Although just about everyone knows something about DNA, I’d like to take a few moments to introduce you to RNA, the real power behind the throne.
Protein represents what biologists call phenotype – the living, breathing, metabolizing part of life. DNA is information. Other than acting as a blueprint and occasionally remembering to replicate itself, it doesn’t have a single real world obligation. It is RNA that acts as the bridge between DNA and protein, translating the message of DNA into the reality of proteins. All the basic functions of the cell require RNA. Copies of the desired DNA gene message are first copied onto one type of RNA, which is then read by a machine composed in part by some more RNA to create proteins by linking amino acids which are delivered by another type of RNA.
Let’s start the second part of our story with the sweet, if short life of Messenger RNA, or mRNA.
At a certain point in its life, the cell may get an urge to make some sort of protein or enzyme. Let’s say that you have developed an untidy habit, like smoking cigars. As anyone who has ever tried one can tell you, the first experience with nicotine is usually far from pleasant, with dizziness and nausea the usual end result. This reaction occurs because the new smoker has yet to habituate himself to the poisons in the cigar and has not yet developed a way to detoxify and break them down. Over time the continued smoking of cigars sends an environmental message to cells of the liver telling them that they need to make higher levels of the enzymes used to detoxify tobacco toxins. This message (“hey, he’s trying to kill us out there!”) travels to the cell nucleus, where special machinery locates the section along the DNA that contains the gene to produce these detoxifying enzymes, snips it open and unravels that part of the DNA to expose the blueprint.
At that point an enzyme called RNA polymerase comes along, reads the DNA code and makes an RNA copy by linking together similar building blocks (a stretch of RNA is similar to DNA except that RNA is almost always single-stranded and uses the nucleotide Uracil instead of Thymine). This is called “transcription” and just like a court stenographer transcribes the court proceedings, so RNA transcripts the proceeding necessary to make a protein. The RNA strand, called Messenger RNA, (mRNA) is then extensively primped and tweaked to clean it up and get it just right. From here it is about to embark on the ride of its life.
Once everything is set to go, the mRNA is shot through the one of the many pores which act as gates between the cell body and the nucleus. Once out into the cell proper it is carried to the real workhorses of protein synthesis, the ribosomes. Using a railroad analogy, you can think of a ribosome as a dispatcher in the rail yard, whose job it is to assemble an entire freight train. Each time the phone rings the dispatcher gets his next order:
“Fetch the Baltimore and Ohio flatbed with the Honda Hybrids on it. Attach it to the Union Pacific 3985 locomotive.”
“Next, locate and attach the milk tanker from Happy Cow Farms.”
And on and on, until you have one of those interminably long freight trains that take twenty minutes to pass by the railroad crossing as you desperately try to get to the airport.
Just like our rail dispatcher, ribosomes get the information from messenger RNA, by zipping along the code like an old fashioned ticker-tape, reading the code called 'codon triplets' to determine which amino acid to fetch, then linking that amino acid to the prior one, and fetching the next instruction, etc. until it gets a stop message.
In this job the ribosome is assisted by a different type of RNA called Transfer RNA which acts like a crusty old rail yard worker, bringing the appropriate amino acid to the ribosome. At some point the protein is finished up and released, and the messenger RNA decomposes back to the basic building blocks of DNA and RNA, called nucleotides, and ready to do it all over again.
From there the sky is the limit. Proteins are interesting in a lot of ways but perhaps most interesting in their folding tendencies, a molecular origami if you will. Depending on the amino acid sequence and length proteins will fold into a myriad number of complex three dimensional shapes, and it is these shapes that give them their unique powers over the environment.
For example a protein of a certain shape may function as an enzyme, taking sugar molecules and attaching them together, turning single sugars onto cellulose, an important dietary fiber. The protein that results from our string of amino acids might be an insulin molecule, helping to control the owner’s blood sugar, or even a protein that helps DNA do its job, perhaps even part of another ribosome!
As I said, the sky is the limit.
The RNA Queen is so basic to life that many scientists think that perhaps life originated with it, and not with DNA: That DNA came along later as a way to 'memorialize' the work of RNA.