Imagine we have a beautiful article with us. What if we want to make a copy of it? The best way would be to make use of a cast or a mould.
Similar strategies are used by ourselves. The most important task for a cell is to synthesize proteins based on the codes in DNA. But how are these codes read?
What's the process that helps decode this language? And are proteins synthesized merely by the sequence of "A T G and C"? The answer to all questions lies in the central dogma of life.
But what exactly is the central dogma? Well, DNA makes copies of itself by the process of replication. Post replication in every cell the code of the DNA is cracked and transcribed into a messenger RNA.
This mRNA is then read and the code is translated into proteins. This interesting concept is nothing but the central dogma. We know that the replication process is semiconservative in nature.
So now our focus will be on the two processes, namely transcription and translation. Let's begin with transcription first. As we can see here, the process in which mRNA is synthesized based on the parent DNA template, is called transcription.
The part or segment of DNA which encodes for a particular protein is called gene. So how are proteins made from the sequence of base pairs in the gene? Let's go in the reverse direction now.
Proteins are a specific sequence of amino acids. So the various amino acids come together in a specific series to form a chain that makes up the proteins. Now to get the amino acids in a series we need a set of instructions.
These are present in the form of mRNA. And where does the mRNA come from? It is synthesized from a special template.
Can you guess what this template is? Give it a shot. That's right, the template is a gene.
So a segment of the DNA that encodes for protein is copied first. The copy is in the form of mRNA. This process of making mRNA is called transcription.
Now can you tell me why this process is needed? Well, it's quite simple. If there is no transcription then how will mRNA be synthesized.
And if that's not done, then how will the proteins be made? Hence, transcription is vital. But it's not as easy as it seems.
There are several enzymes and sub steps involved in the process. We are referring to the steps like opening of the DNA double helix at a specific point. Assembling the set of enzymes gathering of nucleotides and so on.
After all these steps, an mRNA is formed. The mRNA is like an exact copy of the DNA sequence that coats the protein. The only difference between the two, is that mRNA is single stranded and the base thymine is replaced by uracil.
Now that the mRNA is ready. It's all set to enter the cytoplasm. That is where the next step begins.
It's the translation process. Let's have a look at this interesting process in the upcoming part. Imagine you're appearing for an exam, but the paper presented to you is in a language that you don't understand.
You are prepared for the exam but interpretation of the questions is a problem. What can be done? Some help from a translator would work wonders right?
So translation is the simplest way to solve this problem. Clearly a wise move would be to decode the unrecognisable language and then attempt the exam. Now imagine this cell to be in a similar situation.
Let's assume that this cell encounters a set of instructions for performing a task. But the instructions are written in a coded language. So all it needs to do is to translate this language and then perform the task.
Any idea which process are we talking about? That's right. We are talking about the translation process.
In continuation with the process of transcription from the previous part, let's now understand the next step of the central dogma of life. As the name suggests, translation means converting something into a simpler understandable form. And what's getting translated here?
That's correct. The code hidden in the mRNA molecule. The process of transcription helps in copying the code written on the DNA.
That means it forms the mRNA. Now the mRNA comes out of the nucleus through the nuclear pore and enters the cytoplasm. Here the process of translation begins.
To begin with let's find out the directionality of this mRNA molecule. This is the 5 prime end of the mRNA and this is the 3 prime end. And how do we know this?
Are we randomly stating this? No, it's not random. We can recognize these ends with the help of specific markers.
The three prime end has a continuous series of the "A" nucleotides. This region is called the poly A tail. The 5 prime end on the other hand has a cap of methyl group.
Thus this end is said to be methylated. But why is the directionality needed? Well, it's extremely important.
The assembly that carries out transcription binds to the mRNA near the 5 prime end. So how does the assembly know that this is the 5 prime end? The easiest way to recognize the 5 prime end is by the presence of this methyl cap.
This is how the assembly understands to bind near the 5 prime end and not to the poly A tail. Now what exactly is this assembly that we're referring to? Well, that's the ribosome unit.
Two units of ribosomes, one small and one large, form an assembly that helps in the process. Ribosomes are basically a complex assembly which acts like a molecular machine for protein synthesis. Remember the small dots attached to the surface of the rough endoplasmic reticulum?
We find them in the diagram of a typical cell. These are nothing but Ribosomes. So the ribosomes are usually made up of two subunits a smaller and a larger one.
The smaller subunit binds to the mRNA at the initiation site. Once this is done the larger subunit binds and completes the assembly. Now the other type of RNA called tRNA steps in to get the remaining work done.
At the end a chain of amino acids is formed and released from the ribosomal unit. So doesn't this chain of amino acids look familiar? You got that right.
It's our protein. The chain is further modified to achieve a proper configuration so that the protein becomes functional. This is how the translation process works.
The code of mRNA is translated and the protein chain is synthesized out of it. The process of transcription and translation together are called as gene expression. The code of a gene is transcribed first and then translated into the form of proteins.
There is an extremely important thing to note here. Whatever we've talked so far is all about the processes in eukaryotes. Such high level of organization is found only in eukaryotes.
But we know what eukaryotes are right? A cell with a well defined nucleus and a highly organised system is a eukaryotic cell. On the other hand cells which lack of proper membrane bound nucleus and organization are called prokaryotic cells.
Since they lack the proper membrane bound organelles and nucleus they do not undergo such complex and sophisticated processes. So the replication process in prokaryotes is slightly different. This was a brief idea about the transcription and translation process as seen in the eukaryotes.
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