Understanding the Code of Life (Part 5: Genetic Code)
The best way to understand DNA is to simply look at it. So let's do that right now:
Here we have a couple of images of the famous double-helix of DNA. These images shows us the basic structure of a DNA molecule. DNA consists of two strands which wind around each other to form what looks like a twisted ladder.
What we are currently interested in are the 'steps' of that ladder. These steps are actually a molecule known as a nucleotide. What interests about this molecule is that each nucleotide contains one of four chemical bases. These bases are:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Thymine (T)
Don't worry too much about the chemical names (in general we'll refer to them by their letter abbreviations A, C, G & T). What we need to know is the function of these four different chemicals bases. What are they doing here? What purpose do they serve?
A Biological Hard Drive
These chemical bases help store information – biological information. Think of DNA as a biological hard drive. Just like a hard-drive contains a series of 1s and 0s to store digital information, so too DNA uses four different chemicals (A, C, G and T) to store digital information; namely, the information of which amino acids to use for a given protein and in which order.
And just like a computer uses ASCII code to sync those 1s and 0s to letters of the alphabet, so too the cell uses a genetic code to sync the As, Cs, Gs and Ts to amino acids. In fact, we can write a similar table for the relationship between the chemical bases of DNA and amino acids as we did between 1s and 0s and the letters of the alphabet, as follows:
In the above table, the top row lists different types of amino acids and the bottom row lists the coded representation of that amino acid (known as a codon) via a sequence of three nucleotides bases.
Implementing the Code
Here we have in front of us the code of life. By stringing together long chains of the right nucleotides in the right order, DNA can direct the construction of proteins. All that is needed is a mechanism to a) read the appropriate sequence of nucleotides, b) translate that sequence into the language of amino acids and c) implement the instructions contained within.
Again, an analogy with a computer may help. Imagine you downloaded and stored the entire works of Shakespeare to your computer hard drive. What do you need to do in order to print out a chapter of one of those works?
To start with, you need to search for, find and retrieve the relevant section of 1s and 0s on your hard-drive that represent that particular chapter. You then need to 'send' those 1s and 0s to your printer, where they are 'translated' into the English language and printed onto a piece of paper.
This is basically what happens within our cells. When our bodies need more hemoglobin, the cell needs to search for, locate and retrieve the relevant section of nucleotide bases (the As, Cs, Gs and Ts of the DNA language). It then needs to send those As, Cs, Gs and Ts (or, more accurately, a copy of them) elsewhere in the cell where it translates them into the amino acid language of proteins. It then 'prints' out that sequence of amino acids and folds them into a working protein.
Of course, print is not really the right word. What it actually does is join them together in the proper sequence. The point is, though, a) DNA is a source of biological information and b) there is a means of retrieving and implementing the information contained within DNA.
Escher Would Be Proud
Now here is where things become interesting. We just talked about searching for, retrieving and sending information located within DNA. How is that done? What in the cell performs that function? The answer is rather surprising – proteins.
Now, at first thought that may not sound so surprising, but let's think about this for a second. It takes proteins to retrieve, send and process the information contained within DNA. But those very proteins themselves need DNA to come into existence.
So you need DNA in order to form proteins and you need proteins in order to process the information in DNA. It basically reminds one of the famous Escher painting Drawing Hands
Each hand is drawing the other hand at the same time. But how is that possible? Obviously one hand has to draw the other hand first. But how can one hand draw the other hand until the other hand draws that hand. They each need to draw each other into existence before they can draw the other hand into existence.
Similarly, in the first cell(s) that ever existed. You can't start to process the information in DNA until you have DNA processing proteins. But those DNA processing proteins don't exist until the DNA has been processed.
Is your head spinning yet?
In this manner DNA differs significantly from a computer. With a computer, there can be one manufacturer of the computer, another one of the printer and a third party which programs the software that runs the computer and printer.
It doesn't work that way in the cell. In the cell, everything is made by the same 'manufacturer'. DNA contains the information on how to make all the different proteins that the cell needs, including the proteins needed to process the information contained within DNA.
In fact, DNA doesn't merely code for the machinery of the cell, but it actually codes for the code itself. Remember the codons that we mentioned above – the three letter nucleotide base representation of amino acids. We may rightly wonder where that representation came from. How is it determined which amino acid is symbolically represented by a particular codon (i.e., by a particular sequence of three nucleotide bases)?
In short, where does the genetic code come from?
Well, the answer comes from another protein with a rather foreboding title: Aminoacyl tRNA synthetase. Without going into the technical details, it is this protein which associates an amino acid with a particular codon.
And how does that protein know which amino acid to associate with a particular codon? Well, the information is stored in DNA, of course. What this all means is that it is DNA which determines the meaning of the genetic code which is used to give meaning to DNA.
Basically, it's Escher all the way down.
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