A Very Sophisticated Code no 3 Alternative Splicing
Alternative splicing Alternative splicing is the solution to a problem that has puzzled Scientists for some time. It has been recognised that the number of genes in complex creatures like ourselves does not seem to be enough to build the large number of proteins that exist in the huge array of different cell types our bodies are constructed from.
To illustrate the puzzle, consider that the humble Yeast has around 6,000 genes, the tiny roundworm c.elegans has around 19,000 genes and the Mustard plant has around 26,000 genes and the human genome contains only around 30,000 genes. The total number of proteins in the human body is an area of active research, but some researchers put their estimates into the billons. So where does the information necessary to build these extra proteins come from? The answer is alternative splicing.
To understand the marvel of alternative splicing it is first necessary to understand what regular RNA splicing is. (it first needs to be recognised that this is another area of active research and a bare bones description is here presented)
Prokaryotic (bacterial) DNA is transcribed (copied) as a complete strand. it is read and copied in a linear fashion just like this text. However more complex eukaryotic cells found in all multicellular life are not like that. The original RNA message contains stretches of code (known as Introns) that will not be part of the completed protein and therefore need to be removed before the process of translating the information into a protein can begin. The following 90 second video gives an simple introduction to the process.
The molecular machine that carries out this process is called a spliceosome, it is a large and extraordinarily complex molecule. It is made up of as many as 300 distinct proteins and five separate RNA subunits that come together to perform the various steps necessary to complete the task.
It may at first seem strange that the information encoded in the introns is even there in the first place however, we will see shortly, they play a vital part in the regulation of alternative RNA splicing.
So what is Alternative Splicing?
To help us analyse just what the alternative splicing process does it is valuable to take a look at the work of a film editor. When a film director has finished the filming process the editor has the task of assembling, or splicing together, the various film clips together in a coherent order. This involves selecting which clips to use, which to discard and which order that they will be best presented to effectively tell the intended storyline. Some directors even film alternative endings to the film which can be selected later during the editing process. This process involves active decision making. The work of the editor will have a dramatic effect on the finished product. In fact when you consider just how many short clips are filmed, the amount of potential variation that is available by the editing process is simply enormous.
What we have just described here is a process of alternative splicing. Basically the same process takes place in the eukaryotic cell. After a gene is first transcribed (copied) it contains arears of code (called exons) that can be used to produce a finished protein. These are separated by introns that largely need to be removed to form a mature mRNA ready to be translated into a finished protein.
However just like with a human video editor, the alternative splicing process has the capacity to change the end product and this is exactly what occurs in our cells.
Scientists have discovered a least 7 distinct modes for the alternative splicing process, this combined with the fact that many genes are made up of a large number of Introns and Exons results in a huge variety of finished proteins, all with varying properties that can be constructed from the same gene.
It is currently believed that around 95% of all multi exon containing genes within the human genome are alternatively spliced.
It was once thought that Introns were not functional, in fact they were at one time dubbed “Junk DNA”, this assumption has now been overturned. Science now realises that Introns play an important role in the regulation of gene expression and they are part of the control process that governs how genes are alternatively spliced.
For more information on how introns function please see the superb video in the following link.
Alternative splicing clearly demonstrates the remarkable level of organisation written into the informational instructions contained within biological systems.
Conclusion Under the heading of a Very Sophisticated Code we have considered just four examples of the mind boggling engineering feats of coding that are to be found within our genome. We have looked at how,
The basic genetic code is supremely optimised to reduce the damaging effect of copying errors (mutations)
Overlapping genes efficiently contribute to data compression.
Additional functional instructions nested within same data string using a second language. Functional instructions are alternatively spliced together to produce multiple protein products from same gene. We could have also looked at Proof reading and error correction systems or remarkable unfolding subject of epigenetics, but I feel the point has already been powerfully made already. The semiotic coding contained within biology is breathtakingly sophisticated, way beyond any computer coding mankind has been able to write.