When the structure of DNA was discovered in the middle of the 20th century, it was soon found that only some 2% of the molecule consists of genes coding for proteins. The majority of the DNA was considered to be ‘junk’ left over from the evolutionary progress of protozoa to today’s life forms. However, since the turn of this century, research has revealed that the whole of the DNA can be transcribed into RNA, and that these code for proteins that have a variety of vital functions. There is no ‘Junk DNA’.
Much of the non-protein-coding genes make transcription factors that add or remove methyl groups to or from cytosine bases on the protein coding genes. The effect of this is to turn the genes off or on. This is necessary since different types of cells such as brain, heart or bone need different combinations of active genes.
When a protein coding gene (of which we have some 25,000) produces messenger RNA to be coded into proteins, the sections of mRNA can be spliced together in a number of different sequences to produce a seemingly infinite variety of proteins to do different jobs in the different cell types. These can be everything from bodily structures to enzymes to facilitate cell reactions.
There are trillions of cells in our bodies, each type having its own customised configuration of DNA, its genome. As with all processes, the copying of DNA leads to mistakes. If these mutations were not repaired, the cells would soon die. There is a system of monitoring the integrity of the DNA and rapidly repairing breaks by threading it through a complex machine built by the DNA. Damaged parts are removed and the correct sections inserted.
Researchers discover more and more of the complexity of our genome all the time. Clearly, far from being junk left behind by a non–existent evolutionary history, the genomes of all life forms are exquisitely designed and crafted machines. We are fearfully and wonderfully made, as the Psalmist put it three millennia ago.