DNA Transcription is the process of copying genetic information from one strand of DNA into RNA. Like DNA Replication, the principle of complementarity governs the process of transcription, except that adenosine base pair with uracil instead of thymidine (as RNA does not contain thymidine base).
But unlike DNA replication where the total DNA of an organism gets duplicated, in transcription, only a segment of DNA and only one of its strand is copied into RNA.
There can be two reasons for both the strands of DNA not being copied during transcription:
- If both strands act as a template, they would code for RNA molecule with different sequences. And in turn, if they code for protein, the sequence of amino acids in the protein will be different. Hence, one segment of DNA would be coding for two different proteins.
- The two RNA molecules if produced simultaneously would be complementary to each other, hence would form a dsRNA. This would prevent translation of RNA into protein.
The segment of DNA taking part in transcription is called transcription unit. It has three components:
- A promoter
- The structural gene
- A terminator
Template Strand and Coding Strand:
The DNA-dependent RNA polymerase catalyses the polymerization in only one direction 5′–>>3′ polarity. The other strand with polarity 3′–>>5′ act as a template and is called template strand or non-coding strand. The other strand with polarity 5′–>>3′ is called coding strand or sense strand or non-template strand.
Promoter sequences are present upstream towards 5′ end of the structural gene of the transcription unit. It is a DNA sequence that provides a binding site for RNA polymerase. It is the presence of a promoter in a transcription unit that defines the template and coding strands.
The terminator is present at 3′ end downstream of the coding strand and it defines the end of the process of transcription.
Transcription Unit and the Gene:
A gene is defined as the functional unit of inheritance. Genes are located on the DNA. Cistron is defined as the functional unit of the gene, it is a segment of DNA coding for a polypeptide.
The structural gene in a transcription unit is monocistronic (mostly in eukaryotes) and polycistronic (mostly in prokaryotes). Monocistronic gene synthesizes one type of polypeptide whereas polycistronic gene synthesizes different proteins.
The monocistronic gene has interrupted coding sequences called introns and sequences expressed in mature RNA are called exons.
Transcription In Prokaryotes:
The transcripting enzyme DNA-dependent RNA polymerase can transcribe all types of RNA i.e mRNA, rRNA and tRNA. All three RNAs are needed to synthesize a protein in a cell.
RNA polymerase is a holoenzyme that is made of polypeptides. There are 5 subunits in RNA polymerase– alpha (α), beta (β), beta’ (β ‘), sigma (s), and omega (w). The enzyme without sigma subunit is referred to as core enzyme. The process of transcription is completed in 3 steps:
- Initiation: It is catalyzed by the sigma factor or initiation factor. It binds to promoter site of DNA and confers specificity. In the absence of the sigma factor, transcription starts non-specifically by core enzyme at any base of DNA.
- Elongation: The RNA polymerase (core enzyme) is only capable of catalyzing the process of elongation.
- Termination: Rho factor is required for the termination of transcription.
The RNA polymerase binds to the promoter region of DNA and the process of transcription begins. It uses nucleoside triphosphate as substrate and polymerizes in a template-dependent fashion following the rule of complementarity. Once the polymerase reaches the terminator region, the nascent RNA and RNA polymerase falls off and it results in termination of transcription.
Transcription In Eukaryotes:
There are three types of transcripting enzyme, i.e. RNA polymerases in the nucleus in addition to RNA polymerase found in organelles. There is a clear cut division of labour. Functions of different RNA polymerases in eukaryotes are:
- RNA polymerase I: 5.8S, 18S, 28S rRNA synthesis
- RNA polymerase II: HnRNA, mRNA synthesis
- RNA polymerase III: tRNA, ScRNA, 5S rRNA and SnRNA synthesis
The nascent RNA synthesized by RNA polymerase II is called hnRNA or primary transcript. It contains both unwanted base sequence (introns) alternated with useful base sequences (exons).
The primary transcript is converted into functional mRNA after post-transcriptional processing involving three steps:
- Modification of 5′ end by capping: Capping of 5′ end occurs rapidly after the start of transcription. An unusual nucleotide i.e. methyl guanosine triphosphate is added to the 5′ end of hnRNA. It is catalyzed by guanyl transferase. Cap is essential for the formation of mRNA-ribosome complex. Translation is not possible if the cap is lacking because the cap is identified by 18SrRNA of ribosome unit.
- Tailing: It is the addition of adenylate residues about 200-300 at the 3’end in a template-independent manner on newly formed hnRNA with the help of Poly A polymerase.
- Splicing: It is the process of removal of introns and joining of exons in a defined order. Introns are removed by a small nuclear RNA (SnRNA) and SnRNPs (Snurps).
The fully processed hnRNA is called mRNA and it is transported out of the nucleus for translation.