How it works: The DNA to Protein Pathway!

In this installment of How it works, where I talk about various different aspects of science, I am going to focus on micro ribonuclease, also known as miRNA!  These tiny molecules have often been confusing to scientists – for clarity, read on!

One of the most important, if not the absolute most important, pathway in a cell is the creation of proteins.  Proteins are the building blocks of life, and make up most of your cells, carrying out various functions, providing structure, support, helping to create new molecules, digest food, fight off disease, and many more functions!  But where do they come from?

First an introduction: we all know about DNA, the building blocks of life, the code that contains the instructions for creating any organism, the double-stranded helix that looks like an incredibly long and twisted ladder.  But how does DNA turn into a baby, or a dog, or a palm tree?  Well, it needs to go through some conversion!

The first conversion step is the creation of RNA, which is very similar to DNA.  While DNA is double stranded and made up of the bases A, T, G, and C, RNA is only single stranded, and instead of using T (thiamine), it uses U (uracil).  The creation of RNA from DNA is known as transcription, while the conversion of the RNA into proteins (which make up the various parts of the cell and are what makes it ‘tick’) is known as translation.

This may seem confusing, so let’s use an analogy!

Think of DNA as the blueprints of a skyscraper.  These blueprints are very valuable, since they contain all the instructions for how to build this massive building!  Also, because they include every single detail of the construction, there are a lot of pages of blueprints.  There’s only one copy of these blueprints.  Now, if all the construction workers building the skyscraper had to keep on handing off this giant book of blueprints to each other, things would very quickly get disorganized and not much work would be accomplished.

So, what if we made things a little easier on our construction workers?  Instead, let’s just make photocopies of the relevant pages, and give each worker the pages for his specific assignment, copied out of the blueprints.  This way, you can have tons of copies of the blueprints floating around, each one short and to the point, and the workers can destroy their copies when they’re done, keeping the original blueprints intact.

In this case, all of those copies are like RNA!  This RNA is actually referred to as messenger RNA, or mRNA, because it carries the message of how to build a protein.  So, there’s a guy sitting next to the blueprints in the main building making photocopies – what he’s doing is known as transcription.  All the workers that get the copies then go on to build parts of the building – just like mRNA leads to the creation of proteins.  This is known as translation.

Still with me?  Good!

As I mentioned, there’s only one copy of the DNA.  In humans, this isn’t quite true – there are two copies, one copy coming from each parent.  Sorry to lie to you about that.  But still, 2 copies for an entire cell is not a lot, so the DNA remains inside the nucleus of the cell, where it is safe and protected.  Inside this nucleus, a protein known as RNA polymerase (remember this from our discussion of PCR?) is responsible for reading along an exposed strand of DNA and synthesizing the single-stranded RNA copy.

That single-stranded RNA copy isn’t ready to be turned into a protein yet, however.  It needs to have some modifications made to it!  One step is that some parts of the code may be unnecessary, and certain protein complexes will chop out these unnecessary sections (known as introns) and reattach the two ends of the RNA.  Also added on to the RNA is a “cap” at one end, that helps keep the RNA from being prematurely broken down by enzymes.  At the other end of the RNA is a “tail”, which contains instructions about how the RNA should be processed, as well as sites for miRNA binding (more on that later).

After the newly made mRNA has had these edits made to it, it is considered to be a mature mRNA, and is ready to be sent out into the big, wide world outside the nucleus!  So out it goes, into the cytoplasm.  Here, it floats around until it encounters a large protein complex known as a ribosome.  The ribosome attaches onto this RNA and, like a computer program, reads its code!  I will talk more about DNA code in another update, but the code specifies a specific order of amino acids.  As the amino acids are assembled in this order, they coil together and become a specific protein.  So this ribosome reads the RNA code and assembles the amino acids in the specified order, thus creating a protein.  This is translation.

Now, that mature mRNA hangs around for a while outside the nucleus, constantly being used by ribosomes to make copy after copy of protein.  But it can’t stay forever, or else the ribosomes would waste all their energy making extra copies of the protein that it specifies!  So, over time, that tail is shortened more and more.  The tail is representative of the mRNA’s age – the shorter the tail, the older the mRNA.

Eventually, that mRNA has such a short tail that its code is exposed!  At this point, another protein, known as RNase, steps in.  This enzyme, RNase, is designed for the specific purpose of destroying RNA without a tail.  It latches on to the mRNA and chops it up into pieces, which are then recycled and brought back inside the nucleus, where they will be reused to make new mRNA!

And that, ladies and gentlemen, is the pathway for making proteins, from the DNA (the blueprints) to immature mRNA (the unedited blueprint copies) to mature mRNA (the copies with proper notes for the workers) to protein (the actual product specified on the blueprints).  Not that bad, is it?

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