DNA is the blueprint of life. It is the information on the basis of which all our cells function. When we read about cells and their functions, we come across terms such as proteins and enzymes (proteins that function as catalysts), but have you ever wondered how the cells know which protein to make? Or what the exact structure of the protein actually is? This is the job done by our DNA. It basically acts as a storage unit that keeps the sequence of amino acids for every protein in the form of nucleotide sequences (amino acids are the basic units that make up the protein and their sequence determines which protein it is!). You would better understand the process if you read through our article on DNA Replication, as it covers the structure and directionality of DNA. So give it a read here!
So how do you think our DNA will tell the cell about this sequence? By sending a message! No, not an SMS or Whatsapp text, but rather a message in the form of nucleotides. The sequence of these nucleotides is unique to each protein, but these nucleotides aren’t formed of DNA, they use RNA to do this. Also, unlike the DNA, this RNA sequence is single-stranded. Thanks to our creative minds, we named this RNA as mRNA, i.e., messenger RNA.
The process of producing mRNA sequences from DNA is known as Transcription. So let’s take a look at how this happens!
We know that DNA is double-stranded, but only one strand is known as the coding strand (also known as the sense strand)! Thus, the sequence of the mRNA should be similar to the coding strand! How do we achieve that? By using the complementary non-coding strand (also known as the anti-sense strand) as the template! This is very important, so keep it in your head. The coding strand is not used as the template. Now, let’s take a look at the process step by step.
The process of making the RNA sequences is performed by RNA Polymerase, along with the help of a transcription factor. The transcription factor helps the RNA Polymerase to recognize and bind to a sequence called the promoter sequence. The polymerase unwinds around 14 base pairs of DNA and forms an RNA polymerase-promoter open complex. This single-stranded form of DNA is known as the ‘transcription bubble’.
Now for the Polymerase to continue its job and add nucleotides to make the mRNA, it needs to detach from the promoter. This happens through a process known as Abortive initiation. During this process, the Polymerase creates short mRNA transcripts that are released before the polymerase detaches itself from the promoter.
Even after detaching from the promoter, it was observed that the polymerase that remained did not move. So the question that arises is how does the polymerase read the template without moving? The answer to that is DNA Scrunching (scrunching means to crush or squeeze). While the polymerase is stationary, it pulls the DNA template into the transcription complex. The template passes through the polymerase’s active site, allowing transcription to take place without any movement required! The unwound DNA accumulates in the complex, which is why it’s called DNA scrunching. Then the polymerase re-winds and releases the downstream portion of the unwound DNA.
The polymerase travels along the template (which runs in the direction of 3′ to 5′). This means that RNA Polymerase makes the mRNA in the direction 5′ to 3′. It reads the template and adds the complementary nucleotides to the RNA sequences. This process continues until it reaches a terminating sequence.
The process of transcription can have multiple Polymerases on a single template and the transcription could take place for multiple rounds. This allows for a large number of mRNA to be created from a single template. Elongation also has a proofreading mechanism, which helps in replacing wrongly inserted nucleotides and keeping the sequence accurate!
The termination has two types, the first is Rho-independent and the second is Rho-dependent (Rho factor is a protein). Let’s look at them one by one.
- Rho-independent transcription termination – In this process, the polymerase reaches a termination sequence of guanines and cytosines. This is followed by a sequence of repeating adenines. Then, a loop is formed in the guanine-cytosine region, and as guanine forms three hydrogen bonds with cytosine, it takes longer for the RNA Polymerase to join these. All this puts a strain on the adenine region and causes the strand to break off, releasing the polymerase!
- Rho-dependent transcription termination – The Rho protein moves along the RNA sequence until it reaches the termination sequence. Once it reaches the termination sequence, the Rho protein destabilizes the interaction between template and RNA sequence.
Eukaryotes vs Bacteria
There are some differences between how transcription occurs in eukaryotes and bacteria.
- RNA Synthesis in eukaryotes takes place in the nucleus.
- Eukaryotes have three types of RNA polymerases (one each for mRNA, tRNA, and rRNA)
- Eukaryotes have at least 5 transcription factors that help RNA polymerases bind to the promoter while bacteria use only one sigma factor.