Protein Coding and Formation — Transcription
In my earlier posts, I explained all about DNA replication and how it works. But what is the use of the DNA replicating? What functional purpose does this process have? The answer is everything. Proteins play an extremely vital role in the human body and the DNA is the one responsible for the creation of it. Scientifically, this process of creation is known as “coding”. In simpler terms, the DNA contains the blueprints, which help construct the proteins. The way that the cells in our body are able to read the blueprint and use that to create proteins is what I’m going to write about in this post. This is done in a two step process — Transcription and Translation.
Let’s start with step one — transcription. So now, the DNA has replicated, that is, it has split into two semi-conservative strands (one newly created strand connected with a pre-existing/parent strand). But what happens after this? How do the strands go from a bunch of nucleotides to becoming a protein? Proteins are created in ribosomes. In order to create the proteins, the ribosomes require instructions.
Imagine a factory with workers. The workers need to be given instructions to be able to create a product. Same goes for the ribosomes. The instructions are given by the DNA and the blueprint it possesses. Image 3 shows the ribosomes reading the mRNA to make proteins.
The process of how the blueprints gets from the nucleus (location of the DNA) to the ribosomes is called transcription.
The “code” that a DNA contains is the sequence that its nucleotides are arranged in. A protein called RNA Polymerase creates a complementary strand of this sequence the exact same way DNA polymerases creates nucleotides complementary to the original DNA. The resulting strand has the exact same nucleotide arrangement of the original DNA and is called as messenger RNA (mRNA for short). The only difference is instead of thymine, the mRNA has uracil. Thus wherever the original DNA had thymine in it, the mRNA substitutes it with uracil.
So right now, the mRNA has an inverted copy of the blueprint. This is done as the ribosome has the ability to bring the inverted blueprint back to its original form (this process is called as translation). This is similar to film photography. When you take a picture, a film strip (which is light sensitive) gets exposed to light which leaves an imprint. This imprint has its colors inverted i.e it is in its “negative” form. In order to convert the film strip into a tangible picture, it must be projected onto paper. When this is done, the “negative” image gets inverted again and becomes positive i.e it becomes normal. The same pattern applies here.
DNA (original sequence) → mRNA (inverted sequence) → Ribosomes (original sequence)
After copying the DNA sequence in its inverted form, the mRNA molecules travel through pores present in the nucleus, which allows it to enter into the cytoplasm. From there, it travels to the ribosomes for translation.
