Stages of Transcription

Introduction

Transcription is a process that takes place in all living organisms, where genetic information in the form of DNA is converted into a functional molecule of RNA. RNA is a polymer of nucleotides that plays a key role in the synthesis of proteins.

Transcription is the first step in gene expression and is a complex process that involves a series of stages. In this tutorial, we will explore the different stages of transcription in detail.

Initiation

The initiation stage of transcription begins with the recognition of a specific region of DNA called the promoter by RNA polymerase, the enzyme that carries out transcription.

• The promoter sequence determines the starting point and direction of transcription.

• Once the promoter is recognized, RNA polymerase binds to the DNA and unwinds the double helix to expose the template strand.

• This unwinding of DNA is important because it allows the polymerase to read the base sequence on the template strand and synthesize a complementary RNA strand.

• Once the polymerase has bound to the promoter and opened up the DNA double helix, it can begin the process of transcription.

Elongation

The second stage of transcription is elongation. In this stage, RNA polymerase moves along the template strand, adding nucleotides to the growing RNA molecule.

• The enzyme catalyzes the formation of phosphodiester bonds between adjacent nucleotides in the RNA strand, using the exposed DNA strand as a template.

• As RNA polymerase moves along the DNA, the double helix reforms behind it, creating a new segment of unwound DNA ahead of the enzyme.

• This process of RNA synthesis continues until the polymerase reaches the end of the gene or transcript.

• The elongation stage of transcription is highly regulated and requires the coordinated activity of many proteins and enzymes. For example, RNA polymerase must be able to recognize and accurately incorporate the correct nucleotide into the growing RNA chain.

• This process is aided by the presence of several protein factors, including elongation factors and RNA polymerase-associated proteins.

Termination

The third stage of transcription is termination. The termination stage of transcription involves the recognition of a specific DNA sequence called the terminator, which signals the end of transcription.

• The RNA polymerase detaches from the DNA, releasing the newly synthesized RNA molecule. The RNA molecule then undergoes further processing to become functional RNA.

• There are two types of termination in prokaryotes: intrinsic termination and rho-dependent termination.

  • Intrinsic termination occurs when the RNA polymerase encounters a DNA sequence that signals the end of the gene, causing the RNA polymerase to stop synthesizing RNA and fall off the DNA template.

  • In contrast, rho-dependent termination requires the activity of a protein factor called rho, which binds to the RNA transcript and forces the RNA polymerase to dissociate from the DNA.

• In eukaryotes, termination of transcription is a more complex process that involves the coordinated activity of several protein factors.

• After the RNA polymerase transcribes the entire gene, the pre-mRNA molecule is cleaved and polyadenylated at its 3’ end.

• This polyadenylation process involves the addition of a long string of adenine nucleotides to the 3’ end of the RNA molecule, which is important for the stability and processing of the transcript.

• The RNA molecule is then released from the DNA template and undergoes further processing, including splicing, editing, and export from the nucleus.

Let Us Understand the Different Stages of Transcription with an Example

Consider a gene that codes for the protein insulin

• The Initiation stage begins when the promoter sequence for the insulin gene is recognized by RNA polymerase. The polymerase binds to the promoter and separates the two strands of DNA. The template strand for insulin is then exposed, and the process of transcription can begin.

• In the elongation stage, RNA polymerase moves along the insulin template strand, adding nucleotides to the growing RNA molecule. As it moves along, the polymerase reads the DNA sequence and synthesizes a complementary RNA strand. This process continues until the polymerase reaches the end of the insulin gene.

• The termination stage occurs when the RNA polymerase encounters a specific DNA sequence that signals the end of the insulin gene. The polymerase detaches from the DNA, releasing the newly synthesized RNA molecule. The RNA molecule then undergoes further processing to become functional insulin.

Another Example of the Stages of Transcription

Human Gene for The Protein Hemoglobin

• In this case, the initiation stage begins when RNA polymerase recognizes the promoter sequence for the hemoglobin gene. The polymerase binds to the promoter and begins to unwind the double helix.

• In the elongation stage, RNA polymerase moves along the hemoglobin template strand, adding nucleotides to the growing RNA molecule. As it moves along, the polymerase reads the DNA sequence and synthesizes a complementary RNA strand. This process continues until the polymerase reaches the end of the hemoglobin gene.

• The termination stage occurs when the RNA polymerase encounters a specific DNA sequence that signals the end of the hemoglobin gene. The polymerase detaches from the DNA, releasing the newly synthesized RNA molecule. The RNA molecule then undergoes further processing to become functional hemoglobin.

Conclusion

In conclusion, transcription is a complex process that involves the recognition of specific DNA sequences, the unwinding of the double helix, the addition of nucleotides to a growing RNA molecule, and the release of the newly synthesized RNA molecule.

The stages of transcription are initiation, elongation, and termination, and these stages are critical for the accurate and efficient expression of genetic information.

Understanding the different stages of transcription is essential for researchers and students studying gene expression, and for the development of new therapies and treatments for a wide range of diseases.