One of the most notable milestones in molecular biology is the discovery of DNA as genetic material. More research on DNA and a deeper understanding of its chemical properties have led us to know more about inheritance, genetic codes, and so on. Earlier, proteins were believed to be the carriers of inheritance, but the discovery of DNA has led us to unravel how DNA codes for the proteins. It undergoes a three-stage process- Replication, transcription, and translation to code the information.
• Replication- It is the process of copying of genetic information by DNA strands during cell division.
• Transcription- It is the first step of gene expression wherein a segment of DNA is copied into RNA.
• Translation- It is the next step in which coding proteins as per the DNA codes takes place.
We will learn more about DNA transcription in this chapter.
Transcription can be defined as the process of the first step of gene expression that involves the copying of information on DNA strands into the RNA molecules.
The process of gene expression begins with transcription. The genetic information flows from DNA to RNA and into the proteins and this flow of information takes place in a sequential process of transcription and translation. Transcription takes place when there is a need for a particular gene product at a specific time or in a specific tissue or spot.
During the process of transcription, only one strand of DNA is copied. This strand undergoing the process is known as the template strand. The RNA molecules thus produced are single-stranded RNA called the messenger RNA or mRNA. The DNA strand that corresponds to the mRNA is called the coding strand.
The objective of transcription is to make a copy of RNA from the DNA sequence. The RNA transcript carries the information which is then used to encode a protein.
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Transcription takes place in three distinct steps. Every step is catalyzed by enzymes. Enzymatic catalyzation is very crucial at every step of this process. Three different phases of transcriptions are explained below-
The process of transcription is catalyzed by the enzyme RNA polymerase. This enzyme attaches itself to the DNA molecules and moves along with it until it recognizes a promoter sequence. The promoter sequence is the sequence of a fragment of DNA where the process begins. There may be multiple promoter sequences present in a DNA molecule. After binding to the promoter sequence on the DNA molecules, RNA polymerase unwinds a portion of the DNA double helix. The unwinding causes the base pairs to get exposed.
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One of the strands of the uncoiled DNA acts as the template for new mRNA molecules. The other strand acts as the coding template. The base sequence of mRNA is almost similar to the coding DNA strand base pairs except for thiamine which is replaced by Uracil. Ribonucleotides are added to the template strand. And thus, the elongation of the RNA molecule starts taking place.
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The phase of elongation continues and the RNA molecule continues to grow until it reaches a termination sequence. Enzyme RNA polymerase encounters this sequence of termination and the transcription stops. The DNA template is then released by RNA polymerase.
The mRNA transcribed up to this point is called the pre- mRNA. It must be processed further to convert it into mature RNA. This process takes place through the following steps-
It involves the capping of methylated guanine to protect the mRNA. The addition of methylated guanine occurs at the 5′ end of the mRNA transcript. It protects the immature molecule of mRNA from degeneration by RNases.
This step involves the addition of a poly(A) tail to the 3′ end of mRNA. The poly(A) tail comprises multiple molecules of adenosine monophosphate. This is necessary for stabilizing the RNA. Stabilizing the RNA is very important as it is much more unstable than DNA
This step allows one genetic sequence to code for different proteins. It is carried out in two steps
• Introns which are the non-coding sequences, get removed by spliceosome excision.
• The exons which are the coding sequences are joined together by ligation.
Splicing conserves the genetic material.
And also, this way, several proteins can be made from a single pre-mRNA.
With the final step of transcription a mature mRNA is obtained. It allows the next step of translation to occur which also leads to protein synthesis.
1. Explain the Role of RNA Polymerase in Transcription.
RNA polymerase is the primary enzyme involved in transcription. It helps the synthesis of a complementary RNA strand from the segment of the DNA molecule. The DNA-dependent RNA polymerase enzyme binds to the promoter sequence on DNA and catalyzes the polymerization in the 5’ to 3’ direction on the template DNA strand. Once it reaches the terminator sequence the process stops and the newly synthesized RNA strand is released.
2. What is the Transcription Unit?
Transcription Unit is a stretch of DNA that gets transcribed into a new RNA. It comprises a sequence of nucleotides in DNA, that codes for a single RNA molecule, along with the sequences necessary for its transcription; it also contains a promoter, an RNA-coding sequence, and a terminator sequence.
3. Explain the Mechanism of Transcription in Prokaryotes.
The transcription process is less complex and different in organisms without the nucleus- i.e. Prokaryotes. The RNA polymerase enzyme of prokaryotes is relatively simple with just five subunits. Repressors are released from the sites on DNA that block the action of RNA polymerases on receiving signals from other molecules in the cell indicating that the gene needs to be expressed. Thus, transcription begins following similar steps of initiation, elongation, and termination.