Transcription is the first step in gene expression, where DNA instructions are copied into RNA. In prokaryotes, this process is carried out by RNA polymerase, which reads DNA and synthesises RNA in a highly efficient, three-step process: initiation, elongation, and termination. Understanding transcription is key to microbiology, molecular biology, and biotechnology, as it underpins gene regulation, protein synthesis, and genetic engineering applications.

What Is Transcription and Why Does It Matter?

Every living cell stores the instructions for making proteins in DNA, but these instructions cannot be used directly. Transcription converts DNA into a functional RNA message, which can then guide protein synthesis.

In prokaryotes, bacteria and other organisms without a nucleus, transcription is simple, fast, and highly efficient. It allows cells to respond rapidly to environmental changes by turning genes on or off.

Step 1: Initiation – Starting the Process

Initiation is the first phase of transcription, where the transcription machinery assembles and begins RNA synthesis.

RNA Polymerase: The Key Enzyme

In prokaryotes, a single enzyme, RNA polymerase, carries out transcription. It binds to DNA and synthesizes RNA using base pairing rules, always building the strand in the 5′ to 3′ direction.

The Role of Promoters

RNA polymerase requires a signal to know where to start. This signal comes from a promoter, a DNA sequence upstream of the gene. Promoters guide RNA polymerase to the correct start site.

In bacteria, promoters typically contain -10 and -35 regions, which help position the polymerase for accurate transcription initiation.

Step 2: Elongation – Building the RNA Strand

Once RNA polymerase is positioned, elongation begins, and the RNA molecule is synthesized.

The Transcription Bubble

The DNA helix unwinds near the gene, creating a transcription bubble. RNA polymerase reads the template strand and synthesizes a complementary RNA strand. The coding strand matches the RNA sequence (T replaced by U).

RNA Synthesis Speed

RNA polymerase adds nucleotides at about 40 bases per second, continuing until the entire gene is transcribed.

Step 3: Termination – Ending the Transcription

Termination is when RNA polymerase stops transcription and releases the newly made RNA molecule. Prokaryotes have two main termination types:

Rho-Dependent Termination

A protein called Rho moves along the RNA strand. When RNA polymerase slows at a specific sequence, Rho catches up and displaces the RNA from the DNA, ending transcription.

Rho-Independent Termination

No protein is required. The RNA forms a hairpin loop at a C-G-rich region, stalling RNA polymerase. A U-A-rich sequence follows, weakening RNA-DNA interactions, and the RNA molecule is released.

Why Understanding Transcription Matters

Transcription in prokaryotes is tightly regulated and efficient, allowing rapid gene expression adjustments. Mastering initiation, elongation, and termination provides insight into:

  • Gene regulation and control
  • Antibiotic mechanisms (many target bacterial transcription)
  • Genetic engineering applications

For students and researchers in microbiology or molecular biology, understanding transcription is foundational to exploring life at the molecular level.