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Q14(a):
Briefly describe the following: Transcription

Solution :

Definition and Theoretical Foundation

Transcription is the process of copying genetic information from one strand of the DNA into RNA. Unlike replication, which results in the duplication of the entire genome, transcription is selective; only a segment of DNA and only one of the two strands is copied into RNA. This process is governed by the principle of complementarity [Per the Watson-Crick base pairing rules].

Step 1: The Transcription Unit

A transcription unit in DNA is defined by three regions in the DNA:

  • Promoter: Located at the $5'$ end (upstream) of the structural gene. It provides the binding site for RNA polymerase.
  • Structural Gene: The segment of DNA that codes for the RNA transcript.
  • Terminator: Located at the $3'$ end (downstream) of the coding strand, defining the end of the transcription process.

Step 2: The Mechanism of Transcription

The process occurs in three distinct phases within the nucleus of eukaryotes or the cytoplasm of prokaryotes:

  1. Initiation: RNA polymerase binds to the promoter site with the help of a transcription factor (in prokaryotes, the $\sigma$ factor). This causes the local unwinding of the DNA double helix to form a transcription bubble.
  2. Elongation: The RNA polymerase utilizes ribonucleoside triphosphates (rNTPs) to synthesize the RNA strand in the $5' \to 3'$ direction. The enzyme moves along the template strand ($3' \to 5'$ orientation), adding bases complementary to the DNA template ($A$ pairs with $U$, $T$ with $A$, $C$ with $G$, and $G$ with $C$).
  3. Termination: Upon reaching the terminator sequence, the nascent RNA strand and the RNA polymerase are released from the DNA template. In prokaryotes, this is often mediated by the $\rho$ (rho) factor.
Coding Strand (5') Template Strand (3') RNA (5' → 3') RNA Polymerase

Key Analytical Distinctions

Feature Description
Template Strand The strand with $3' \to 5'$ polarity which acts as a template for mRNA synthesis.
Coding Strand The strand with $5' \to 3'$ polarity that does not code for RNA but has the same sequence as the RNA (with $T$ replaced by $U$).

Final Solution: Transcription is the enzymatic synthesis of RNA from a DNA template, mediated by RNA polymerase, proceeding in a $5'$ to $3'$ direction and governed by the complementarity of nucleotide bases.


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