One of the most important scientific discoveries was the discovery of DNA. A deeper understanding of the DNA structure has revealed answers to questions that have persisted for millennia.
The Central Dogma describes how DNA passes on information to mRNA and proteins are produced in three stages: replication, transcription and translation. Transcription is the process of copying information from DNA into RNA after it has replicated its two strands.
Let us now take a closer look at transcription definition, transcription classification and its properties.
What is transcription?
It is the process of coding of message by DNA in the form of mRNA. The mechanism is somewhat similar to DNa replication but with a difference that only one strand of DNA is used as template. RNA polymerase is the key enzyme required for this process.
RNA produced by this step has all the information which it now uses for the synthesis of proteins. This step of formation of proteins is called translation.
RNA Polymerase
The key enzyme involved in transcription is RNA polymerase. It creates a complementary RNA strand from single strand of DNA. The promoter is bound by DNA-dependent RNA polymerase, which catalyses polymerisation on the template strand in the 5′ to 3′ direction. The process ends when it reaches the terminator sequence, and the freshly synthesised RNA strand is released.
A transcription unit is a segment of DNA that has been converted into an RNA molecule. Its role is to code for at least one gene. Assume that if a gene codes for a protein, transcription produces mRNA. A coding sequence may be present in a protein transcribed by the DNA transcription unit. Transcription has a lower copying fidelity than DNA replication.
Different regions of a transcription unit in DNA
Here are the different regions of the transcription unit in DNA:
Gene with a structural function
The structural gene’s two DNA strands have different names. The strand with 3′-5′ polarity becomes the template strand since RNA polymerase only catalyses polymerisation in one direction, 5′-3′. Even though it does not code for anything, the other strand with 5′-3′ polarity is shifted during transcription, and is referred to as the coding strand. The promoter and terminator sections of a transcription unit are located on either side of the structural gene.
Promoter
It is a DNA sequence found at the coding strand 5′ end (upstream). It is the RNA polymerase binding site, and it is this site that instructs the polymerase to begin transcription. A transcription unit template and coding strand are also defined by the presence of the promoter.
Terminator
It is a DNA sequence found at the 3’ end of coding strand (downstream). It sends out a stop signal and marks the end of the transcription process. There could be more regulatory sequences upstream or downstream of the promoter.
Stages of transcription
Transcription is carried out in a series of enzymatically catalysed processes, i.e.
- Initiation
- Elongation
- Termination
Initiation
RNA polymerase binds to DNA and proceeds down the strand until it finds a promoter sequence to recognize. The transcription start locations are what they are called. The double helix of DNA unwinds, exposing all of the bases on each DNA strand. This serves as a starting point for formation of a new strand of mRNA.
Elongation
Ribonucleotides are added to the template strand, which allows mRNA to expand. It adds a complementary RNA nucleotide to the developing RNA transcript for every nucleotide identified on the DNA template.
Termination
When the RNA polymerase reaches the terminator region, transcription ceases. Both the RNA transcript and the RNA polymerase fall off at this point. The RNA polymerase forms a brief association with the ‘termination factor’ to end transcription.
RNA Processing
Pre-mRNA is the RNA that has been transcribed. It is then processed further to become mature RNA. The following are examples of RNA processing:
- Capping
- Polyadenylation
- Splicing
Capping
A methylation guanine cap is inserted to preserve the mRNA. It entails:
- Methylated guanine is added
- It is found at the 5′ end of the mRNA transcript
- It guards against mRNA degradation
Polyadenylation
The mRNA is additionally protected from destruction by the poly-A tail. It entails:
- Endonucleases cleave mRNA at a specified position.
- PolyA polymerase is an enzyme that aids in the addition of multiple adenine nucleotides.
Splicing
- Spliceosome excision is used to delete non-coding sequences, like introns.
- Ligation joins the coding sequences or exons together.
As a result, a single pre-mRNA can produce several proteins. At the end of transcription, mature mRNA is obtained.
What is replication?
A double-stranded DNA molecule divides into two identical daughter strands during DNA replication. As a result, each daughter cell contains the same genetic information as the parent cell during cell division. DNA polymerase is the primary enzyme involved in DNA replication. Each strand receives additional nucleotides.
In a cell, both DNA replication and transcription require making a new copy of the DNA. DNA transcription is involved in the conversion of DNA to RNA, whereas DNA replication is the process of making another copy of DNA. Both processes are involved in the synthesis of new nucleic acids, either DNA or RNA. The newly created nucleic acids are identical in appearance but differ in function. One is responsible for cell division, while the other is responsible for gene expression.
Conclusion
Transcription is one of the first steps in the gene expression process. During transcription, a single DNA strand is copied to form mRNA. mRNA will then form proteins by the process of translation. Transcription is the process of passing on genetic information from DNA to mRNA.