Transcription in Prokaryotes: Process, Steps, Diagram

Definition Of Transcription

Transcription is a biological process where the information from DNA gets replicated into messenger RNA by using the enzyme RNA polymerase. It is a fundamental step in converting genetic information into functional proteins.

In prokaryotes, gene expression and regulation occur at transcription level. This allows prokaryotes to respond to changes in their environment by producing those proteins that are essential for survival and adaptation.

Prokaryotic Transcription Process

Prokaryotic transcription has three steps including initiation, elongation, and termination. The steps along with the help of specific proteins and DNA sequences ensure accurate and efficient transcription.

Prokaryotic cells lack membrane-bound organelles. Unlike eukaryotes, the genetic material is not enclosed in a nucleus. Instead it is present in a region called the nucleoid. The examples include organisms like bacteria and archaea.

Components Involved In Transcription

Transcription is a complex process that requires several components. The components help in initiation, elongation, and termination of RNA synthesis. The efficiency of transcription depends on the presence of these components. It also gives insight into how genes are expressed within a cell.

• DNA template for the synthesis of RNA. In prokaryotes, DNA is usually a single, circular, double stranded molecule.

• RNA Polymerase is responsible for synthesizing RNA from the DNA template.

• Promoter Regions are specific DNA sequences that mark the beginning of a gene and specify the binding site for RNA polymerase.

Steps of Transcription Process In Prokaryotes

Transcription in prokaryotes is simpler due to absence of a membrane bound nucleus. This allows transcription and translation to occur simultaneously in the cytoplasm. It starts at specific promoter sites on DNA and produces mRNA. The three well-defined steps of the transcription process in prokaryotes are:

Initiation

Role of Sigma Factor: This is a subunit of RNA polymerase to recognize and bind promoter regions on DNA. It ensures that RNA polymerase attaches to an appropriate start site for transcription.

Binding to the Promoter Region: The RNA polymerase and sigma factor together forms a complex that binds to the promoter region. This binding induces unwinding of the DNA to form the open complex. This unwound segment of DNA is where the initiation of transcription will occur.

Elongation

The Transcription Bubble Formation: As RNA polymerase travels along the DNA, it unwinds the double helix continuously and forms a 'transcription bubble.'

Synthesis of mRNA: The RNA polymerase reads the template strand of the DNA and synthesizes a complementary strand of RNA by adding ribonucleotides. The strand of RNA elongates in a direction from 5' to the 3'

Termination

Rho-dependent Termination: Rho protein binds to the RNA and translocates on it toward the RNA polymerase. When it catches up with the polymerase, it causes the release of the enzyme from the RNA, thereby terminating transcription.

Rho-independent Termination: This depends on specific sequences in the RNA, which form a hairpin structure followed by a series of uracil nucleotides. This structure causes RNA polymerase to pause and fall off the DNA thus terminating transcription.

Diagram: Transcription In Prokaryotes

The diagram given below illustrates the three main stages of transcription: initiation, elongation, and termination. It shows how RNA polymerase binds to the promoter, the formation of a replication bubble, and rho-dependent termination.

Regulation Mechanisms Of Prokaryotic Transcription

Prokaryotic transcription is regulated mainly at the initiation step to ensure genes are expressed only when needed. All genes have regulatory DNA sequences that are used to switch the gene on or off. The regulators are influenced by the environment. This helps conserve energy and maintain cellular efficiency.

Operons And Their Role

Lac Operon: It is the most common example of the operon system in prokaryotes. It controls the transcription of genes involved in the transport and metabolism of lactose. The system contains a promoter, operator, and structural genes. It is regulated by both positive and negative control mechanisms.

Positive And Negative Regulation

Positive control refers to the binding of RNA polymerase to the promoter, thereby enhancing the rate of transcription.

Negative control refers to binding to the operator region and preventing the RNA polymerase from initiating transcription.

Attenuation Mechanism

A kind of regulatory mechanism under which the transcription is terminated prematurely. It depends on the secondary structure formation in mRNA. This helps in the accurate control of gene regulation and expression according to the environmental conditions.

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MCQs on Transcription in Prokaryotes

Q1. The transcription of DNA to a molecule of messenger RNA occurs:

1. On the ribosomes

2. In the cytosol

3. In the nucleus

4. only during cell division

Correct answer: 3) In the nucleus

Explanation:

The RNA polymerase enzyme is essential to the production of mRNA during transcription because it moves through the DNA strand and puts the matching ribonucleotides together. An essential bridge in the transfer of genetic information from DNA to protein synthesis is created as a result, producing a complementary mRNA strand.

Option (A) is incorrect because the ribosome is the site of protein synthesis in the cell.

Option (B) is incorrect

Option (D) is incorrect

Hence, the correct answer is option 3) In the nucleus.

Q2. Select the incorrect match.

1. RNA polymerase II – hn RNA

2. Rho factor – Initiation of transcription

3. DNA gyrase – Topoisomerase activity

4. Aminoacyl tRNA synthetase – Activation of amino acids

Correct answer: 2) Rho factor – Initiation of transcription

Explanation:

• Transcription is not initiated by the Rho factor instead it is terminated.

• Rho factor aids in stopping the transcription process once the RNA has been entirely transcribed. In bacteria, RNA polymerase synthesizes RNA during transcription.

• The Rho factor has no direct role in the process of transcription initiation, which is mediated by RNA polymerase binding to the DNA promoter region.

Hence, the correct answer is option 2) Rho factor – Initiation of transcription.

Q3. The enzyme that performs transcription is ____________.

1. DNA polymerase

2. DNA ligase

3. RNA polymerase

4. Reverse transcriptase

Correct answer: 3) RNA polymerase.

Explanation:

1) DNA polymerase that catalyzes DNA replication

2) DNA ligases catalyze the formation of phosphodiester bonds at single-strand breaks in double-stranded DNA.

3) DNA-dependent RNA polymerase catalyzes transcription on one strand of the DNA

4) Reverse transcriptase catalyzes the RNA-dependent polymerization of DNA and is responsible for maintaining genetic information in the more stable form of DNA.

Hence, the correct answer is option 3) RNA polymerase.

Also Read:

• MCQs on Transcription in Prokaryotes

• RNA World

• Bioinformatics