed, which contains instructions for the formation of a particular hnRNA/mRNA, is called a gene. A gene is a segment of a DNA strand that contains the base sequence for the production of a specific hnRNA/mRNA molecule.

Transcription process

Four steps are involved.

1. A portion of the DNA double helix unwinds, exposing some bases(a gene).

The unwinding process is governed by the enzyme RNA polymerase.

2. Free ribonucleotides, one nucleotide at a time, align along one of the exposed strands of DNA bases, the template strand, forming new base pairs. In this process, U rather than T aligns with A in the base-pairing process. Ribose, becomes incorporated into the new nucleic acid backbone.

3. RNA polymerase is involved in the linkage of ribonucleotides, one by one, to the growing RNA molecule.

4. When the RNA polymerase enzyme encounters a sequence of base that is “read” as a stop signal the transcription ends. The newly formed RNA molecule and the RNA polymerase enzyme are released, and the DNA then rewinds to re-form the original double helix.

Last modified: Wednesday, 22 February 2012, 9:30 AM