Learners may believe that only a tiny fraction of the DNA in the human genome (the 23 pair of chromosomes) is functional, and the rest is just junk that has accumulated during evolution.
The chromosomes are comprised of vast chains of DNA (deoxyribonucleic acid, a a material* that has repeating molecular structure that incorporates 'bases' that can be included in various sequences), including sections, genes, which are indirectly responsible for the production of the myriad proteins which are key functional molecules in living structures and mechanisms.
When 'the' human genome was first mapped out in detail, scientists were surprised to find that the genes that (indirectly) coded for the proteins made up only a small fraction of the DNA in the genome. The rest, sometimes said to be about 98%, was labelled as 'junk DNA'.
This label was perhaps unhelpful, and was likely influenced by the 'central dogma' of molecular biology (the scheme: DNA ➝ RNA ➝ protein) which focused on the role of genes in giving rise the proteins essential to living species. This was seen as the key explanatory idea for how the complexity of living things was represented in a physical substrate (DNA) that allowed inheritance (so a single cell zygote has the potential to grow into an adult human being (or polar bar, or eagle, etc. – or even into a caterpillar and then a very differently organised butterfly) like its parents, with its many tissues and different cell types – as the information represented by the sequence of bases in the DNA act in effect as if the instructions for development.
It is now believed that some of the DNA found in a genome is the result of retroviruses that are able to use RNA (ribonucleic acid) to code for DNA and insert genes into infected cell genomes. (It is sometimes claimed metaphorically that the 'genome contains fossil viruses'.)
However, research is increasingly revealing that much of the DNA which does not code for proteins, is nonetheless important in the functioning of cells, and that indeed sometimes mutations in these so-called 'non-coding' regions (that is regions that do not indirectly code for proteins – but may still code for RNA) may be responsible for some diseases. It seems possible that a good deal of the 'junk DNA' has effects on cell functioning, for example moderating protein production, and is far from inert – and so should not be seen as junk.
So, far only some of the junk DNA has been found to be functional, but it is likely that over time research will uncover further regions that are involved in cell functioning. How much might genuinely be junk is still a matter for speculation.
* We normally think of DNA as a substance but strictly a substance has a unique molecular structure. There is a vast number of possible DNA structures (indeed a vast number of actual ones!), so strictly we might say there are many DNA substances.
[Please be aware that a word may have different nuances, or even a different meaning, according to context.]« Back to Index
