Understanding DNA Transcription: The Key to Gene Expression

Transcription of DNA & Central Dogma

The discovery of DNA was one of the most significant findings in the field of science. Today, deeper insight into the DNA structure has unravelled answers to questions that have been around for eons.

The Central dogma explains how the DNA codes for the proteins which proceed in three stages, namely, replication, transcription and translation. Once DNA replicates its two strands, the information is copied into RNA by the process called transcription. Let’s learn more about the process of transcription.

What is Transcription?

Transcription is the first step in the process of gene expression, where a specific segment of DNA is copied into RNA. This process allows the genetic information encoded in DNA to be used to produce proteins, which are essential for countless cellular functions.

The Stages of Transcription

Transcription involves three main stages: 

1. Initiation
2. Elongation  
3. Termination.

 1. Initiation

Transcription begins when RNA polymerase, the enzyme responsible for synthesizing RNA, binds to a specific region of the DNA called the promoter. This region is located upstream of the gene and signals where transcription should start. Various transcription factors also play a role in assisting RNA polymerase in recognizing and binding to the promoter.

2. Elongation

Once RNA polymerase is bound to the promoter, it unwinds the DNA helix and begins synthesizing a complementary RNA strand. This process involves the addition of ribonucleotides, which are the building blocks of RNA, one by one, based on the sequence of the DNA template. Unlike DNA replication, which uses deoxyribonucleotides, transcription uses ribonucleotides that contain ribose sugar.

 3. Termination

Transcription continues until RNA polymerase reaches a termination signal in the DNA sequence. This signal tells the enzyme to stop adding nucleotides and release the newly synthesized RNA molecule. 

RNA Processing

The transcribed RNA is known as the pre-mRNA. It is processed further to convert it into mature RNA. RNA processing include:

• Capping

• Polyadenylation

• Splicing

Capping

A methylated guanine cap is added to protect the mRNA. It involves:

• Addition of methylated guanine

• It occurs at 5′ end of mRNA transcript

• It protects the mRNA from degradation

Polyadenylation

The poly-A tail also protects the mRNA from degradation. It involves:

• The endonucleases cleave the mRNA at a specific sequence.

• The enzyme polyA polymerase facilitates the addition of several adenine nucleotides.

Splicing

• The non-coding sequences, i.e., the introns are removed by spliceosome excision.

• The coding sequences or the exons join together by ligation.

Thus several proteins can be made from a single pre-mRNA. A mature mRNA is obtained at the end of transcription.

 Types of RNA Produced

The main type of RNA produced during transcription is messenger RNA (mRNA), which carries the genetic information from the DNA to the ribosomes, where proteins are synthesized. Other types of RNA, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), are also involved in protein synthesis and play critical roles in the overall process of gene expression.

The Importance of Transcription

Transcription is crucial for several reasons:

Gene Regulation: The level of transcription of a gene determines how much of that gene's product is made, allowing cells to respond to environmental changes and maintain homeostasis.

Protein Synthesis: Transcription is the first step in the pathway that leads to the production of proteins, the workhorses of the cell.

Development and Differentiation: During development, specific genes are turned on or off at different stages, influencing cell fate and function.

 Conclusion

Transcription is a vital process that bridges the gap between our genetic code and the functional proteins that carry out essential cellular tasks. Understanding transcription not only illuminates how life operates at a molecular level but also opens doors for advancements in fields like genetics, medicine, and biotechnology.