Process of Transcription: Know Stages, RNA Polymerase

Transcription is the first step in gene expression. It is the process where the information in DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase. It makes sure that the instructions in DNA are converted into readable sequences for protein synthesis. It is a key step on the molecular basis of inheritance.

This Story also Contains

1. What is Transcription?
2. Where does Transcription take Place?
3. Central Dogma of Molecular Biology
4. Key Components Involved in Transcription
5. Stages of Transcription
6. Post-Transcriptional Modifications in Eukaryotes
7. Mechanisms of Transcription Regulation
8. Applications and Importance of Transcription
9. Recommended Video for The Process of Transcription
10. MCQs on Process of Transcription

It is important to study transcription as it bridges the gap between the genetic material and the proteins in the cell. It also explains gene expression and regulation, and how mutations lead to diseases. It plays a key role in genetic engineering and biotechnology, where gene expression is manipulated and used for research or other purposes.

What is Transcription?

Transcription is a process by which a DNA template gets copied into RNA. This process is important in gene expression because it leads to the production of mRNA, which is utilized to make proteins. It is a key part of the central dogma in molecular biology.

Where does Transcription take Place?

Transcription occurs in different locations depending on whether the organism is a prokaryote or a eukaryote. In eukaryotic cells, it occurs inside the nucleus. In prokaryotic cells, it occurs in the cytoplasm.

Central Dogma of Molecular Biology

The central dogma explains the flow of genetic information within a biological system. DNA is transcribed into RNA, which is then translated into proteins. This process helps in the transmission of genetic information in cells.

Role of Transcription in Central Dogma

• Transcription is one of the initial steps in the central dogma, where it converts the already replicated DNA into mRNA.

• Transcription is the process by which the information contained in the DNA is copied into a complementary RNA sequence.

• It helps in protein synthesis by the cell based on genetic information.

Key Components Involved in Transcription

Transcription is a process that uses several essential factors to work effectively. These factors help to read genetic code and form RNA strands. It ensures that the genetic information is copied accurately. Any mistake in this step can cause genetic mutation. The key components involved in transcription are:

DNA Template

• A strand of the DNA template is used by RNA polymerase to synthesise RNA.

• The antisense strand of DNA acts as a template for RNA synthesis.

• The sense strand is identical to the RNA sequence produced (except that thymine is replaced with uracil).

RNA Polymerase

• RNA polymerase is the enzyme that catalyses transcription.

• RNA polymerase binds to the DNA and synthesises RNA by adding nucleotides.

• Prokaryotes have only one type of RNA polymerase, eukaryotes have three types: I, II, and III.

Promoters and Transcription Factors

• Promoters are DNA sequences that mark the beginning of transcription. Transcription factors are proteins that bind to specific DNA sequences, enhancing the binding of RNA polymerase to DNA.

• Promoters lie upstream of the gene and thus play a crucial role in the initiation of transcription.

• Transcription factors modulate this binding of RNA polymerase to the promoter, hence gene regulation and gene expression.

Stages of Transcription

Transcription occurs in steps to make sure the conversion from DNA to RNA is precise. Each stage plays a vital role and controls how genes are expressed. It consists of three well-defined stages that include initiation, elongation, and termination:

Step 1: Initiation

• The process of initiation is the beginning of RNA synthesis.

• RNA polymerase binds to the promoter region of the DNA.

• The transcription initiation complex is formed, and the process of unwinding od DNA double helix begins.

Step 2: Elongation

• In elongation, RNA polymerase travels along the DNA to synthesize RNA.

• RNA polymerase adds nucleotides to the growing mRNA strand in a 5' to 3' direction.

• As RNA polymerase moves along the DNA, the double helix unwinds ahead of it and rewinds behind it.

Step 3: Termination

• Termination signals the end of transcription and the release of the RNA molecule.

• It is these specific terminator sequences in the DNA that cause the RNA polymerase to stop and release the mRNA transcript.

• In eukaryotes, termination is followed by other processing steps.

Diagram: Transcription Process in prokaryotes

Post-Transcriptional Modifications in Eukaryotes

After transcription in eukaryotes, the RNA undergoes several changes to become functional mRNA. RNA processing helps in stability, proper export, and precise protein synthesis. Such processes are also important for gene regulation and expression. The post-transcriptional modifications are:

5' Capping

• The 5' cap is attached to the 5' end of the mRNA molecule. It protects the mRNA from degradation.

• A modified guanine nucleotide is added to the 5' end of the mRNA.

• This cap is essential for mRNA stability and recognition by the ribosome.

Polyadenylation

• This process adds a tail composed of adenine nucleotides to the 3' end of the mRNA.

• The poly-A tail protects mRNA from enzymatic degradation.

• It also facilitates the export of mRNA from the nucleus to the cytoplasm.

Splicing

• RNA splicing is a step where the non-coding regions are removed from the pre-mRNA and the coding regions are joined together. The spliceosome complex cuts out introns and joins exons together.

• This process is essential to produce a functional mRNA transcript.

Alternative Splicing

• Alternative splicing allows a single gene to yield multiple variants of mRNA. This allows for increasing protein diversity.

• The joining of different sets of exons leads to the formation of different types of mRNAs.

• Different types of proteins originate from a single gene.

Mechanisms of Transcription Regulation

Transcription regulation involves regulatory factors that control the activity of RNA polymerase. It ensures that genes are expressed at the right time and in the right amount. Proper regulation is essential for normal cell functioning. The following mechanisms are included in the regulation of transcription:

Transcription Factors and Enhancers

• Transcription factors and enhancers are the most prominent players involved in gene regulation and expression.

• The modulation of the rate of transcription comes about by the binding of transcription factors to definite sequences of DNA.

• Enhancers raise the effectiveness of the transcriptional procedure through their interaction with the promoter region.

Epigenetic Regulation

• Epigenetic changers may also affect transcription without altering the DNA sequence.

• DNA methylation and histone acetylation result in activation or repression of transcription.

• Such modifications are heritable and play a role in gene regulation.

Feedback Loops

• Cells adopt the feedback loops system to regulate transcription as well as to maintain homeostasis.

• Positive feedback is when the product produced through gene expression increases the rate of transcription.

• The negative feedback is a turned-off expression of genes due to their products.

• Examples include the regulation of hormone production and metabolic pathways.

Applications and Importance of Transcription

Transcription is a crucial step to express genetic information present in the DNA. Understanding the process of transcription helps in diagnosing genetic disorders and developing gene therapies. It also forms the basis for many techniques of biotechnology like gene cloning. The various applications of transcription are :

Studies of Gene Expression

• In studying gene function and expression, one needs to understand transcription.

• Transcription analysis studies how genes are regulated.

• Such knowledge can help understand the nature of human genetic disorders and developmental processes.

Medical Implications

• It is an area of medical research because errors in transcription may lead to diseases.

• Misregulation of transcription leads to cancer, genetic disorders, and other diseases.

• Targeting transcription factors has become a strategy for developing new therapies.

Applications in Biotechnology

• Transcription plays a huge role in genetic engineering or synthetic biology.

• Scientists control transcription to produce recombinant proteins and genetically modified organisms.

• It is used in developing gene therapies and studying gene function.

Recommended Video for The Process of Transcription

MCQs on Process of Transcription

Q1. The enzyme DNA-dependent RNA polymerase catalyses the polymerisation reaction in _______ direction.

1. Only 5' to 3'

2. Only 3' to 5'

3. Both 1 and 2

4. None of these

Correct answer: 1) Only 5' to 3'.

Explanation:

Convention in defining two strands of the DNA - The other Strands which as the polarity 5' to 3' and the sequence same as RNA are displaced during transcription. This strand is referred to as the coding strand.

All the reference points while defining a transcription are made with coding strands.

Hence, the correct answer is option 1) Only 5' to 3'.

Q2. A transcription unit in DNA is defined primarily by the three regions in DNA and these are for upstream and downstream ends:

1. Repressor, Operator gene, Structural gene

2. Structural gene, Transposons, Operator gene

3. Inducer, Repressor, Structural gene

4. Promotor, Structural gene, Terminator

Correct answer: 4) Promotor, Structural gene, Terminator

Explanation:

A transcription unit is a distinct DNA segment transcribed into RNA. It is composed of three main regions, each with a unique role in the transcription process:

1. Promoter Region:
Position: Before the gene on the 5’ end of the DNA template.
Role: It is the RNA polymerase and transcription factor attachment site.
Characteristics: It features conserved sequences like the eukaryotic TATA box or prokaryotic Pribnow box, and may include upstream elements affecting transcription.
2. Structural Gene:
Position: After the promoter.
Role: Contains the actual DNA sequence that gets transcribed into RNA types such as mRNA, tRNA, and rRNA.
Key Aspect: In eukaryotes, it includes exons (coding regions) and introns (non-coding regions). The RNA polymerase synthesizes RNA complementary to the DNA's 3’ → 5’ direction.
3. Terminator Region:
Position: Beyond the structural gene on the 3’ end of the DNA template.
Role: Indicates transcription's conclusion and causes RNA polymerase release from DNA.
Characteristics: These can be defined by specific RNA sequences or structures like hairpin loops in prokaryotes.

Hence, the correct answer is option 4) Promotor, Structural gene, Terminator.

Q3. Name the site where the upstream sequences are located.

1. Before the start point

2. After start point

3. The right border of DNA

4. In the middle of DNA

Correct answer: 1) before the start point.

Explanation:

• The term "promoter region" usually refers to the upstream sequences that come before the transcription start point.

• Certain DNA sequences, such as the TATA box or Pribnow box, are found in this region and are recognized by RNA polymerase to start transcription.

• By aiding in the binding of transcription factors, these sequences guarantee that gene expression begins correctly.

Hence, the correct answer is option 1) before the start point.

Also Read:

• MCQs on Process of Transcription

• Genetic Material

• Difference Between Prokaryotic and Eukaryotic Replication