DNA Transcription to mRNA

DNA is a double-stranded structure found in eukaryotes (multi-cell organisms) whereas RNA (Ribonucleic acid) are single-stranded structure found in prokaryotes (single-cell organisms like bacteria). During transcription, the DNA is converted into RNA inside the nucleus of a cell. Once converted the messenger RNA (mRNA) comes out of the nucleus and the translation process starts in the cytoplasm of the cell to produce protein.

If the DNA is considered the genetic code, it has to be read and understood to process those instructions to carry out how the cell and thereby the entire organism functions. This process of reading the genetic code in DNA happens in two ways – transcription and translation.

DNA is a double-stranded structure found in eukaryotes (multi-cell organisms) whereas RNA (Ribonucleic acid) are single-stranded structure found in prokaryotes (single-cell organisms like bacteria). During transcription, the DNA is converted into RNA inside the nucleus of a cell. Once converted the messenger RNA (mRNA) comes out of the nucleus and the translation process starts in the cytoplasm of the cell to produce protein.

Even though the DNA has two strands, only a single strand is used as a template, always. The strand used as a template is called the template strand (non-coding) or an anti-sense strand. The coding happens in the other strand which is called the non-template (coding) strand or sense strand– the sequence of this strand resembles the same as the newly produced RNA. Within the same chromosome, a template strand can also act as a non-template strand for other genes.

When the DNA looks like the trunks of a tree under the electron microscope, the RNA looks like branches emerging out of them. DNAse and RNAse are enzymes that when applied eliminate the DNA (trunk-like structures) and RNA (branches), respectively.

RNA Polymerase

Three types of RNA polymerases, namely −

• RNA polymerase 1

• RNA polymerase 2 and

• RNA polymerase 3.

Each of them used to transcribe different RNA. Each of these polymerases attaches to several other proteins called transcription factors. Each polymerase has a specific transcription factor and core promotor site of its own. The transcription factors (smaller proteins) help the polymerase to attach to the promotor site. Only when the polymerase recognizes its specific core promotion site does it gets attached to it at that particular spot.

Transcription Stages

The five stages in the transcription process are activation, initiation, elongation, modification and termination.

When a DNA strand undergoes activation and initiation stages, it’s called an upstream sequence. When the DNA strand undergoes the termination stage then it is called a downstream sequence.

RNA polymerase 2 needs four different proteins, namely, transcription activators (Sp1 & CTf1), architectural regulators, coactivators and base transcription factors, to attach itself to the core promotor site.

Activation

The transcription activators Sp1 and CTf1 will bind themselves to the enhancer region of the genome, at the GC box and CAAT box, respectively and sends a signal to the core promotors to start the activation process.

The architectural regulators (also called High Mobility Group, HMG) attach themselves to the DNA and bend the DNA to enable the enhancers to come closer to the core promotor to signal them to start initiation. The coactivators or mediators act as a bridge between the enhancers to signal the core promotor to start initiation.

Initiation

During initiation, the RNA polymerase attaches itself to the core promotor of the DNA to start reading it. The initiation process is carried out by several basic transcription factors namely TF2A, TF2B, TF2D, TF2E, TF2F and TF2H. CTD (Carboxy terminal domain) is a tail-like structure attached to the RNA polymerase.

The Tatabox Binding Protein (TBP) present in the TF2D recognizes the Tatabox and helps TF2D attach with the tatabox which is stabilized by TF2A. Followed by TF2A, TF2B also attaches itself to the tatabox while the other three – TF2F, TF2E and TF2H – help the RNA polymerase to attach to the core promotor.

Elongation

TF2H will help in splitting the double-helix of DNA into a single strand. TF2H helps in phosphorylating the RNA polymerase making it leave the core promotor site to move along the single DNA strand freely. Hence, starting from the core promotor site, the RNA polymerase moves along the DNA, with occasional pauses at checkpoints, until it reaches the termination site. As it moves along, a single strand of an RNA transcript is produced before the RNA polymerase dissociates itself from the DNA.

Modification

The mRNA sequence produced by the elongation process contains two areas – exons, strips which code for a protein and introns which are non-coding strips. The intron strips must be removed from the mRNA sequence while attaching the split exons to form the mRNA strand for translation to occur. The 5’ and 3’ ends are also added to their respective ends. This whole process is called intron splicing.

Intron splicing is carried out by a protein-RNA complex called the spliceosome which also attaches the exon segments. The completed mRNA sequence now exits from the cell nucleus and reaches the cytoplasm.

Termination

Two proteins called CPSF (cleavage and polyadenylation specificity factor) and CSTF (cleavage stimulation factor) attach themselves to the mRNA to stop the RNA polymerase from further modifying the double-strand DNA. The mRNA detaches itself from the polymerase.

Once the transcription process is done, the translation process starts in the cytoplasm of the cell to create protein. The complex process of transcription involves several proteins, enzymes and lysine among others to carry out every step during the attachment of polymerase, detecting the specific site, etc., before producing the mRNA.

FAQs

Qns 1. What are the proteins involved in the initiation process called?

Ans Transcription activators (Sp1 & CTf1), architectural regulators, coactivators help begin the initiation process during transcription.

Qns 2. What is intron splicing?

Ans Once the mRNA is generated by elongation process, the non-coding intron regions of the mRNA must be removed and the split exons, which are the coding regions, must be attached together in order to carry on with the translation process outside the cell nucleus. This process of removing the introns from the RNA strand is called intron splicing.

Qns 3. Where does translation occur?

Ans After transcription is carried out in the cell nucleus, the mRNA thus produced will travel from the nucleus through nuclear pores and reaches the cytoplasm of the cell where translation occurs.

Qns 4. What are the proteins involved in the termination process?

Ans Two proteins called CPSF (cleavage and polyadenylation specificity factor) and CSTF (cleavage stimulation factor) attach themselves to the mRNA to stop the RNA polymerase from further modifying the double-strand DNA. The mRNA detaches itself from the polymerase.

Qns 5. What are the proteins involved in the initiation process?

Ans During initiation, the RNA polymerase attaches itself to the core promotor of the DNA to start reading it. The initiation process is carried out by several basic transcription factors namely TF2A, TF2B, TF2D, TF2E, TF2F and TF2H. CTD (Carboxy terminal domain) is a tail-like structure attached to the RNA polymerase.