Messenger RNA is used to to synthesize proteins from DNA. Genes are transcribed into mRNA, and then passed to ribosomes which translate the genetic information into proteins.

mRNA: What does it mean?
a) most Rain Not Acidic
b) many Rats Now Angry
c) mary Resents Noxious Arithmetic
d) messenger Ribo-Nucleic Acid

...Yes, the last answer is the correct one.

mRNA is the postman in our cells. Think of it like Hermes or Mercury in ancient Greco-Roman mythology, the one that rushes from one place to another carrying information vital to our bodies.


What's RNA mean?

RNA stands for ribonucleic acid. Our bodies have two methods of storing information. One of this methods is by using DNA and the other is by using RNA.

Think of DNA as a recordable CD, which cannot be easily erased, and is a durable and reliable form of storing information. RNA on the other hand, is like that crummy piece of paper we jot stuff down on. It has a very short lifespan compared to DNA, and is easily "lost". These differences are important to the functioning of our body.

All of us (assuming you're human) came from a fertilised egg. This fertilised egg contained all the DNA that is currently present in every cell of our body. The DNA contains instructions for each and every type of cell that makes us who we are. So the programme for the development of our noses, and fingers, and toes are all stored in the DNA. this is why it has to be like a recordable CD, unchangeable and durable.

However, looking at our bodies, we realise that even though all the cells contain the same DNA, they look different. Our eyes, our lungs and our fingers all took different paths in their development. (That's why we're not one big stomach.) These differences were determined by RNA. As RNA is short-lived, it does not interfere with the next stage of development.

RNA is made up of 4 bases: namely, Adenine, Uracil, Guanine and Cytosine. Imagine them to be LegoTM bricks that build up the Great Wall of RNA.


Cells, nose and toes

To know the importance of mRNA, we first have to understand what is in a cell. We will define a cell here to mean a non-bacteria cell, or a cell most like the ones in our bodies (eukaryotic cell).

The cell contains the nucleus, which holds the DNA. The nucleus is surrounded by a nuclear membrane, or nuclear envelope. Outside the nuclear envelope is the cytoplasm, in which floats different types of "processing stations", where proteins are manufactured. The type of proteins manufactured, and the quantity made, determines how we look like. Whether we have red or blonde hair, for example. And the cell is itself surrounded by an envelope called the cell membrane.

The nuclear membrane has several pores. This enables our messenger to run about carrying information from the nucleus, where the information is stored, to the outside of the nucleus, where the information is processed.

The DNA that resides in your nucleus is quite special. Your body knows that for a nose to function properly, it does not need the functions of a toe. (Imagine your nose having a toenail!) Therefore, those genes that are special to the nose is turned ON, and those of the toes are turned OFF. This occurs by the combined effort of several proteins and minerals in your body.

For those genes that are turned ON, the corresponding mRNA of that gene is made. So the presence of a particular mRNA in your cell determines how your cells would look like and function. And this is why your nose does not look like your toes.


Skeletons in the closet

Like the skeleton that we all possess, mRNA has a defined structure.

All nucleic acids (DNA and RNA) have a direction in which it is read, much like the way we read English (from the left to the right, from up to down). The direction in which nucleic acids are read is from the 5' (five-prime) end to the 3' (three-prime) end. The whats, whys and how-comes of this directionality will not be discussed here. Just imagine the 5' end to be the head, and the 3' end to be the bottom of the mRNA.

So when we discuss the structure of the mRNA, we will also naturally start with the head, following all the way to the tail.


 5'   UTR        MESSAGE                UTR               3'
   C~~~~~~~AUG~~~~~~~~~~~~~~~~~(STOP)~~~~~~~~AAAAA//AAAAA


At the head of the mRNA, there is a cap. (No, really, it does wear a cap! Like boys! In the days of gender discrimination!) The cap is known as the 5' 7-methyl guanosine cap, and serves to identify the RNA as an mRNA (there are several other types of RNA, which do not concern us right now).

Just below the cap (and head) is the 5' untranslated region (5' UTR). The UTR is a part of the mRNA which is not made into protein.

After the 5' UTR comes the ACTUAL MESSAGE (we've reached the body). The message starts off with a standard opening - AUG (adenine-uracil-guanine). It's like saying "Hello!" to start a conversation. This is how the cell knows where to sit up and pay attention. From this point on, all this information will be used to make a protein.

The message ends politely with another mRNA code equivalent to "Goodbye!" (Actually there are 3 codes that can be used, like variations on a theme.) Then another UTR, the 3' untranslated region (3' UTR) follows directly after the message ends.

And finally, the mRNA ends with a splendid poly-adenine (poly-A) tail (yes, like a fish!). This is a long (100-250 bases) tail made up entirely of adenine-adenine-adenine-adenine... you get the idea. The longer this tail is, the more stable and longer-lasting is the mRNA. Remember, we don't always want a super-stable mRNA strand, so its good that the lifespan is determined by this tail.

So now you know thousands of these small boy-like fish-like message-carrying structures run about in your cells. How are they made?


Hie ho, hie ho, its off to work we go!

We can now examine how our cells get the information they need just the way they need it.

First, another look at the original blue-print, the DNA. The genes in the DNA actually contains a lot of pieces of junk in between the real information. These junk are called introns; they interrupt the message. The useful bits are called the exons.

mRNA is made in a process known as transcription. An enzyme known as RNA polymerase attaches itself to a gene and starts reading it. Assuming the gene is not turned off, it will then proceed to make mRNA.
     .-(DNA)---gene-------------------------.  
    /                                        \
---'                                          '----



       
     .-P------------------------------------.  
    /                                        \
---'                                          '----
   
   RNA Polymerase attaches.



     .-------------P------------------------.  
    /  ~~~~~~~~~~~~                          \
---'                                          '----

   
   RNA Polymerase moves along and transcription occurs.
mRNA (~~~) is made.

mRNA then undergoes some modifications during transcription. This includes the addition of the cap structure to identify it as an mRNA.

Sometime during and after transcription, mRNA undergoes splicing, whitch removes the introns and leaves the exons. mRNA contains only the information that is needed, without all the junk that is in the DNA.

Also after transcription, the poly-A tail is added on. The mRNA is then transported out of the nucleus.

Outside the nucleus, most of the mRNA migrates to one of the "processing stations", called the rough endoplasmic reticulum (RER) where the information contained by the mRNA is made into protein. This process is known as translation. (This is appropriate considering that its almost exactly like translating one language into another.)


Overworked, underappreciated

Once the mRNA leaves the nucleus, it starts work almost immediately. Tens of ribosomes (translating machinery) attach themselves to the mRNA and start reading off it, making several strands of proteins all at once.

Also, the mRNA itself starts undergoing degradation. This is the equivalent of a human being undergoing the aging process. The mRNA is chewed away by several types of enzymes (ribonucleases, exonucleases and endonucleases). Both young and old mRNA undergo this process. You might imagine that every single mRNA that leaves the nucleus fears for its life, and that its single duty is to allow ribosomes to make as many proteinous copies of it as possible before it dies.

Some numbers are needed to show just how short-lived mRNA are. In prokaryotes (bacteria) the entire lifespan of an mRNA might be over in 5-10 minutes. In eukaryotes (think mammals, or humans) mRNA takes a longer time to be degraded. Most eukaryotic mRNA might take about 5 minutes to be transcribed and about 15-20 minutes for post-transcriptional modifications to be made before leaving the cell. Their half-lives in the cytoplasm range from 4-24 hours. In other words, within 4-24 hours, a certain population of mRNA would have been cut by half.

Live fast and die: that's the motto of the mRNA.



Sources:
Genes VI, Chapter 7, by Benjamin Lewin. (Printed 1997)
mRNA -- http://www.biochem.uwo.ca/meds/medna/mRNA.html
Visualization of mRNA Transcription -- http://www-compiler.csa.iisc.ernet.in/~mbk/visual.html

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